The Effect of Temperature, Lecithin Content, Voltage, Resistivity, Viscosity, and
Surface Tension on Droplets/cm² During Electrostatic Spraying of Oil
THESIS
Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the
Graduate School of The Ohio State University
By
Didem Peren Aykas, B.S.
Graduate Program in Food Science and Technology
The Ohio State University
2012
Master’s Examination Committee:
Dr. Sheryl A. Barringer, Advisor
Dr. V.M. Balasubramaniam
Dr. John Litchfield
Dr. Gonul Kaletunc
ii
Copyright by
Didem Peren AYKAS
2012
iii
ABSTRACT
In the food industry it is important to evenly coat the surface of food when oil soluble
flavor, color or nutrients are added. Soybean oil was sprayed on oil sensitive paper at 4, 22 and
47⁰C, 0 to 40kV with 0 to 15% lecithin to determine droplet distribution. Lecithin decreased
electrical resistivity, increased viscosity, and first increased then decreased surface tension.
Increasing temperature decreased resistivity, viscosity, and surface tension. Increasing voltage
decreased the drop size and increased the number of droplets. At 0 kV and 0% lecithin oil did
not atomize therefore the drop sizes were significantly larger and the number of droplets/cm2
lower than with electrostatic atomization. 15kV was the threshold voltage for atomization.
Increasing lecithin content up to 10% increased the number of droplets/cm2, while further
lecithin addition increased, decreased, or had no effect. Increasing voltage and temperature,
and decreasing resistivity, viscosity and surface tension produced a higher number of
droplets/cm2. Voltage had the greatest effect on the number of droplets followed by lecithin
content and temperature. The Weber number for the onset of droplet formation was
determined to be 0.01. Droplet size range or span decreased with increasing voltage and lecithin
content. Thus, higher voltage, higher temperatures, and 10% lecithin produced the lowest
resistivity and the smallest droplets.
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PRACTICAL APPLICATIONS
Liquid electrostatic coating can produce more uniform and efficient coating than
traditional methods in the snack food industry. From the economic standpoint, high value
additive loss will be less and costs will be lower; also products will have higher quality. From the
health standpoint, products can be made that have less oil sprayed on the surface.
OBJECTIVES
There were 3 main objectives in this study;

To determine a system which produces high numbers of droplets

To determine the effect of temperature and lecithin content on resistivity, viscosity, and
surface tension of soybean oil

To determine the effect of voltage, resistivity, viscosity, and surface tension on the
number of droplets/cm2 and on span of the droplet diameters
iii
ACKNOWLEDGMENTS
I would like to thank you my advisor, Dr. Sheryl A. Barringer, for her guidance, support,
patience and advices entire of my education at The Ohio State University. Her encouragement
and help have been priceless to me.
I would like to thank to my committee members, Dr. V. M. Balasubramaniam, Dr. John
Litchfield, and Dr. Gonul Kaletunc for their invaluable guidance and patience throughout my
study. Especially i would like to thank Dr. Gonul Kaletunc for her permission to use her
Rheometer, and for her kind help, advice and patience in assisting me with data collection and
analysis.
I received invaluable collaboration from all the current and past members of my lab
mates. I would like to thank especially Offy for her wonderful support and also, Ruslan, Paige,
Amal, and Kwang. I would also like to thank Huseyin Ayvaz, Jeremy Somerville, and Gabriel
Sanglay.
I’m grateful to the Republic of Turkey Ministry of Education for providing me the
opportunity to complete my M.S. study at The Ohio State University.
In addition, I would like to thank American Lecithin Company for their gracious
contribution in supplying lecithin for my studies.
iv
Very special thanks to my parents Belgin and Erdem, and my brother Berk for their
unconditional love and support. Finally, I want to thank to my love Emre for all of his patience
and love throughout this long journey.
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DEDICATION
To my parents Belgin and Erdem
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VITA
April 28, 1986 ....................................................... Born – Izmir, Turkey
2004 -2008 ........................................................... B.S. Food Engineering, Celal Bayar University,
Turkey
2010 to present.................................................... M.S, Department of Food Science and
Technology, The Ohio State University, US
FIELDS of STUDY
Major Field: Food Science and Technology
vii
TABLE OF CONTENTS
ABSTRACT......................................................................................................................................... ii
PRACTICAL APPLICATIONS .............................................................................................................. iii
OBJECTIVES ..................................................................................................................................... iii
ACKNOWLEDGMENTS ..................................................................................................................... iv
DEDICATION .................................................................................................................................... vi
VITA ................................................................................................................................................ vii
FIELDS of STUDY ............................................................................................................................. vii
TABLE OF CONTENTS..................................................................................................................... viii
LIST OF TABLES ................................................................................................................................. x
LIST OF FIGURES .............................................................................................................................. xi
1. INTRODUCTION ............................................................................................................................ 1
2. LITERATURE REVIEW .................................................................................................................... 4
2.1 ELECTROSTATIC ATOMIZATION ................................................................................................. 4
2.2 FACTORS AFFECTS LIQUID ELECTROSTATIC ATOMIZATION....................................................... 9
2.2.1 RESISTIVITY............................................................................................................................ 10
2.2.2 VISCOSITY .............................................................................................................................. 13
2.2.3 SURFACE TENSION ................................................................................................................ 16
2.2.4 VOLTAGE ............................................................................................................................... 21
2.2.4.1 WEBER NUMBER ................................................................................................................ 24
2.2.5 LECITHIN................................................................................................................................ 25
2.2.6 SOYBEAN OIL ......................................................................................................................... 28
2.3 OXIDATION AND OFF-FLAVOR FORMATION ............................................................................ 29
2.4 OIL SENSITIVE PAPER ............................................................................................................... 32
2.5 METHODS TO CLASSIFY THE DROPLETS BY SIZE ...................................................................... 32
3. MATERIALS AND METHODS ....................................................................................................... 37
viii
4. RESULTS AND DISCUSSION......................................................................................................... 47
4.1 ELECTRICAL RESISTIVITY ........................................................................................................... 47
4.2 VISCOSITY ................................................................................................................................. 49
4.3 SURFACE TENSION ................................................................................................................... 50
4.4 EFFECT OF VOLTAGE, LECITHIN CONTENT, AND TEMPERATURE ON THE NUMBER OF
DROPLETS/cm2 ............................................................................................................................... 51
4.5 EFFECT OF RESISTIVITY, VISCOSITY, SURFACE TENSION, AND WEBER NUMBER ON THE
NUMBER OF DROPLETS/cm2 .......................................................................................................... 55
4.6 EFFECT ON SPAN ...................................................................................................................... 61
5. CONCLUSION .............................................................................................................................. 62
REFERENCES ................................................................................................................................... 63
APPENDIX: TABLES ......................................................................................................................... 71
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LIST OF TABLES
Table 1. Experimental surface tension results at different temperatures of Methanol (Souckova
and others 2008). ........................................................................................................................... 19
Table 2. Sunflower oil surface tension with changing concentration of lecithin (Khan and others
2012). ............................................................................................................................................. 20
Table 3. DV0.5 and Relative Span values of different methanol-water formulations (Haq and
others 1983). .................................................................................................................................. 34
Table 4. DV0.5 and Relative Span values of different glycerol-water formulations (Haq and others
1983). ............................................................................................................................................. 35
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LIST OF FIGURES
Figure 1. Effect of electric field on the surface area of the droplet .............................................. 25
Figure 2. Molecular structure of phosphatidyl choline (Weyland and Hartel 2008) ..................... 27
Figure 3. Distribution of droplets relative to VMD – total volume of the droplets on the left hand
side is equal to the total volume of the droplets on the right hand side (Matthews 2000) ......... 33
Figure 4. Illustration of liquid electrostatic coating machine ........................................................ 38
Figure 5. Applicator nozzle with metal electrode which is between two halves of insulating
material .......................................................................................................................................... 39
Figure 6. Effects of lecithin content on resistivity at 4, 22, and 47⁰C............................................ 48
Figure 7. Effects of lecithin content on viscosity at 4, 22, and 47⁰C. Viscosity was measured at
shear rate 62 s-1 during decreasing shear ...................................................................................... 50
Figure 8. Effects of lecithin content on surface tension at 4, 22, and 47⁰C .................................. 51
Figure 9. Effect of voltage on the number of droplets/cm² at 0, 5, 10, and 15% lecithin and 4, 22,
and 47⁰C......................................................................................................................................... 54
Figure 10. Pictures of oil sensitive papers after coating: (A-1) 4⁰C 10% lecithin 25kV, (A-2) 22⁰C
10% lecithin 25kV, (A-3) 47⁰C 10% lecithin 25kV, (B-1) 47⁰C 0% lecithin 40kV, (B-2) 47⁰C 5%
lecithin 40kV, (B-3) 47⁰C 10% lecithin 40kV .................................................................................. 55
Figure 11. Effect of resistivity on the number of droplets/cm2 at 0 to 40kV. 0% lecithin results
were not included .......................................................................................................................... 56
xi
Figure 12. Critical resistivity vs. voltage. 0% lecithin results were not included ........................... 57
Figure 13. Effect of viscosity on the number of droplets/cm2 at 0 to 40kV. 0% lecithin results
were not included .......................................................................................................................... 58
Figure 14. Effect of surface tension on the number of droplets/cm2. 0% lecithin results were not
included.......................................................................................................................................... 59
Figure 15. Effect of Weber number on the number of droplets/cm2 at 0, 5, 10, and 15% lecithin
and 4, 22, and 47⁰C ........................................................................................................................ 60
Figure 16. Effect of voltage on span at 5, 10, and 15% lecithin and 4, 22, and 47⁰C. 0% lecithin
results were not included .............................................................................................................. 61
xii
1. INTRODUCTION
In an electrostatic atomization system, electrical forces are employed to increase the
surface area of the liquid jet and hereby the surface energy to a critical point where the liquid
jet becomes unstable and disintegrates into small droplets, or atomizes (Bailey 1974). There are
many industrial applications such as fuel injection, paint spraying, production of ceramic
powders and aerosol standards, food coating applications and dispersion of pesticides (Okuda
and Kelly 1996; Ganan-Calvo and others 1997). Electrostatic spraying has a series of benefits
compared to conventional spraying techniques. Since the droplets are charged, agglomeration
of droplets is prevented due to repulsion, producing homogenous dispersion of droplets, ease of
controlling the droplet size by altering flow rate or conductivity of the liquid, the motion of the
droplets can be manipulated precisely by controlling electric and aerodynamic forces. In
addition, there is a decrease in the amount of waste due to attraction between charged sprayed
droplets and the object being coated, and more uniform coating over the surface (Coffee 1982;
Okuda and Kelly 1996; Ganan-Calvo and others 1997).
The applied voltage, electrical resistivity, viscosity, surface tension and flow rate of the
liquid determine the formation of droplets in electrostatic spraying (Hughes and Pavey 1981).
The average droplet diameter decreases with increasing voltage (Coffee 1982). There is an
1
interaction between voltage and resistivity; at higher resistivity the required voltage for
atomization is higher (Wilkerson and Gaultney 1989).
Electrical resistivity is affected by temperature, viscosity of the solution, and type and
concentration of ions in the solution (Adamczewski 1969). The most important factor in coating
effectiveness is the electrical resistivity (Downer and others 1994). Generally, temperature of
the sample and its resistivity are inversely proportional (Palaniappan and Sastry 1991). The
resistivity of the solution depends on the mobility of ions which is decreased by high viscosity.
Therefore, the resistivity of a solution is decreased by reducing viscosity (Adamczewski 1969).
Increasing lecithin content decreases the resistivity of the solution; therefore it is one of the
ionic emulsifiers which is added into liquid to be atomized (Evans and Reynhout 2000; Abu-Ali
and Barringer 2005). Good atomization can be achieved if the resistivity of the liquid is kept
within the range of 105 to 109 Ωm (Abu-Ali and Barringer 2005). Decreasing resistivity decreases
the size of the droplets of soybean oil (Wilkerson and Gaultney 1989), because with decreasing
resistivity the charges on the surface of the molecule increases (Bailey and Balachandran 1981)
producing a repulsive force that exceeds the surface tension, forcing the droplets to divide.
Viscosity is affected by temperature and lecithin content. Viscosity decreases with
increasing temperature (Wilkerson and Gaultney 1989) because increasing temperature
increases the kinetic energy of the molecules (Schmidt 1997; Nik and others 2007). Sunflower
and other types of oil become more viscous when lecithin is added (Bhattacharya and others
1998; Pernetti and others 2007; Rao 2007), but in confectionary coating lecithin decreases the
viscosity of the samples (Marthina and Barringer 2012).
2
The electrical forces which arise from the added charge attempt to break the liquid jet
into small drops while the surface tension opposes them in order to keep liquid as one huge
drop (Hines 1966). This ratio is known as the Weber number, indicates the ratio between the
electric field and the surface tension stress (Eow and others 2001). Surface tension is affected by
temperature and lecithin content. Increasing temperature decreases the surface tension of most
liquids (Kahl and others 2003). Addition of surface-active agents reduces the surface tension
resulting in decreased drop diameter at a given voltage (Nawab and Mason 1958). Since the
viscosity is directly related to the surface tension the effect of viscosity on the size of atomized
droplets is appreciable (Jayasinghe and Edirisinghe 2004). Reduction in either viscosity or
surface tension of the liquid significantly increases the number of droplets/cm2 (Matthews
1979).
3
2. LITERATURE REVIEW
2.1 ELECTROSTATIC ATOMIZATION
Electrostatic atomization is a developing technology in the food industry which employs
liquid or powder electrostatic atomization systems (Abu-Ali and Barringer 2008). In the food
industry, with powder electrostatic atomization systems, largely snack foods are coated; potato
chips, cakes, donuts, shredded cheeses, crackers, and pretzels are examples (Sumonsiri and
Barringer 2011). There are numerous advantages of powder electrostatic coating such as
reduced cost by decreasing the loss of raw material, dust reduction, and increased consumer
acceptance in terms of taste and appearance (Hughes 1997; Sumonsiri and Barringer 2011).
Currently, snack food producers struggle with the problem of coating edible substrates
with flavorings, colorants and stabilizers. Current methods have a series of drawbacks. More
than one step may be needed. Usually solid seasoning must be dissolved in a bonding agent
such as oil or water in order to apply it. The utilization of oil as bonding agent is in the contrast
with market demand of using minimum fat in production. Most of the food items cannot
tolerate adding water. Solid seasoning adheres to the snack food weakly, although applying a
two-step approach which leads to waste of seasoning material and higher costs. Generally, 10%
of the seasoning mixture is lost. The flavor and ingredients are usually applied on substrates
4
unevenly. Practically, a carrier such as vegetable oil or fluid fat base material must be employed
to demulsify the flavoring, coloring, stabilizing, and incidental additives which are in
concentrated form in order to spray them. But, high electrical resistivity values of vegetable oil
and fluid fats is a very important drawback which make them unsuitable for electrostatic
atomization and spraying. This handicap can be handled by using additives which decreases the
resistivity of carrier (Evans and Reynhout 2000).
Interest in electrostatics arose from power generation by polarizing dielectric materials
with electric fields. But the scientists realized that power generation applications are
impractical. In the following years effects of electrostatic forces on charged particles was
discovered which is based on manipulating the motion and the adhesion of charged particles
(Abu-Ali 2004). Electrostatics can be defined as an interaction between charged particles. These
particles can be stationary or moving due to their interaction between them. The source of
these interactions comes from the particles themself and their position and not by their motion
(Hendricks 1973). Basically electrostatic coating depends on creating electrically charged coating
material and a grounded coating target. Thus, the coating material is attracted to the grounded
target due to electrical forces which are a consequence of Coulomb’s Law (Hughes 1997).
Coulomb’s Law can be defined by two properties; 1) Particles with the same polarity repel each
other while particles of opposite polarity are attracted to each other, 2) The mathematical
representation of this attraction or repulsion forces are as follows (Hughes 1997; Abu-Ali 2004);
5
and
and 2,
are the charges of the particles 1 and 2, r is the distance between charges 1
is the permittivity of vacuum which has the value of 8.854 x 10-12 F/m, and
is the
permittivity of the medium which is hosting the particle .
The charge on particles can be related to an excessive amount of electrons or having
fewer electrons which are carried by the particle. Therefore, the charge on particles is only
associated with the number of electrons of the particle (Hughes 1997).
The electrohydrodynamic process is defined as dispersing a liquid into fine droplets by
using an electrostatic force to overcome the surface tension of the liquid (Watanabe and others
2003). Among the other techniques it has the ability to produce fine droplets which have a
comparatively narrow size distribution (Liu 2000).
Regardless of atomization process type, there are three ways of supplying energy to the
atomizer unit. These are: new surface formation energy, energy required to overcome viscous
forces, and energy losses, respectively. New surface formation energy can be derived from the
rate of atomization and size distribution of droplets, if both are known. It is hard to decisively
determine the amount of energy which is needed to overcome the viscosity of liquid in order to
form new surfaces. On the other hand, viscosity can be used as a parameter in the atomization
process (Bailey 1974).
Electrostatic atomization is one of the methods for liquid atomization. To carry out
electrostatic atomization of liquid, electrical forces are employed for the purpose of exceeding
the surface tension of the liquid. With the electrostatic atomization process droplets obtained
can be as small as nanometer size. Droplet size and charge can be regulated by properties such
as applied voltage and flow rate of the liquid. During the electrostatic atomization process,
6
liquid is sprayed from the nozzle, and this liquid is exposed to electrical shear stress when it
comes out from the nozzle. In order to obtain electrostatic atomization an electric potential
should be applied to the nozzle and it must be kept constant at the same potential level which
enables obtaining uniform droplet size. The liquid starts to be pulled out from the nozzle with
the help of electric charges and a meniscus and a jet are formed at a close distance from the
departure point of the nozzle. Electrostatic repulsion forces can eject the liquid out of the
meniscus or liquid can be dispersed into droplets when the jet formation occurred. There is no
mechanical energy application other than to form droplets in electrospraying. The droplets
which are formed by electrospraying are highly charged; almost half of the Rayleigh limit
(Jaworek 2008). The liquid droplets have an equilibrium state between the maximum charge
which could be carried by them and the surface tension of the droplets. This equilibrium state
can be disrupted by injecting more charge into the droplets which results in accumulation of
excessive charge at the surface that overcomes the surface tension and cause formation of fine
droplets. This is known as the Rayleigh limit. Rayleigh proposed a relationship between applied
voltage, radius of the droplet and charge carried by the droplet. (eq 2) :
Where; V is voltage (V), Q is charge (Coulombs), and a is radius (meter) of the droplets.
The charge density which is calculated by this formula can be used in the quantitative
statement of the Rayleigh limit (Rayleigh 1882). When the Rayleigh limit is exceeded surface
tension of droplets spraying of liquid starts (Jaworek 2008). Electrostatic atomization is an
effective process, which can produce droplets down to the order of 100 nm or 10 nm according
to some research, and also their replicates give close numbers (Jaworek 2007; Jaworek 2008).
7
When the spraying occurs the droplets do not tend to gather; on the contrary they tend to
spread apart (Jaworek 2008).
To achieve fine droplets from spraying of fluid, atomization is used. Forming fine
droplets is important for many practices such as crop and paint spraying, fuel injection, or some
medicinal practices (Michelson 1990). Volume of chemical pesticides needs to be reduced due
to environmental and economic reasons. On the other hand, while reducing volume of a
pesticide, droplet sizes must also be reduced to perform an effective distribution (Wilkerson and
Gaultney 1989).
Electrostatic atomization is also known as electrohydrodynamic atomization (EHDA) or
electrospraying. In this process, electrostatic forces affect the liquid and the liquid forms fine
dispersed droplets (Watanabe and others 2003). The first discussion of this phenomenon is
initially in the 1600s. William Gilbert, after Stephen Gray (1731-1732) discussed some of their
observations on this topic. Stephen Gray is the person who for the first time wrote down
qualitative investigations about this topic. Gray realized a charged stick can pull the liquid
droplets into a conic shape, meanwhile, a small pillar ascends from its top and fine droplets
occur. In 1915, Zeleny examined this phenomenon, and in 1964 Taylor explained this
phenomenon technically (Ahern and Balachandran 2006). There are many atomization
techniques for liquids but the most functional ones are; pressure atomizers, two-fluid atomizer,
rotary atomizer, effervescent atomizer, electrostatic atomizer, ultrasonic atomizer, whistle
atomizer, as an ample and every technique has characteristic advantages (Liu 2000). Advantages
of electrostatic atomization are these: when an electric field is applied it’s easy to form fine and
uniform droplets. Because of the droplets having the same electrical polarity there is no
8
conjoining of droplets, and narrow size distribution of droplets occurs (Liu 2000; Watanabe and
others 2003). When compared with mechanical spraying of pesticides, electrostatic spraying
gives more narrow size distributed droplets and more efficient deposition (Inculet and others
1981). Liquid electrostatic atomization is widely used in many areas such as the paint industry,
pesticide applications, lubrication of mechanical systems, and printing systems (Inculet and
others 1981; Liu 2000; Abu-Ali and Barringer 2005), air conditioning systems, tablet, pill, and
capsule coatings with polymer solutions in pharmaceutical industry, water atomization to aerate
water and separate detrimental impurities, spray and coat food products such as, candies and
bakery products, and to humidify cheese storage rooms (Bayvel and Orzechowski 1993). Also
silica particles and zinc sulfide nanoparticles are formed with electrostatic atomization
(Lenggoro and others 2000; Mori and Fukumoto 2001; Watanabe and others 2003).
Furthermore, it has been applied to highly viscous liquids such as sodium alginate, kcarrageenan, carboxymethyl cellulose, and polyethylene glycol and desired results are obtained
with samples which are challenging because of high viscosity of these materials (Watanabe and
others 2003). Effective pesticide usage is important to reduce the costs of pesticides and to
prevent adverse environmental effects. Therefore, there are multiple studies about pesticides
and its electrostatic atomization.
2.2 FACTORS AFFECTS LIQUID ELECTROSTATIC ATOMIZATION
In liquid electrostatic coating applications, liquid properties and process parameters
determine the effectiveness of the process. Liquid properties are electrical resistivity, viscosity,
and surface tension of the solution. On the other hand process parameters which can determine
9
the effectiveness of the process are applied voltage and electric field, and flow rate of the liquid
(Jayasinghe and Edirisinghe 2004; Abu-Ali and Barringer 2008).
2.2.1 RESISTIVITY
Conductivity of a material is the measure of how easily charges pass through the
material. Resistivity, measured in ohm-meter (Ωm), is the reciprocal of conductivity, and as the
material’s ability to oppose the electrical flow (Abu-Ali 2004). A solution’s resistivity is affected
by a number of factors such as type and concentration of ions in the solution, and viscosity of
the solution. Viscosity affected the solution’s resistivity by affecting the mobility of ions in the
solution. As the temperature of a solution is increased, kinetic energy of the ions within the
solution increases which results in an increase in the mobility of ions in the solution. Therefore
temperature has a direct effect on conductivity (Adamczewski 1969). Lecithin is an ion
producing agent, and ion concentration in solutions does not depend on temperature, but ion
mobility increases by increasing the solution temperature. In a chocolate study, where the
temperature was increased from 35 to 50⁰C, all the samples other than cocoa liquor show
decrease in electrical resistivity (Gorty and Barringer 2011). The electrical resistivity of tomato
juice and orange juice decrease linearly with increasing temperature from 25 to 85⁰C
(Palaniappan and Sastry 1991). Resistivity of the liquid is the most critical factor in terms of
effective coatings in electrostatic atomization (Downer and others 1994).
While the conductivity of a solution depends on the internal components such as the
concentrations of ion producing agents, conductance of the solution can be altered by external
factors such as temperature and intensity of the electric field being applied. Ionic mobility does
not show significant field dependence but the applied electric field can give rise to a higher
10
concentration of ionic charge carriers. This can be related to low activation energy for formation
of ionic charge carriers from complex ionic compounds (O'Dwyer 1973).
Resistivity; which is the reciprocal of conductivity, is the only factor that introduces an
upper limit for stable atomization processes. Resistivity is the most influential factor among the
factors which effect droplet size and charge to mass ratio (Wilkerson and Gaultney 1989). High
resistivity leads to lower charge to mass ratio which results in larger droplets. On the other
hand, electrode voltage required for atomization is affected by resistivity; if the resistivity
decreases; required voltage for atomization is lowered (Wilkerson and Gaultney 1989). The
fundamental feature of liquids which makes it favorable for electrostatic atomization is its
resistivity (Burayev and Vereshchagin 1972). High resistivity prolongs the time for efficient
charging of liquid which is detrimental for good atomization (Burayev and Vereshchagin 1972).
In a study with soybean oil, resistivity was found which has the greatest effect on droplet size
and charge to mass ratio; on the other hand it was the only liquid variable which can determine
an upper limit for atomization (Wilkerson and Gaultney 1989).
The volumetric resistivity is one of the most important properties of a liquid which is
used in electrostatic painting practices. It defines the liquid’s capability for atomization under
the applied electric field. If the volumetric resistivity (
of liquids are in the range of 5 x 104 – 5
x 106 Ωm, liquids can be sprayed as fine droplets. Outside this range, the atomization process is
severally hampered due to insufficient charging time of particles by the existing electric forces
(Burayev and Vereshchagin 1972). For atomization, it has been reported that a liquid with
resistivity higher than 1 x 1010 Ωm will not atomize adequately (Wilkerson and Gaultney 1989).
Other studies have shown lower limits for oil atomization; 105 to 109 Ωm (Gaultney and others
1987), 105 to 108 Ωm (Abu-Ali and Barringer 2005), or 106 to 108 Ωm (Law 1984). High volumetric
11
resistivity leads to formation of poorly charged particles which means that particles cannot
overcome surface tension; therefore the atomization process is hindered. This is the reason
behind poor atomization of liquids with low conductivity (Burayev and Vereshchagin 1972).
Downer and others (1994) showed the importance of resistivity in the atomization process by
conducting research on oil diluted pesticides. When the resistivity of a liquid is lowered, the
amount of charge that can be carried by the liquid is increased. Also the liquid can be charged
faster by increasing its resistivity. Both of these lead to atomization of the liquid to highly
charged fine droplets which have small volumes and diameters.
In the electrostatic systems, resistivity of the solution must be in a certain range. Pure
vegetable oil’s resistivity is 1014 Ωcm at 25⁰C (Oommen 2002), and this value is too high for
atomization (Abu-Ali and Barringer 2005). To achieve vegetable oil atomization, surfactants
should be added at 3-20% into the oil (Abu-Ali and Barringer 2005) because they have an ability
to lower the resistivity. Pure soybean oil’s resistivity is outside of the atomizable range but
addition of 13% lecithin decreased its resistivity more than 3 logs, 1011 to 108 Ωm, entering the
atomizable range (Abu-Ali and Barringer 2005). During the electroshydrodynamic spraying of
chocolate, if the electrical resistivity decreases, droplet sizes will decrease and also % area
coverage will increase. When 1.5%lecithin replaced the same amount cocoa butter in a
chocolate sample, resistivity decreased from 7.0 x 109 to 0.1 x 109 Ωm. A chocolate sample
containing 0.35% lecithin has a higher resistivity than one containing 1.5% lecithin sample (Gorty
and Barringer 2011). By decreasing resistivity, improved atomization processes can be achieved.
Fluids which have a resistivity greater than 1x1010 Ωm, cannot atomize adequately. A 2% antistatic additive into bleached soybean oil decreased its resistivity most among 0.9 and 75% crude
soybean oil, 0.2% antistatic agent, 20% surfactant+0.2% antistatic agent additive. Bleached
12
soybean oil and 2% antistatic agent mixture’s resistivity reached 1x107 Ωm. The mixture gives
the smallest drop sizes and the highest charge to mass ratio among other samples (Wilkerson
and Gaultney 1989). In a confectionary coatings study, addition of lecithin significantly
decreased resistivity. 1% lecithin addition decreases resistivity of all samples 1 to 2 logs, 5%
lecithin addition decreases resistivity an another log, but because of the pure lecithin’s
resistivity is close to 5% lecithin added samples resistivity, further lecithin additions are not
decreased resistivities much more (Marthina and Barringer 2012). Drop sizes are dramatically
increasing when resistivity is increased over 3.7x108 Ωm among chocolate samples with different
fat and lecithin contents (Marthina 2011).
2.2.2 VISCOSITY
Viscosity is one of the key characteristic properties of all liquids. As liquids flow, an
internal friction resists flow and viscosity is the measure of this resistance (Viswanath and others
2007). Attractive forces between the molecules determine the molecule’s viscosity. Increasing
temperature decreases viscosity, because increasing temperature increases molecule’s kinetic
energy and increasing kinetic energy overcomes the attractive forces, therefore molecules can
easily move to past one another. Viscosity affects the solution’s ion mobility, at higher solution
viscosity; ion’s ability to move from one point to another is reduced highly and this situation
reduces the solution’s ability to conduct charge (Schmidt 1997; Nik and others 2007). The
increasing molecular mobility decreases viscosity. On the other hand increasing attractive forces
between molecules give a rise to viscosity (Dogan and Kokini 2007). Increasing temperature
from 35 to 50⁰C decreases viscosity of chocolate (Gorty and Barringer 2011). In a soybean oil
study, droplets sizes increased with increasing viscosity at the same applied voltage. By
13
increasing the voltage, the differences between droplet sizes decreased at the different
viscosities (Wilkerson and Gaultney 1989). While the temperature of crude oils increases their
kinematic viscosity decreases significantly; this decrease is greater in heavy crude oils than in
light ones. The decrease of viscosity is observed in both individual samples and their mixtures
(Al-Besharah and others 1989). Viscosities of oils such as soybean, corn, cottonseed, linseed,
olive, peanut, rapeseed, coconut and safflower decrease with increasing temperature (Rao
2007). Both shear rate and temperature have an effect on oil viscosity reduction but
temperature has greater effect on viscosity than shear rate. Viscosity values of all vegetable oils
diminish with increasing temperature (Nik and others 2007). Bitumen viscosity which is around
10000 Pa.s at 0⁰C decreases to almost 0.3 Pa.s with increasing temperature to 100⁰C (Long and
others 2007).
Lecithin has a very high viscosity, 61.5 Pa.s (Marthina and Barringer 2012); therefore
lecithin content affects liquid’s viscosity. Addition of 1.5% versus 0.35% lecithin in chocolate
increases its viscosity (Gorty and Barringer 2011). In confectionary coatings, lecithin addition
significantly lowers the viscosity (Marthina and Barringer 2012). Addition of lecithin in sunflower
oil produces viscous solutions (Pernetti and others 2007). When lecithin content of the lecithinsoybean oil mixture is increased, viscosity increases. At 103 s-1 shear rate, viscosity of oil without
any additives was 0.0827 Pa.s; with 25% lecithin added, the sample’s viscosity was 0.1287 Pa.s
(Bhattacharya and others 1998). Addition of 2% soy lecithin increased peanut oil viscosity from
0.0345 Pa.s to 0.0414 Pa.s at 30⁰C (Rao 2007). The viscosity of poloxymer gels is the function of
lecithin content. Higher the lecithin contents of the gel greater the viscosity (Bentley and others
1999).
14
Viscosity affects the droplet size during electrohydrodynamic atomization. Droplet sizes
are increased with increasing viscosity of the sprayed solution (Hartman and others 2000). High
viscosity hinders atomization of liquid (Bayvel and Orzechowski 1993). With some highly viscous
solutions such as petroleum fuels, a heating process can be used to decrease their viscosity to
bring them atomizable range. However for heavy fuels, viscosity is not decreased into the
atomizable range with heating (Bayvel and Orzechowski 1993). In Gorty and Barringer’s (2011)
study, both during electrostatic and non-electrostatic atomization viscosity had a noteworthy
effect on the drop sizes, as viscosity increased drop sizes were increased. If viscosity of a
solution is increased, it will prevent droplet formation by preventing liquid disintegration;
therefore percent area coverage decreases (Gorty and Barringer 2011). In another study
changing water, glycerol, and citric acid content of the atomized mixture, its density, viscosity,
and surface tension characteristics changed and increasing viscosities increased the drop sizes
and wider droplet size distribution occurs (Jayasinghe and Edirisinghe 2002). The drift of spray
from pesticides during spraying applications is an unintentional dispersion of pesticides. Drift
causes environmental contamination and achievement costs of applications. Small droplets are
more liable to drift than larger ones. Therefore some thickeners, gels, and foaming gels are
using to increase viscosity and surface tension of the solution to increase droplet sizes and to
prevent drifting (Matthews 1979). In a study which has been made with different solutions
showed that, increasing viscosity increases drop sizes and decreases relative span slightly (Haq
and others 1983).
15
The shear stress and viscosity can be determined at the average shear rate within the
nozzle during the spraying process, which can be calculated from equation 3 (Marthina 2011).
Flow characteristics of liquids are related to their viscosities (Viswanath and others
2007). If an emulsion has weak intermolecular interaction, a mixture shows low viscosity and
Newtonian behavior, but if it has strong interaction, it shows high viscosity and non-Newtonian
behavior. Soybean oil without any additives shows Newtonian behavior, whereas lecithinsoybean oil mixture shows pseudoplastic behavior (Bhattacharya and others 1998). If a liquid
shows linear relationship when plotted to shear rate and shear stress, this liquid is a Newtonian
liquid and at the same temperature viscosity remains same regardless of the shear rate. For
pseudoplastics or shear thinning liquids, viscosity decreases with increasing shear rate (Singh
and Heldman 2009). This is a favorable characteristic in terms of atomization, because reduction
in viscosities will decrease effect of viscosity on atomization (Lefebvre 1989).
2.2.3 SURFACE TENSION
Surface tension is the characteristic of liquids which comes from their desire to minimize
their surface area (Tro 2011). While a liquid wants to decrease its surface area, the smallest
16
possible surface area to volume ratio is in the sphere shape and surface tension pulls a drop of
the liquid into this shape. Formation of a sphere shape minimizes the total molecules at the
liquid surface so that system’s potential energy minimizes. But because of forces such as gravity,
liquid cannot generate perfect spheres. The equilibrium shape is determined by forces such as
surfaces tension, gravity and electric field. Gravity and electric field are denoted as body forces.
While surface tension wants to make a spherical shape, body forces affect to elongate or flatten
the drop on the body forces direction (Bateni and others 2005). At the surface of the liquid,
molecules make a fewer number of bonds than the interior liquid molecules because surface
molecules have relatively fewer neighbors to interact with. Surface molecules are less stable
than the interior ones and also surface molecules have higher potential energy. If the surface
area of the liquid is increased by an external force as in atomization, interior molecules should
be accelerated to the surface of the liquid. The movement of interior molecules to the surface
requires an increase in total energy of the liquid which means creating more molecules with
high potential energy. The liquid tends to minimize its surface area in response to an increase in
concentration of molecules with high energy. To break a big drop into smaller droplets, a
specific amount of energy has to be provided. This energy also overcomes liquid’s tendency to
minimize the liquid’s surface area (Tro 2011). In electrostatic atomization high surface tension is
one of the restrictive factors (Burayev and Vereshchagin 1972).
Surface tension is increasing with increasing intermolecular forces. Benzene’s surface
tension at room temperature is 28 mN/m while water’s is 72.8 mN/m. The reason is that,
dispersion forces between benzene molecules are dramatically weaker than hydrogen bonds
between water molecules (Tro 2011). Additionally, at 20⁰C surface tension of milk is 42.3 to 52.1
mN/m, skim milk is 52.7 mN/m, cream (34% milk) is 45.5 mN/m, cotton seed oil is 35.4 mN/m,
17
coconut oil is 33.4 mN/m, olive oil is 33.0 mN/m, sunflower oil is 33.5 mN/m, wine (Chardonnay)
(10.8% ethanol) is 46.9 mN/m, diluted wine (Chardonnay) (2.7% ethanol) is 60.9 mN/m (Sahin
and Sumnu 2006).
Besides intermolecular forces, temperature affects surface tension. Most liquid’s
surface tension is decreased with increasing temperature nearly in a linear manner (Kahl and
others 2003; Wohlfarth 2008). Because with increasing temperature cohesive forces between
molecules decreases and surface tension decreases (Lindeburg 2006). Temperature effects on
surface tension are shown in table 1. According to this table, methanol’s surface tension is
decreasing when temperature is increasing.
18
Table 1. Experimental surface tension results at different temperatures of Methanol (Souckova
and others 2008)
Temperature
Surface Tension
(K)
(mN/m)
279.11±0.02
23.87±0.04
282.59±0.02
23.63±0.04
288.30±0.02
23.14±0.02
293.37±0.04
22.73±0.02
296.97±0.01
22.43±0.03
301.54±0.02
21.99±0.01
307.31±0.03
21.43±0.02
311.76±0.06
21.00±0.05
317.66±0.06
20.43±0.05
322.90±0.09
19.83±0.07
326.56±0.05
19.42±0.06
330.01±0.10
19.09±0.03
333.81±0.06
18.63±0.05
In the electrospraying process, electrostatic forces are employed to overcome surface
tension forces. Thus, disintegration of liquids into small droplets is achieved (Khan and others
2012). As an emulsifier, lecithin has a property of decreasing surface tension and in systems
which consist of water and lecithin both soybean lecithin and egg lecithin can decrease surface
tension. Surface tension decreases with the addition of lecithin, and lowers to a minimal value,
and then it becomes independent from concentration (Palacios and Wang 2005). Sunflower
19
oil’s surface tension is changed mildly by addition of lecithin (Khan and others 2012). According
to table 2; surface tension increases up to 3% lecithin addition, while a decline in the surface
tension is observed at 5% lecithin to 10%. By 15% and 20% addition, an increase and decline is
observed, again.
Table 2. Sunflower oil surface tension with changing concentration of lecithin (Khan and others
2012)
Concentration Surface Tension
(w/w%)
(mN/m)
0
31.0
1
31.5
3
31.6
5
30.5
10
26.1
15
26.3
20
25.4
In electrostatic applications, surface tension has a crucial importance on droplet
formation. The electric field which is required to obtain an unstable drop is partially related to
surface tension. At the point where the unstable state begins, there is a proportionality between
the square of the electric field and the surface tension (Abu-Ali 2004). Surface tension is the
binding force which keeps a volume of liquid together. Surface tension is also connected with
the activation energy of transport process and molecular migration in liquids with viscosity and
ionic mobility, recombination and diffusion coefficients (Adamczewski 1969). Liquids which have
higher surface tension form larger droplets than liquids which have lower surface tension when
20
they atomized (Bayvel and Orzechowski 1993). The surface tension effect on liquid electrostatic
coating is more related to droplet formation and droplet sizes. In literature, there is no
experimental result with surface tension and its effects on evenness or amount delivered of
coating (Abu-Ali 2004). Surface tension has an influence on the extent of covering on crops. Low
surface tension liquids have a small contact angle when they sprayed and they will spread better
on the crop surface; therefore liquids which have lower surface tension will produce higher
covering degree on surfaces (Bayvel and Orzechowski 1993). In a study on pesticide atomization,
surface tension’s statistically significant effect on droplet size (α=0.05) was found, but it has the
least significant effect among conductivity, viscosity, voltage, and mass flow rate (Wilkerson and
Gaultney 1989). In terms of coating reproducibility, surface tension has also no significant effect
(Abu-Ali and Barringer 2008). In a study, where methanol and water, glycerol and water, and
water, diquat and NalcoTrol have been used in different proportions showed that; drop sizes are
increased with increasing surface tension. On the other hand, relative span is not changed
properly, it changed in a small amount when surface tension is increased (Haq and others 1983).
2.2.4 VOLTAGE
The strength of electric field which is generated around the material directly depends on
the amount of charge on the material, which is proportional to the current that passes through
the material. Since voltage determines the current, it directly affects the amount of charge on
material and strength of electrical field (Abu-Ali 2004).
There is a limit where the droplet sizes start to increase with increasing voltage which
corresponds to a point where the reproducibility starts to decrease (Abu-Ali and Barringer
21
2008). 30kV is the approximate value where droplets have the smallest diameter. The most
important factor for liquid electrostatic coating reproducibility is voltage (Abu-Ali and Barringer
2008). The voltage is the primary factor which has the most noteworthy effect on reproducibility
alone, and also has an interactive effect with other factors. In this study, from 0 kV to a range of
30-35kV the reproducibility increases progressively. Abu Ali and Barringer’s (2008) electrostatic
coating experiments showed the highest reproducibility in the 30-35kV range, and after 35kV
reproducibility started to decrease. This phenomenon was explained by Franz and others (1987);
applied voltage initially increased charge to mass ratio, but the effect of Corona, discharge,
decreased the charge to mass ratio with a further increase of voltage (Franz and others 1987).
Both studies have consistent results about charge to mass ratio compared to each other. In
another study mean droplet sizes were decreased with increasing voltage due to the effect of
voltage on atomization (Downer and others 1994). In an electrostatic atomization system,
droplet volume median diameter (VMD) decreased from 160 to 60 µm by increasing the voltage
from 14 to 25kV (Wilson 1982).
In order to obtain more even spread of oil on the surface the voltage must be increased
which leads to smaller droplet size or higher charge to mass ratio. At the optimum voltage,
repulsion between the droplets of liquid and their attraction to ground are both at the highest
level. Above the optimum voltage charge to mass ratio decreases while droplet size increases
(Abu-Ali and Barringer 2008). A liquid with high conductivity cannot retain enough charge to
atomize because charges tend to move towards the ground. Charges can move faster in a
solution which has high conductivity, therefore they move apart quicker which causes an
increase in their relative distance and so reduction in the momentary charge concentration.
Conductivity must be minimized in order to create atomization, because high conductivity
22
decreases reproducibility at low voltages. However, effect of conductivity on the atomization
process lessens with increasing voltage. 40kV is the point that the conductivity lost its effect on
reproducibility because at such high voltage conductivity of the medium becomes insignificant
(Abu-Ali and Barringer 2008). When electrostatic atomization is used for pesticide spraying, 2.5
times greater deposition is obtained that of non-electrostatic atomization on plant leaves; 9
times greater deposition is obtained with electrostatic atomization on plant stems (Coffee
1979).
Viscosity has a considerable effect on charge diffusion time which means it affects the
amount of charge that is retained in the liquid. Therefore, its interaction with the voltage
directly influences the reproducibility of the coating. A higher viscosity of liquid means higher
charge concentration which leads to more reproducible coating (Abu-Ali and Barringer 2008).
While all the other factors are fixed, increasing the flow rate reduces reproducibility due
to insufficient time which is required to reach higher charge concentration. At high flow rates
more liquid passes through the system which means the same amount of charge is distributed
among the higher volume of liquid. Therefore the momentary charge concentration per unit
volume decreases which leads to lower reproducibility and poor atomization. Effect of flow rate
on reproducibility is significant at all voltages, but it lessens slightly at high voltages (Abu-Ali and
Barringer 2008). The threshold voltage can be determined mainly by two factors, the fluid’s
resistivity and flow rate (Wilkerson and Gaultney 1989).
Due to amount of emulsifier in the solution, conductivity and the viscosity of the
mixture are altered. Since emulsifier concentration does not affect reproducibility directly, it
implicitly affects reproducibility by modifying viscosity and conductivity of the solution. Thus,
interaction between emulsifier and voltage is the combination of conductivity, viscosity, and
23
their interplay with voltage (Abu-Ali and Barringer 2008). Increasing voltage increased charge to
mass ratio and the number of droplets/cm2 non-linearly (Wilkerson and Gaultney 1989).
2.2.4.1 WEBER NUMBER
Deformation of a droplet can be defined as change in surface area by increasing electric
field strength which also decreases the stability of a droplet. Figure 1 provides information
about formation of interfacial surface tension with increasing electric force. The ratio between
deforming electric force and the interfacial tension reaches a critical point when electric force
increases, which leads to formation of smaller droplets. As indicated in the surface tension
section, this critical point is where the surface tension and square of the electric field force
become proportional by a constant when the droplet is critically unstable. This critical point is
defined by a dimensionless parameter called the electrical Weber number. Weber number
proposes a relationship between electric field, droplet radius, permittivity, and interfacial
surface tension (Ha and Yang 1999).
In equation 7,
is the dimensionless Weber number,
radius, is the permittivity of the continuous phase,
is the undeformed drop
is the electrical field strength, and is the
interfacial surface tension.
An alternative method of calculating the Weber number indicates a relationship
between density of the liquid, characteristic length, velocity, and surface tension (Berthier
2008).
24
In equation 8,
is the dimensionless Weber number,
is the density of the liquid,
is
the velocity of the liquid, is the characteristic length, and is the surface tension.
Figure 1. Effect of electric field on the surface area of the droplet
2.2.5 LECITHIN
Lecithin is 1, 2-diacyl-sn-glycero-3-phosphocholine (IUPAC-IUB Commission on
Biochemical Nomenclature 1977). It has a light brown color (Weyland and Hartel 2008) and
contains phosphatides, phytoglycolipids, triglycerides, phytosterols, tocopherols, and free fatty
acids (Szuhaj 1983). Lecithin is one of the main components in biological membranes (Pearson
and Pascher 1979). Lecithin is generally derived from seeds such as sunflower, canola, rapeseed,
corn, or soybean, or from eggs (Magil and others 1981; Shchipunov 1997; Shchipunov 2002).
Generally commercially produced lecithins are prepared from soybeans or eggs, and they are
named soy lecithin or egg lecithin (Magil and others 1981). Soy lecithin and egg lecithin have
differences in their fatty acid and phospholipid contents (Magil and others 1981). If lecithin is
25
produced from a seed it is usually produced from the soybean, the richest source of lecithin.
Lecithin has continuous availability, perfect properties of emulsification, color, and taste
(Nieuwenhuyzen 1976; Orthoefer 1998). Lecithin is a by-product of the refinement process of
the seed (Shchipunov 1997; Shchipunov 2002). It is a non-toxic and safe ingredient (Shinoda and
others 1991).
Lecithin is one of the important members of the surfactant family. Researchers have
been studying lecithin for a long time because of its important properties such as it is one of the
main parts of the biological membrane’s lipid matrix, it is naturally formed and a biocompatible
surfactant, and in solutions it can create micelles and liposomes, can be self-organizing into
liquid-crystalline structures, can dissolve and stabilize dispersions, and it can wet and cover
surfaces (Shchipunov 1997; Orthoefer 1998; Shchipunov 2002). In aliphatic, halogenated,
aromatic hydrocarbons, alcohols, esters, ethers, water, and aqueous solutions lecithin is
miscible and dissolvable, but it is not soluble in acetone and some ketones (Shchipunov 1997;
Shchipunov 2002). For this reason, acetone is used in purification processes (Shchipunov 2002).
Lecithin has very wide usages because of its functional properties. It can be used as an
emulsifier, antispatter, instantizing, wetting, dispersing, release agent, viscosity modifier, and
diet supplementing agent (Orthoefer 1998; Shchipunov 2002).
Lecithin is an amphiphilic molecule; which has both lipophilic and hydrophilic properties,
and this amphiphilic structure is responsible for its surface active components. The phosphatidyl
group in its structure is the hydrophilic component which tends to be in the aqueous phase. Its
fatty acid chains are the lipophilic component and tends to be in the lipid phase. The
26
phopsphatidyl choline is the main component of the lecithin and structure is given in the figure
2. In this molecular structure R1 and R2 are the alkyl chains (Weyland and Hartel 2008).
Figure 2. Molecular structure of phosphatidyl choline (Weyland and Hartel 2008)
Lecithin is an ionic emulsifier and this property gives it the ability of carrying charge
(Abu-Ali 2004). Adding lecithin into oil decreases resistivity and assists good atomization. In one
study it was found that addition of 13% lecithin into the soybean oil gives best atomization. Also
that amount of lecithin decreased the soybean oil’s resistivity by 3 logs; from 108 to 1011 Ωm
(Abu-Ali and Barringer 2005).
Lecithin is commonly used in the food industry. The main use is in the margarine
industry, followed by baking, confectionary, snack food, instant food, cheese, meat and poultry,
dairy, imitation dairy, and ice cream industries, and packaging aid industry (Shchipunov 2002).
For nonfood applications, it is used in animal feed, cosmetics, soaps, paints, ink and polymer
coatings, metal processing, masonry and asphalt products, and textiles (Szuhaj 1983;
Shchipunov 2002). Wide ranging lecithin applications are due to its appropriate colloid features
and expanded functional benefits (Shchipunov 2002).
27
Lecithin exists in every living cell, including both animals and plants, and is related to all
of the vital functions such as metabolism, breathing, energy production, energy transportation,
and nerve function (Orthoefer 1998). Recent research showed that lecithin usage has positive
effects on cholesterol metabolism, brain functions, reproductive health, liver and hearth health,
memory improvement, physical activity increasement, and cancer risk reduction (Knuiman and
others 1989; Orthoefer 1998).
2.2.6 SOYBEAN OIL
Soybean oil is commonly used in most electrostatic coating practices (Abu-Ali 2004).
Soybean oil has high resistivity, by this way benefited from its insulation properties, and also it
doesn’t dissolve ionic materials or gases well, and this situation diminishes its potential for
unaccounted for variations in conductivity (Abu-Ali 2004).
For good atomization resistivity must be below a certain range. Pure soybean oil’s
resistivity is too high to achieve atomization therefore researchers add some compounds to the
oil. By adding ionic emulsifiers like lecithin and charged molecules which have a hydrophobic
end like alcohols, resistivity of the pure soybean oil can be lowered. With these additions pure
soybean oil which has insulator characteristics can be altered to a dielectric solution (Abu-Ali
2004).
Resistivity of soybean oil increases with decreasing temperature. As the temperature
goes from 100⁰C to -5⁰C the resistivity increases from 0.251 to 100 Tohm.cm on the logarithmic
scale (Tekin and Hammond 1998). Apart from the temperature, polar compounds decrease the
resistivity, but especially water, phospholipids, and monoglycerides have the greatest effect. By
28
the addition of 400 mg/kg phospholipid into alumina-purified soybean oil its resistivity
decreased from 23.7 to 0.016 Tohm.cm (Tekin and Hammond 1998). Water addition decreases
the resistivity too much so water addition does not achieve atomization. According to Abu-Ali
and Barringer (2005) pure oil needs 3-20% surfactant addition to reached the oil atomizable
range. Also in their study, liposome containing water in oil emulsions were formed to allow
atomization. Pure soybean oil does not atomize, instead jet formation occurs. This characteristic
assisted to search out the effects of the solution properties on atomization (Abu-Ali 2004).
2.3 OXIDATION AND OFF-FLAVOR FORMATION
Off-flavor compounds in oils are produced during the oxidation process. Oxidation
makes oil less acceptable or unacceptable for consumers and also oil becomes unacceptable
regarding its food ingredient usage. Oxidized oil’s essential fatty acids’ are destroyed, also toxic
compounds and oxidized polymers are produced. The oxidation of oil has a great importance to
the palatability, toxicity, and nutritional value. In terms of edible oils during process and storage,
oxidation may occur. Several chemical mechanisms might be responsible of this formation.
Energy input is required in edible oil oxidation, and light or heat can cover this need. Besides the
energy input, fatty acid composition, types of oxygen (1O2 or 3O2), existence of minor
compounds such as metals, pigments, phospholipids, free fatty acids, mono and diacylglycerols,
thermally oxidized compounds, and antioxidants can affect oxidation (Choe and Min 2006). If
two oils with different unsaturated fatty acid concentrations are compared, the one which has
higher unsaturated fatty acid will oxidize more rapidly than the one that has less unsaturated
fatty acid (Parker and others 2003).
29
While unsaturated fatty acid number increases, oil becomes more sensitive to oxidation.
Soybean oil contains 15.2% saturated fatty acids, 23.3% mono-saturated fatty acids, 53.4% diunsaturated fatty acids, and 8.1 tri-unsaturated fatty acids. On the other hand, peanut oil
contains 18.4% saturated fatty acids, 61.4% mono-saturated fatty acids, 20.1% di-unsaturated
fatty acids, and 0.1 tri-unsaturated fatty acids. Therefore while soybean oil is more sensitive to
oxidation, peanut oil is more resistant to oxidation (Le Dréau and others 2009).
Iodine number or iodine value is the amount of iodine in g absorbed by 100 g oil, fat, or
wax (Nielsen 2010). Iodine number measures the degree of unsaturation in a given amount of
oil, fat, or wax. Induction period is the length of time until detect rancidity in oil or it is the lag
period of lipid oxidation (O'Keefe and Murphy 2010). Soybean oil, safflower, or sunflower oil,
which have iodine numbers higher than 135, have shorter induction period than coconut or
palm kernel oil, which have iodine numbers less than 20 (Tan and others 2002). During the
oxidation of oil, peroxides are produced and, peroxide value gives the amount of produced
peroxides. The peroxide value can be defined as the miliequivalents (mEq) of peroxide per
kilogram of sample (O'Keefe and Murphy 2010). Oils which have higher than 20 peroxide values
indicate poor quality and most probably have off flavor fats and oils. For soybean oil, peroxide
values of 1-5 indicate low levels of oxidation, 5-10, and 10 indicates medium and high levels of
oxidation, respectively (O'Keefe and Murphy 2010). 4 different frying oils used and fried potato
chips were stored at 63⁰C. Among the cottonseed oil, olive kernel oil, soybean oil and palmolein,
soybean oil showed the highest oxidation rate. Soybean oil samples gave the highest peroxide
value where cotton seed oil is ranked second. Soybean oil has the largest amount of linolenic
acid content (C18:3) among samples and in terms of linoleic acid (C18:2), soybean oil is ranked
second just after cotton seed oil (Lolos and others 1999).
30
Plant-sourced lecithins have similar fatty acid contents to their originated plants.
Soybean sourced lecithins have a high content of linoleic and linolenic acids, which are highly
sensitive to oxidation (Suriyaphan and others 2001). In reduced-fat Cheddar cheese with
lecithin, this has been used as a fat extender, but lecithin related off-flavors developed after 3
months ripening. These off-flavors are; (E)-2-nonenal, (E,E)-2,4-nonadienal, (E,Z)-2,4-decadienal,
and (E,E)-2,4-decadienal, which are linoleic acid oxidation products. On the other hand,
hydrogenated lecithin samples remove this problem (Suriyaphan and others 1999). Unmodified
soybean and rice lecithins results in off-flavor in fermented milk. Therefore it has been
concluded that unmodified soy or rice lecithins are not appropriate for fermented milk but
hydrogenated soy lecithin can be used in fermented milk (Suriyaphan and others 2001).
The autoxidation of oil increases with increasing temperature. At low temperatures,
generation of autoxidation products is slower in the induction period (Velasco and Dobarganes
2002; Choe and Min 2006). At high temperatures new oxidation compound formation is very
fast (Velasco and Dobarganes 2002). On the other hand, oxidation rate is related to the
temperature, increasing temperatures increases oxidation. Every 10⁰C increase in temperature
doubles the oxidation rate (Gill 2009). To reduce oxidation rate during storage or processing
temperature should be reduced, light and oxygen should be excluded, antioxidants should be
used and metals should be removed from the system (Choe and Min 2006). To reduce oxidation
and off-flavor formation it is important to keep the soybean oil and soybean lecithin at low
temperature.
31
2.4 OIL SENSITIVE PAPER
Oil sensitive paper can be used to count droplets in electrostatic atomization
applications. Oil sensitive paper has been employing for the rapid computation of oil-based
sprays, ultra-low volume suspensions, oil miscible liquids, undiluted emulsifiable concentrates
and soluble concentrates. In this paper, firm black colored paper is covered with thin white
colored oil soluble wax. When the oil based droplets touch the wax surface, it dissolves the oil
based solution and the black surface is seen at the surface (Syngenta 1996).
2.5 METHODS TO CLASSIFY THE DROPLETS BY SIZE
The droplets which have been generated with the spraying can be as small as 10 nm and
for desired spraying the equipment should be chosen and/or regulated to form the desired
droplet size, including adjusting the equipment settings. In practice, it is almost impossible to
produce all the same size droplets, because they are always in a range (Anonymous 2002).
Instead of defining a droplet by its diameter, its properties can be more accurately expressed by
defining volume related parameters.
Volume Median Diameter (VMD), also known as DV0.5, is one method used to classify
the droplets by size. In spraying systems, diameters of the droplets vary in a broad range which
creates the necessity for determining VMD for certain conditions. VMD is the most common
parameter employed to define droplet size (Matthews 2000). VMD refers to the actual diameter
of the droplet which lies at the mid-point for the total volume of the droplets. The total volume
of droplets with diameters below the VMD is equal to the total volume of droplets which have
higher diameter than VMD (Matthews 2000).
32
Figure 3. Distribution of droplets relative to VMD – total volume of the droplets on the left hand
side is equal to the total volume of the droplets on the right hand side (Matthews 2000)
DV0.1 is the diameter of the droplet in which 10% of the total droplet volume is smaller.
DV0.9 is the diameter of the droplet for which 90% of the droplet volume is smaller or 10% of the
droplet volume is larger. DV0.5 is a widely used descriptive value for drop size. However, a larger
DV0.9 value indicates less droplets per unit area, which means the sprayed volume of the liquid
disintegrated into fewer droplets. In some application areas higher DV0.9 values can indicate
waste of the spraying material. On the other hand, high DV0.1 value indicates more droplets per
unit area which means an increase in the number of droplets with small diameters (Anonymous
2002).
Relative Span (RS) is defined as the ratio between the width of the volume distribution
and the volume median diameter (eq 9). The smaller the RS, the less diversity among droplet
sizes; in other words the smaller the RS the more homogenous the droplet spectra (Anonymous
2002).
33
Span or range shows the range of the droplet diameters (eq 10) (Elversson and others
2003).
(10)
Table 3. DV0.5 and Relative Span values of different methanol-water formulations (Haq and
others 1983)
*
Formulation*
Temperature (⁰C)
5% methanol
Viscosity (cP)
21
Surface Tension
(dynes/cm)
62.6
1.01
DV0.5
(µm)
420
Relative
Span
0.60
15% methanol
20
53.4
1.38
399
0.59
25% methanol
26
48.4
1.56
389
0.52
35% methanol
26
41.2
1.62
358
0.66
50% methanol
26
34.3
1.50
350
0.65
65% methanol
25
31.7
1.41
326
0.67
75% methanol
18
29.4
1.31
287
0.61
90% methanol
18
24.4
0.98
274
0.70
Rest of the formulation includes water
34
Table 4. DV0.5 and Relative Span values of different glycerol-water formulations (Haq and others
1983)
*
Formulation*
Temperature (⁰C)
72% glycerol
Viscosity (cP)
23
Surface Tension
(dynes/cm)
69.5
28.04
DV0.5
(µm)
526
Relative
Span
0.87
68% glycerol
32
71.0
18.44
491
0.88
60% glycerol
26
69.1
9.41
479
0.89
35% glycerol
26
71.0
2.98
496
0.79
25% glycerol
25
70.2
2.21
501
0.76
20% glycerol
23
70.7
1.85
487
0.97
10% glycerol
19
72.1
1.44
431
1.23
Rest of the formulation includes water
The DepositScan program gives DV1, 5, and 9 values (µm), percent coverage, image area
(cm²), deposits/cm², deposition (µL/cm²), object number, image spot area (µm²), and actual
diameter (µm). The program calculates the spot area (A, µm2). A was acquired from the number
of spot image pixels divided by the scanning resolution (Zhu and others 2011). The actual
droplet diameter (d, µm) was calculated from the spot area (A):
Where,
√
Which becomes,
35
DepositScan program calculates percent coverage from total image spot area divided by image
area and multiplied by 100. Image area is used as the oil sensitive paper’s area in cm².
Deposits/cm² is calculated from total object number on paper divided by image area. Deposition
is calculated from a series of calculations; individual droplet volume, Vi, is calculated from eq 13;
Total of the individual droplet volume is divided by image area to calculate deposition.
36
3. MATERIALS AND METHODS
Soybean oil (The Kroger Co., Cincinnati, OH) was prepared by adding 0, 5, 10, or 15%
w/w lecithin (Alcolec-S, American Lecithin Company, Oxford, CT). The solution was mixed
manually with a glass stirring rod for 1 min and the solution was allowed to rest 10 min to allow
the bubbles inside the solution to dissipate.
The coating targets were 5.2x7.6 cm oil sensitive paper (TeeJet, Spraying Systems Co.
Wheaton, IL). The oil was sprayed onto the targets by a TDC liquid electrostatic coating machine
(Terronics Development Corporation, Elwood, IN) (Figure 4). The electrostatic coating system
consisted of a variable speed gear pump (74012-51, Cole-Parmer Instrument Company, Vernon
Hills, IL), hipotronics voltage source (Terronics Development Corporation, Elwood, IN), a
conveyor belt, and an applicator nozzle. The schematic of the nozzle is given in figure 5. The
applicator nozzle consisted of two halves of insulating material with serrated edges. The metal
electrode, which was between the two halves of the nozzle and charged the liquid, had a
thickness of 0.0762 mm and width of 152.39 mm. The experiment was performed at 6 different
voltages: 0kV and electrostatic voltages -15, -20, -25, -30, and -40kV.
37
Figure 4. Illustration of liquid electrostatic coating machine
Each of the coating targets was placed on the aluminum-foil conveyor belt 12.1 cm from
the conveyor belt’s left edge, which was found to give the most reproducible results. The gear
pump was set at speed 2 out of 12 which produced a flow rate of 9.91 g/min. To determine the
flow rate, the oil coming from the nozzle for 1 min was measured. The conveyor belt speed was
8.4cm/sec. To determine the conveyor speed, tape was attached to the starting point of the
conveyor belt, the belt was turned on, a stopwatch was started, and when the tape came to the
end point of the conveyor belt the stopwatch was stopped. The length of the conveyor belt was
205 cm. The total length was divided by the measured time.
38
Figure 5. Applicator nozzle with metal electrode which is between two halves of insulating
material
Immediately after the coating process, the oil sensitive papers were carefully placed on
a copy stand for 10 sec, to allow the color to change from yellow to black. The camera was set
on the copy stand 23.5 cm vertically above the copy stand’s base. After the color change,
pictures were taken with a camera (Nikon Coolpix L120, with 21X optical zoom, Nikon, Inc.
Melville, NY). The pictures were taken with 1x optical zoom (equivalent to 25mm lens, 1/20,
F3.1) with no photo-flash. After the pictures were taken, they were uploaded into the
computer, and analyzed by the DepositScan program (Portable Scanning System for Spray
Deposit Qualification, USDA-ARS Application Technology Research Unit, Wooster, OH). The
optical distinctness between the oil sensitive paper’s background and spray deposits was used
39
to quantify the spray deposit distribution (Zhu and others 2011). There are some limitations of
the program; first, the program can not differentiate overlapped deposits, to determine if it is
formed from several droplets or one. The other limitation is that if the percent coverage is over
20% the results will become inaccurate (Zhu and others 2011). In this study coverage was 1.37
to 23.17%.
The oil was sprayed at 4, 22, and 47⁰C. The 4⁰C experiment was performed in a walk in
cooler. The cooler was at 4.1⁰C and 71% relative humidity. Nozzle, metal electrode, tubes, and
oil samples were held in the cooler for 12 h before the experiment. Oil sensitive papers, coating
machine and pump were brought into the chamber immediately before the experiment to
protect them from the humidity. Samples at 22⁰C were measured at room temperature. For
47⁰C samples, the solution was held in a water bath at 50⁰C with an isotemp immersion
circulator (730-28, Fisher Scientific, Pittsburgh, PA). Heating tape was wrapped around the tubes
carrying the sample to the pump, and from the pump to the nozzle, and insulating foam was
wrapped around the heating tape. A Powerstat variable autotransformer (3PN117C, The
Superior Electric Co. Bristol, CT) was used at setting 70 to heat the heating tape. The empty
system was operated for 10 min with the heating tape on, then the pump was turned on for 12
min more to heat the system. The solution temperature was measured with a micro
thermocouple (HH21, Type K thermocouple, Omega Engineering, Inc. Stamford, CT) at the edge
of the nozzle as the oil exited from the nozzle. Before the start of each coating process, the
desired voltage was set and 30 sec was allowed to reach steady state.
The resistivity of the solutions was measured using a resistivity cell. The resistivity cell
consists of a high density plastic cylinder sample container, two brass electrodes, electrometer
40
(614, Keithley Instruments, Inc. Cleveland, OH), and programmable power supply (Kepco, Inc.
Flushing, NY). The resistivity was measured by determining the current passing through the
sample. 125kV voltage was applied for every sample. Resistivity was calculated by equation 1,
where
is the cell constant, 0.014,
is the applied voltage, and is the current (A) (equation
1).
The resistivity was measured at 4, 22, and 47⁰C. The 4⁰C experiment was performed in a
walk in cooler. Oil samples, plastic cylinder sample container, and electrodes were held in the
cooler for 12 h before the experiment. The electrometer and power supply were brought into
the cooler immediately before the experiment to protect the electric circuits of electrometer
and power supply from the moisture. The 22⁰C experiment was performed at room
temperature. To reached 47⁰C, oil samples, plastic cylinder sample container, and electrodes
were held in a toaster oven (6239, Oster, Miami, Fl) at 125⁰F to achieve the desired sample
temperature. To eliminate leakage from the sample container, rubber cement (No-Wrinkle
Rubber Cement, Elmer’s Products Inc. Westerville, OH) was used, and allowed 1 min to dry. To
perform the actual analyses 5g sample was added to the container with an electronic pipette.
Before the test, to ensure the accuracy of the cell performance, 5g of flake salt (Alberger
Untreated Flake Salt, Cargill Salt, Minneapolis, MN) was placed into the test unit and tapped to
eliminate the air pockets between salts. Salt test results with less than 8.98 %RSD from
1.458x1010 Ωm, were used to check the resistivity cell’s accuracy before testing. After the salt
test the plastic sample container was washed and wiped completely, and the electrodes were
wiped until there was no salt on the electrodes.
41
The critical resistivity was determined by plotted droplets/cm2 vs. resistivity for every
voltage, except 0kV and extrapolating to 0 droplets/cm2. Data for 0% lecithin samples were
excluded because they did not atomize. The extra resistivity at 0 droplets/cm2 or critical
resistivity was plotted versus voltage.
Viscosity measurements were performed with a Rheometer (AR-1000N, TA Instruments,
New Castle, DE) at 4, 22, and 47⁰C, with 40mm 2⁰ steel cone geometry (TA Instruments, Ltd.
Leatherhead, England) which has a 0.0597 stress factor, 28.8 rate factor, and 43 µm truncation.
During the measurements shear stress (Pa), shear rate (1/s), viscosity (Pa.s), time (s),
temperature (⁰C), normal stress (Pa), displacement (rad), gap (µm), normal force (N), normal
stress coefficient (Pa.s²), strain, percentage strain, torque (µN.m), and velocity (rad/s) values
were obtained. First the sample was conditioned at 4, 22, or 47⁰C to equilibrium for 2 min.
Second, steady state flow experiments were done by increasing and then decreasing shear
stress. Test settings were; shear stress set as ramp, from 0.5 to 78.0 or 78.0 to 0.5 (Pa), log
mode, 10 points per decade, 10sec sample period. Percentage tolerance was 5.0, consecutive
within tolerance was 3, and maximum point time was 1 min. For the post experiment step the
system was set at 25⁰C, to bring the system to 25⁰C after the experiment.
After status check and other preparation steps were done, the steel cone was attached
to the system and 43 µm gap were chosen. The sample was loaded with 590µl of oil by an
automatic pipette (Eppendorf Reference Pipette, 100-1000µl, Hauppauge, NY). Gap geometry
was chosen and the experiment started. Each experiment took 40 min. After each experiment
the steel cone was removed from its attachment place and cleaned and also the area where the
oil was added was cleaned. The preparation steps were repeated between every experiment.
42
The solvent trap was not used during the experiments. Every experiment was done in 3
replicates except surface tension.
Sample viscosity was reported at the average shear rate within the nozzle. To determine
volumetric velocity, flow rate was divided by density of the liquid. Area of the nozzle opening
was calculated from height of the nozzle opening multiplied by width of the nozzle. Linear
velocity was calculated by dividing volumetric flow rate by area of the nozzle opening. Shear
rate was calculated from linear velocity divided by height of the liquid which flows over the
fingers of the metal electrode (Marthina 2011). The height of the employed nozzle was 0.0762
mm and width was 152.39 mm. However, it was assumed that the flow was 50% of the width,
and the nozzle height was assumed to be the thickness of the metal electrode used and flow
was 25% of thickness. The calculated volumetric flow rate was converted to the flow rate in
inch3 per minute by multiplying by 0.06102. Calculated area of the nozzle opening was in inches
and was multiplied by 25.4 to convert it to mm. Height of the liquid flowing over the fingers of
the metal electrode was 2 mm, so calculated linear velocity was divided by 2. To convert min to
sec, the result was divided by 60. Shear rate (1/s) was calculated at the end of these steps.
Average shear rate was calculated from the average of all samples at different lecithin content
and temperature. Average shear rate was calculated as 61.6635 s-1. The shear stress and
viscosity for samples at different lecithin content and temperature was calculated with
interpolation at 61.6635 s-1 shear rate.
The surface tension of the solutions was measured using a Drop Shape Analysis System
(DSA1 v 1.9, Kruss, Matthews, NC). A 4 ml sample was poured into the syringe, the filled syringe
was equilibrated to 4, 22, or 47⁰C, and a disposable needle was added to the syringe. Angle of
43
inclination of the prism was 0⁰. The needle’s diameter was measured with calipers (Mitutoyo
Absolute Digimatic, Mitutoyo U.S.A., Aurora, IL.) as 1.82 mm. The exact diameter was entered
into the system in mm with two decimal places. Density of every sample was measured with
volumetric flasks where samples and flasks were equilibrated to the correct temperatures and
the exact numbers entered to four decimals. The exact temperature was entered in Celsius. The
focus wizard of the analysis system was opened to help in setting the sharpness of the image.
Illumination was set to a medium brightness. The knob was turned to move the needle
downward, until it appeared on the computer screen. The needle was moved to the center of
the image. Lens zoom was adjusted by turning the zoom knob, so that the needle occupied 20%
of the frame. The image sharpness was increased to maximize the numerical value of the
median. The experiment started by automatically pushing the syringe, to disperse 165.1µl/min.
The flow button was pushed slowly, to make as large a drop as possible. The zoom and needle
height was regulated again if needed. After the experiment started, the system calculated the
scale of the image from the entered needle diameter and measured width of the needle image,
determined the drop shape, and calculated surface tension by using the Young-Laplace
equation. The Young-Laplace equation identifies the capillary pressure difference between the
interfaces of two static fluids because of the surface tension. These fluids were oil and air. When
the drop is in hydromechanical equilibrium and is hanging from the syringe needle, the surface
tension of the liquid is calculated from the size and the shape of the drop. The Young-Laplace
equation identifies the pressure difference between the outside and inside of the drop, where,
is the pressure difference across the fluid interface, is the surface tension of the liquid and
and
are the principal radii of curvature (equation 2). The surface tension experiment was
done in 16 replicates.
44
The Weber number was calculated from the equation;
. In this equation, ,
the undeformed drop radius, was the DV0.5 value at 0 kV for samples with different lecithin
content and temperature. 0 kV values were used due to partially or completely deformation of
liquid at higher voltages. The permittivity of the continuous phase, , was calculated from
soybean oil’s and lecithin’s dielectric constant.
the electrical field strength, was calculated
from the applied electric potential divided by the distance. Distance was the edge of the nozzle
to the ground. The interfacial surface tension, , was measured as previously described under
surface tension. Drop diameters were divided by 2 to convert to radius and the results were
multiplied by 10¯6 to convert µm to m. For calculations of , soybean oil’s dielectric constant was
3.1167 (Lizhi and others 2008). In the literature there is no study on lecithin’s dielectric constant
so phospholipid’s dielectric constant was used in the calculations as lecithin’s dielectric constant
and it was 2.5 (Seddon and Cevc 1993). For 5% lecithin samples, 95% of the dielectric constant
comes from soybean oil’s dielectric constant and 5% comes from lecithin’s dielectric constant.
Also 10 and 15% lecithin contains samples have 10 and 15% effect from lecithin, respectively.
was calculated by multiplying these results by the permittivity of vacuum which has the value of
8.854 x 10-12 F/m. For calculation of , applied electric potential at 0, 15, 20, 25, 30 or 40 kV was
converted to V and divided by the distance between nozzle and ground, which was 0.26 m.
alues were converted to N/m by multiplying by 10¯³.
The other Weber number was calculated from the equation;
equation
is the density of the sample, which has a unit of kg/m3 and
45
. In this
is the velocity of the
liquid. To calculate the volumetric flow rate, flow rate was measured in g/min and divided by
the density of the liquid. Volumetric flow rate has been calculated in ml/min. It was converted
to m3/min by multiplying with 1 x 10-6. Velocity has been calculated by dividing volumetric flow
rate by the area of nozzle opening. DV0.5 is used as characteristic length called , which is
different for every temperature and lecithin content. In order to convert DV0.5 to m it has been
multiplied with 1 x 10-6. The surface tension, , result of the related sample, which has a unit of
N/m or kg/s2.
Statistical analyses were performed by IBM SPSS Statistics software version 19
(International Business Machines Corp. Armonk, NY). Two way analysis of variance, followed by
a Tukey’s HSD (Honestly Significant Difference) test was used for statistical analysis to determine
if there was a significance differences between samples which exposed to 0, 15, 20, 25, 30, and
40kV in terms of their droplets/cm2 (p=0.05). This analysis was performed to determine if
different temperatures and lecithin contents significantly affects resistivity, viscosity, surface
tension, and droplets/cm2.
46
4. RESULTS AND DISCUSSION
4.1 ELECTRICAL RESISTIVITY
The electrical resistivity of oil with 0% lecithin at 22⁰C was 7.29 x 1012 Ωm (Figure 6). A
liquid with resistivity higher than 1 x 1010 Ωm will not atomize adequately (Wilkerson and
Gaultney 1989). Other studies have shown even lower limits for oil atomization, with a required
resistivity range of 105 to 109 Ωm (Gaultney and others 1987), 105 to 108 Ωm (Abu-Ali and
Barringer 2005), or 106 to 108 Ωm (Law 1984). Thus oil’s resistivity is too high for atomization.
Because of lecithin’s ability to carry charge as an ionic emulsifier, addition of lecithin has been
shown to decrease resistivity in soybean oil, confectionary coating, and chocolate (Abu-Ali and
Barringer 2005; Gorty and Barringer 2011; Marthina and Barringer 2012). Increasing
temperature has also been shown to decrease resistivity in chocolate, tomato juice and orange
juice (Palaniappan and Sastry 1991; Gorty and Barringer 2011). Increasing temperature
increases the kinetic energy of the ions in the solution, increasing the mobility of the ions
(Adamczewski 1969). High mobility of the ionic charge carriers leads to lower resistivity of the
solution because they easily conduct charge. Therefore by adjusting lecithin content and
temperature, resistivity can be decreased into the atomizable range. Addition of an antistatic
agent into soybean and cotton seed oil was also required to decrease the resistivity from 1010 to
107 Ωm, which allowed it to atomize (Law and Cooper 1987).
47
Increasing temperature 4 to 47⁰C decreased resistivity 3 logs at 0% lecithin but only 1
log at 5 to 15% lecithin (Figure 6). Increasing lecithin content from 0 to 5% decreased resistivity
2 to 4 logs, depending on the temperature, but from 5 to 15% lecithin the decrease is less than
half a log. Thus at greater than 5% lecithin, temperature had a slightly greater effect than
lecithin content. None of the samples atomized at 0% lecithin (resistivity > 1011 Ωm), except at
the highest voltage. Samples with higher lecithin contents and higher temperature atomized,
even though the resistivity was as high as 2 x 1010 Ωm. Thus other factors, such as viscosity and
surface tension, are also important to atomization.
1.E+15
4⁰C
22⁰C
Resistivity (Ohmxm)
1.E+14
47⁰C
1.E+13
1.E+12
1.E+11
1.E+10
1.E+09
0
5
10
Lecithin Content (%)
Figure 6. Effects of lecithin content on resistivity at 4, 22, and 47⁰C
48
15
4.2 VISCOSITY
Viscosity decreased with increasing temperature and decreasing lecithin concentration
(Figure 7). Increasing temperature from 4 to 47⁰C decreased viscosity up to 336% while
decreasing lecithin content from 15 to 0% decreased viscosity only up to 45%. Thus temperature
had a greater effect on viscosity than lecithin. There was an interaction between lecithin and
temperature, so that as temperature decreased, the effect of lecithin content increased. As the
temperature increases, the viscosity of the liquid decreases due to an increase in kinetic energy
of the molecules. High kinetic energy facilitates overcoming Coulombic attractive forces among
the molecules to allow atomization. Increasing temperature decreases viscosity of chocolate
(Gorty and Barringer 2011), crude oils (Al-Besharah and others 1989) and many vegetable oils
(Rao 2007). Addition of lecithin increases viscosity in sunflower oil (Pernetti and others 2007),
soybean oil (Bhattacharya and others 1998), peanut oil (Rao 2007), and poloxamer gels (Bentley
and others 1999), while in chocolate and confectionary coatings lecithin can increase or
decrease viscosity depending on the concentration (Gorty and Barringer 2011; Marthina and
Barringer 2012).
49
0.14
22⁰C
0.12
Viscosity (Pa.s)
y = 0.0027x + 0.0898
R² = 0.9978
4⁰C
47⁰C
0.1
y = 0.0015x + 0.0587
R² = 0.9464
0.08
0.06
y = 0.0003x + 0.026
R² = 0.9533
0.04
0.02
0
5
10
15
Lecithin Content (%)
Figure 7. Effects of lecithin content on viscosity at 4, 22, and 47⁰C. Viscosity was measured at
shear rate 62 s-1 during decreasing shear
4.3 SURFACE TENSION
Surface tension decreased with increasing temperature but lecithin first decreased then
increased surface tension (Figure 8). However, the greatest change in surface tension was 1.1
mN/m, or about 4%. Thus neither temperature nor lecithin content had a large effect on surface
tension. Surface tension decreases with increasing temperature for most liquids (Wohlfarth
2008; Khan and others 2012) because the cohesive forces between molecules decrease with
increasing temperature (Lindeburg 2006). The surface tension of sunflower oil increases with
addition of up to 3% lecithin, and then decreases at greater lecithin content (Khan and others
2012).
50
34.0
4⁰C
22⁰C
33.5
Surface Tension (mN/m)
47⁰C
33.0
32.5
32.0
31.5
31.0
30.5
0
5
10
15
Lecithin Content (%)
Figure 8. Effects of lecithin content on surface tension at 4, 22, and 47⁰C
4.4 EFFECT OF VOLTAGE, LECITHIN CONTENT, AND TEMPERATURE ON THE NUMBER
OF DROPLETS/cm2
With increasing voltage, the number of droplets/cm2 increased, which means the
average drop size decreased and more even coating should be produced (Figure 9). In oil with
0% lecithin the oil did not atomize indicating there wasn’t sufficient charge in the sample so
there were few droplets formed and voltage had no significant effect, except at 40kV. Only with
the addition of lecithin did increasing voltage increase the number of droplets. Also, at 0kV none
of the oil samples atomized, so few droplets were formed. Similarly, in crop spraying 2.5 times
greater deposition is obtained on plant leaves and 9 times greater on plant stems when voltage
is applied to improve atomization and decrease droplet size (Coffee 1979).
51
In samples with 5% or more lecithin, increasing voltage greatly increased the number of
droplets/cm2. Increasing voltage applied to the liquid increases the charge concentration on the
surface. When the repulsive force between the like charges exceeds the surface tension the
liquid breaks into droplets, or atomizes (Bailey 1974). Above the threshold voltage, the number
of droplets per unit area will increase with increasing voltage (Gaultney and others 1987). 15kV
was the threshold voltage in the system used. The effect of voltage was greater at higher
temperatures. In mineral oil with N-butanol, increasing voltage from 14 to 25kV decreased
droplet median diameters from 160 to 60 µm (Wilson 1982). Increasing voltage also decreases
droplet sizes of soybean oil and heptane with antistatic fuel additives, and oil with pesticides
(Wilson 1982; Wilkerson and Gaultney 1989; Downer and others 1994; Tang and Gomez 1996).
The effect of voltage on droplet size can be observed in figure 10. At 25kV, (A-3) larger droplets
were produced than at 40kV (B-3). In a study with water volume median diameter (VMD)
decreased with increasing voltage (Laryea and No 2003).
Lecithin content also significantly affected the number of droplets/cm2 (Figure 9). The
effect of lecithin content increased with temperature and voltage. Addition of 5 to 10% lecithin
into the oil increased the number of droplets/cm2; however, addition of 10 to 15% lecithin
increased, decreased or had no effect on the number of droplets/cm2. In a study with soybean
oil, addition of 13% lecithin gave the best coating in terms of transfer efficiency and
reproducibility, which is between the 10 and 15% used in this study (Abu-Ali and Barringer
2005). The reason the number of droplets/cm2 did not always increase between 10 and 15%
could be the negative effect of increasing viscosity and surface tension on atomization. Both
viscosity and surface tension increased with lecithin addition from 10 to 15% lecithin (Figure 7,
8). Increasing viscosity and surface tension have previously been shown to lower the number of
52
droplets produced (Matthews 1979). In figure 10 the effect of lecithin content can be seen in B-1
to B-3. At 47⁰C 0% lecithin and 40kV (B-1) atomization started, but the charge was insufficient to
produce small droplets. With the addition of 5% (B-2) or 10% lecithin (B-3) at the same
temperature and voltage atomization greatly improves and the number of droplets/cm2
increases.
Increasing temperature from 4 to 47⁰C increased the number of droplets/cm2 more
than 4 times at the same lecithin content and voltage (Figure 9). In a study of soybean oil with
antistatic fuel additives, increasing temperature also decreased the droplet size (Wilkerson and
Gaultney 1989). In figure 10, A-1 to A-3 shows the effect of temperature on atomization. At 4⁰C
10% lecithin and 25kV (A-1), atomization is poor because the high resistivity hinders atomization
at low temperatures irrespective of the lecithin content and high voltage. Increasing
temperature to 22⁰C (A-2) and 47⁰C (A-3) improved droplet formation because resistivity,
viscosity, and surface tension was lower at higher temperatures (Figure 6, 7, 8).
Voltage had the greatest effect on the number of droplets/cm2, while lecithin content
and temperature also had significant effects. A study on heptane with anti-static additives
showed applied voltage had the second greatest effect on droplet size after liquid flow rate
(Tang and Gomez 1996). In soybean oil voltage was previously found to have greater effect on
coating reproducibility than resistivity, viscosity, surface tension, and emulsifier content or flow
rate, with the most reproducible coating between 30 and 35 kV (Abu-Ali and Barringer 2008).
53
10% 47⁰C
15% 47⁰C
5% 47⁰C
10% 22⁰C
15% 22⁰C
5% 22⁰C
10% 4⁰C
15% 4⁰C
5% 4⁰C
0% 47⁰C
0% 4⁰C
0% 22⁰C
250
Droplets/cm²
200
150
100
50
0
0
10
20
30
40
Voltage (kV)
Figure 9. Effect of voltage on the number of droplets/cm² at 0, 5, 10, and 15% lecithin and 4, 22,
and 47⁰C
54
1cm
A-1
A-2
A-3
B-1
B-2
B-3
Figure 10. Pictures of oil sensitive papers after coating: (A-1) 4⁰C 10% lecithin 25kV, (A-2) 22⁰C
10% lecithin 25kV, (A-3) 47⁰C 10% lecithin 25kV, (B-1) 47⁰C 0% lecithin 40kV, (B-2) 47⁰C 5%
lecithin 40kV, (B-3) 47⁰C 10% lecithin 40kV
4.5 EFFECT OF RESISTIVITY, VISCOSITY, SURFACE TENSION, AND WEBER NUMBER ON
THE NUMBER OF DROPLETS/cm2
At 0kV there was no trend between resistivity and the number of droplets/cm2 because
the liquid was not charged and resistivity is only important in electrostatic spraying (Figure 11).
Similarly, in confectionary coating at 0kV resistivity does not affect droplet sizes (Marthina and
55
Barringer 2012). At every voltage except 0kV, with decreasing resistivity, the number of
droplets/cm2 increased (Figure 11). At low resistivity the electrical current can more easily
conduct through the solution, increasing the charges on the surface of the molecule so that the
electrical forces dominate over surface tension and atomization is more effective (Bailey and
Balachandran 1981). Resistivity decreased with increasing temperature and increasing lecithin
content (Figure 6, 11). Decreasing resistivity also decreases droplet sizes of soybean oil or oil
with pesticides (Wilkerson and Gaultney 1989; Downer and others 1994).
22⁰C
47⁰C
4⁰C
250
40kV
Droplets/cm²
200
30kV
25kV
20kV
150
15kV
0kV
100
50
0
1.50E+09
1.50E+10
Resistivity (Ωm)
Figure 11. Effect of resistivity on the number of droplets/cm2 at 0 to 40kV. 0% lecithin results
were not included
56
The critical resistivity is the highest resistivity at which atomization can be achieved at a
given voltage. The critical resistivity increased with increasing voltage (Figure 12). At higher
voltages atomization starts to occur at a higher resistivity. Therefore, at 40 kV even at 3.69 x
1010 Ωm atomization can occur while resistivity cannot be above 1.75 x 1010 Ωm at 15kV.
Similarly, the threshold voltage depends directly on the resistivity of the liquid, thus liquids with
high resistivity atomize at higher voltage than liquids with low resistivity (Gaultney and others
1987; Wilkerson and Gaultney 1989).
Critical Resistivity (Ohmxm)
3.69E+10
1.75E+10
1.5E+10
15
20
25
30
35
40
Voltage (kV)
Figure 12. Critical resistivity vs. voltage. 0% lecithin results were not included
At the same applied voltage, decreasing viscosity increased the number of droplets/cm2
(Figure 13). The effect of viscosity on the number of droplets/cm2 increased at higher voltages.
High viscosity hinders atomization and droplet formation (Bayvel and Orzechowski 1993; Gorty
and Barringer 2011). At high viscosity ion mobility is lower; therefore charge conduction is
57
slower (Schmidt 1997; Nik and others 2007) preventing atomization. The viscosity of the liquid
decreased with increasing temperature and decreasing lecithin content (Figure 7, 13). In a study
with water and glycerol mixtures the number of droplets decreased and droplet sizes increased
with increasing viscosity at constant voltage and resistivity (Jayasinghe and Edirisinghe 2002). In
soybean oil, decreasing viscosity decreased droplet sizes (Wilkerson and Gaultney 1989).
47⁰C
22⁰C
4⁰C
250
40kV
Droplets/cm²
200
30kV
25kV
20kV
150
15kV
0kV
100
50
0
0.025
0.045
0.065
0.085
0.105
0.125
Viscosity (Pa.s)
Figure 13. Effect of viscosity on the number of droplets/cm2 at 0 to 40kV. 0% lecithin results
were not included
Generally, decreasing surface tension increases the number of droplets/cm2 however
the correlation is very weak (Figure 14). The number of droplets was affected more strongly by
resistivity and viscosity therefore the number of droplets was not always higher at lower surface
58
tension values. Surface tension should have some effect since atomization occurs when the
electrical forces exceed the surface tension. In a study on pesticide atomization, surface tension
significantly affected droplet size, but it has the least significant effect among resistivity,
viscosity, voltage, and mass flow rate (Wilkerson and Gaultney 1989).
In this study viscosity and resistivity had the greatest effect on the number of
droplets/cm2, with surface tension having the least effect among these factors. In a pesticide
atomization system resistivity was determined to have the greatest effect on droplet size among
voltage, viscosity, surface tension, and flow rate by Wilkerson and Gaultney (1989).
47⁰C
22⁰C
4⁰C
250
Droplets/cm²
200
40kV
30kV
25kV
150
20kV
15kV
100
0kV
50
0
31.10
31.30
31.50
31.70
31.90
32.10
32.30
32.50
32.70
Surface Tension (mN/m)
Figure 14. Effect of surface tension on the number of droplets/cm2. 0% lecithin results were not
included
59
The Weber number indicates the ratio between the electric field and the surface tension
stress (Eow and others 2001). When the electrical field strength exceeds the surface tension of
the liquid, the critical limit is exceeded and atomization starts (Yarin and others 2001; He and
others 2008). The Weber number indicates the critical point where atomization starts. Above
this critical point the number of droplets dramatically increases. The number of droplets/cm2
increases linearly due to a linear relationship between the Weber number and the square of the
electric field (Eow and others 2001). In this study the Weber number which corresponds to the
critical point at which droplet formation occurs was around 0.01 (Figure 15). In aqueous heptane, and oil mixtures, the Weber number of this droplet break-up onset point was between
0.06 and 0.13 (Eow and others 2001).
10% 47⁰C
15% 47⁰C
5% 47⁰C
10% 22⁰C
15% 22⁰C
5% 4⁰C
5% 22⁰C
10% 4⁰C
15% 4⁰C
0% 47⁰C
0% 22⁰C
0% 4⁰C
250
Droplets/cm²
200
150
100
50
0
0
0.01
0.02
0.03
0.04
0.05
Weber Number
Figure 15. Effect of Weber number on the number of droplets/cm2 at 0, 5, 10, and 15% lecithin
and 4, 22, and 47⁰C
60
4.6 EFFECT ON SPAN
Span is the difference between the DV0.9 and DV0.1 values and indicates the range of
droplet sizes formed. A small span means there is a narrow size distribution of the droplets and
more uniform droplets are produced. Span decreased with increasing voltage (Figure 16). At 0%
lecithin, there was little effect of voltage because atomization didn’t occur. The biggest decrease
in the size distribution was observed between 20 and 25kV. Increasing lecithin content
decreased span at 47⁰C, and at 22⁰C and higher voltages. In general, span decreased under the
same conditions that increased the number of droplets/cm2. At high voltages atomization was
more efficient, therefore the number of droplets increased and the droplets had a narrow size
spectrum. In a contrast, in a study with oil in water, a wider size distribution of droplets was
formed at high voltages (Sato and Hatori 1997).
10% 4⁰C
5% 22⁰C
15% 47⁰C
5% 47⁰C
15% 22⁰C
5% 4⁰C
15% 4⁰C
10% 22⁰C
10% 47⁰C
2400
Span (µm)
1900
1400
900
400
15
20
25
Voltage (kV)
30
35
40
Figure 16. Effect of voltage on span at 5, 10, and 15% lecithin and 4, 22, and 47⁰C. 0% lecithin
results were not included
61
5. CONCLUSION
Electrostatic coating systems can make coating applications more effective, uniform,
and with less waste as compare to non-electrostatic coating because droplets were smaller, the
number of droplets/cm2 was higher and the drop size range was smaller. As temperature
increased, the resistivity, viscosity and surface tension of all samples decreased. As lecithin
content increased, resistivity decreased, viscosity increased and surface tension decreased and
then increased. The most important parameters were voltage and resistivity. Under electrostatic
coating, voltage had the greatest effect on number of droplets/cm2. At the highest temperature
(47⁰C), and voltage (40kV) and 10% lecithin the maximum number of droplets/cm2 was
observed. For better coating oil temperature should be increased, and resistivity should be
lowered by adding an ionic emulsifier such as lecithin.
62
REFERENCES
Anonymous 2002. Spray drift management principles, strategies and supporting information.
1st ed. Australia: Csiro Publishing. 71 p.
Abu-Ali J. 2004. Food coating application in: electrostatic atomization, non-electrostatic coating
and electrostatic powder coating. [dissertation]. The Ohio State University.
Abu-Ali J, Barringer S. 2008. Optimization of liquid electrostatic coating. J Electrostat 66(34):184-189.
Abu-Ali J, Barringer S. 2005. Method for electrostatic atomization of emulsions in an EHD
system. J Electrostat 63(5):361-369.
Adamczewski I. 1969. Ionization, conductivity and breakdown in dielectric liquids. London:
Taylor & Francis. 439 p.
Ahern J, Balachandran W. 2006. Experimental electrohydrodynamic nanospray production using
drawn glass capillaries. Particul Sci Technol 24(3):271-280.
Al-Besharah J, Akashah S, Mumford C. 1989. The effect of temperature and pressure on the
viscosities of crude oils and their mixtures. Ind Eng Chem Res 28(2):213-221.
Bailey A. 1974. Electrostatic atomization of liquids. Sci Prog 61(244):555-581.
Bailey A, Balachandran W. 1981. The disruption of electrically charged jets of viscous liquid. J
Electrostat 10(C):99-105.
Bateni A, Susnar S, Amirfazli A, Neumann A. 2005. A novel methodology to study shape and
surface tension of drops in electric fields. Microgravity Sci Tec 16(1-4):153-157.
Bayvel L, Orzechowski Z. 1993. Liquid atomization. 1st ed. Washington, DC: Taylor & Francis. 469
p.
Bentley M, Marchetti J, Ricardo N, Ali-Abi Z, Collett J. 1999. Influence of lecithin on some
physical chemical properties of poloxamer gels: rheological, microscopic and in vitro permeation
studies. Int J Pharm 193(1):49-55.
63
Berthier J. 2008. Ewod microsystems. In: Berthier J, editor. Microdrops and digital microfluidics.
1st ed. Norwich, NY: William Andrew Pub. p 231-233.
Bhattacharya S, Shylaja M, Manjunath M, Sankar U. 1998. Rheology of lecithin dispersions. J Am
Oil Chem Soc 75(7):871-874.
Burayev T, Vereshchagin I. 1972. Physical processes during electrostatic atomization of liquids. J
Fluid Mech 1(2):56-66.
Choe E, Min D. 2006. Mechanisms and factors for edible oil oxidation. Compr Rev Food Sci F
5(4):169-186.
Coffee R, inventor; Imperial Chemical Industries PLC, assignee. 1982 Atomization of liquids. U.S.
patent 4,476,515.
Coffee R. 1979. Electrohydrodynamic energy - a new approach to pesticide application. 1979
British Crop Protection Conference - Pests and Disease 777-789.
Dogan H, Kokini J. 2007. Rheological properties of foods. In: Heldman DR, Lund DB, editors.
Handbook of food engineering. 2nd ed. Boca Raton: CRC Press. p 1-124.
Downer RA, Hall FR, Escallon EC, Chapple AC. 1993. The effect of diluent oils on the electrostatic
atomization of some insecticides. Pesticide Formulations and Application Systems. 13:203-214.
Elversson J, Millqvist-Fureby A, Alderborn G, Elofsson U. 2003. Droplet and particle size
relationship and shell thickness of inhalable lactose particles during spray drying. J Pharm Sci
92(4):900-910.
Eow J, Ghadiri M, Sharif A. 2001. Deformation and break-up of aqueous drops in dielectric
liquids in high electric fields. J Electrostatics 51–52(1-4):463-469.
Evans R, Reynhout G, inventors; Inc Kalamazoo Holdings, assignee. 2000 Electrostatic deposition
of edible liquid condiment compositions upon edible food substrates and thus-treated products.
U.S. patent 6,010,726.
Franz E, Reichard D, Carpenter T, Brazee R. 1987. Deposition and effectiveness of charged sprays
for pest control. ASAE Paper no 86-1005
Ganan-Calvo A, Davila J, Barrero A. 1997. Current and droplet size in the electrospraying of
liquids; scaling laws. J Aerosol Sci 28(2):249-275.
Gaultney L, Almekinders H, Escallon E, Wilkerson J. 1987. Laboratory studies of electrostatic
pesticide applicators. ASAE Paper no 87-1065
64
Gill P. 2009. Insulating oils, fluids, and gasses. In: Gill P, editor. Electrical power equipment,
maintenance, and testing. 2nd ed. Boca Raton, FL: CRC Press. p 193-195.
Gorty A, Barringer S. 2011. Electrohydrodynamic spraying of chocolate. J Food Process Pres.
35(4):542-549.
Ha J, Yang S. 1999. Breakup of a multiple emulsion drop in a uniform electric field. J Colloid
Interface Sci 213(1):92-100.
Haq K, Akesson N, Yates W. 1983. Analysis of droplet spectra and spray recovery as a function of
atomizer type and fluid phsical properties. In: Kaneko TM, Akesson NB, editors. Pesticide
formulations and application systems, third symposium. Philadelphia, Pa. American Society for
Testing and Materials. p 67-82.
Hartman R, Brunner D, Camelot D, Marijnissen J, Scarlett B. 2000. Jet break-up in
electrohydrodynamic atomization in the cone-jet mode. J Aerosol Sci 31(1):65-95.
He J, Liu Y, Xu L, Yu J, Sun G. 2008. BioMimic fabrication of electrospun nanofibers with highthroughput. Chaos Soliton Fract 37(3):643-651.
Hendricks C. 1973. Introduction to electrostatics. In: Moore AD, editor. Electrostatics and its
applications. 1st ed. New York: John Wiley & Sons, Inc. p 8-28.
Hines R. 1966. Electrostatic atomization and spray painting. J Appl Phys 37(7):2730-2736.
Hughes J, Pavey I. 1981. Electrostatic emulsification. J Electrostat 10(C):45-55.
Hughes J. 1997. Electrostatic particle charging: industrial and health care applications. 1st ed.
Taunton, Somerset, England: Research Studies Press, Ltd. 170 p.
Inculet I, Castle G, Menzies D, Frank R. 1981. Deposition studies with a novel form of
electrostatic crop sprayer. J Electrostat 10(C):65-72.
IUPAC-IUB Commission on Biochemical Nomenclature. 1977. Nomenclature of phosphoruscontaining compounds of biochemical importance (recommendations 1976). Proc Natl Acad Sci
USA 74(6):2222-30.
Jaworek A. 2008. Electrostatic micro- and nanoencapsulation and electroemulsification: A brief
review. J Microencapsulation 25(7):443-468.
Jaworek A. 2007. Electrospray droplet sources for thin film deposition. J Mater Sci 42(1):266297.
Jayasinghe S, Edirisinghe M. 2004. Electrostatic atomisation of a ceramic suspension. J Eur
Ceram Soc 24(8):2203-2213.
65
Jayasinghe S, Edirisinghe M. 2002. Effect of viscosity on the size of relics produced by
electrostatic atomization. J Aerosol Sci 33(10):1379-1388.
Kahl H, Wadewitz T, Winkelmann J. 2003. Surface tension of pure liquids and binary liquid
mixtures. J Chem Eng Data 48(3):580-586.
Khan M, Schutyser M, Schroen K, Boom R. 2012. The potential of electrospraying for
hydrophobic film coating on foods. J Food Eng 108(3):410-416.
Knuiman J, Beynen A, Katan M. 1989. Lecithin intake and serum cholesterol. Am J Clin Nutr
49(2):266-268.
Laryea G, No S. 2003. Development of electrostatic pressure-swirl nozzle for agricultural
applications. J Electrostat 57(2):129-142.
Law S. 1984. Physical properties determining chargeability of pesticide sprays. In: Scher HB,
editor. Advances in pesticide formulation technology, monograph series no 254. Washington,
DC: American Chemical Society. p 219-230.
Law S, Cooper S. 1987. Induction charging characteristics of conductivity enhanced vegetable-oil
sprays. T ASAE 30(1):75-79.
Le Dréau Y, Dupuy N, Artaud J, Ollivier D, Kister J. 2009. Infrared study of aging of edible oils by
oxidative spectroscopic index and MCR-ALS chemometric method. Talanta 77(5):1748-1756.
Lefebvre A. 1989. Atomization and sprays. 1st ed. New York: Hemisphere Publishing. 421 p.
Lenggoro I, Okuyama K, Fernandezdelamora J, Tohge N. 2000. Preperation of ZnS nanoparticles
by electrosray pyrolysis. J Aerosol Sci 31(1):121-136.
Lindeburg M. 2006. Mechanical engineering reference manual : for the PE exam. 12th ed.
Belmont, CA: Professional Publications. 1296 p.
Liu H. 2000. Processes and techniques for droplet generation. In: Liu H, editor. Science and
engineering of droplets fundamentals and applications. New York: Noyes. p 19-27.
Lizhi H, Toyoda K, Ihara I. 2008. Dielectric properties of edible oils and fatty acids as a function of
frequency, temperature, moisture and composition. J Food Eng 88(2):151-158.
Lolos M, Oreopoulou V, Tzia C. 1999. Oxidative stability of potato chips: effect of frying oil type,
temperature and antioxidants. J Sci Food Agric 79(11):1524-1528.
Long J, Drelich J, Zhenghe X, Masliyah J. 2007. Effect of operating temperature on water-based
oil sands processing. Can J Chem Eng 85(5):726-738.
66
Magil S, Zeisel S, Wurtman R. 1981. Effects of ingesting soy or egg lecithins on serum choline,
brain choline and brain acetylcholine. J Nutr 111(1):166-170.
Marthina K. 2011. Confectionery coating using an electrohydrodynamic (EHD) system.
[dissertation]. Columbus, Ohio: The Ohio State University.
Marthina K, Barringer S. 2012. Confectionery coating with an electrohydrodynamic (EHD)
system. J Food Sci 77(1):E26-31.
Matthews G. 2000. Spray droplets. In: Matthews GA, editor. Pesticide application methods. 3rd
ed. UK: Blackwell Science. p 74-102.
Matthews G. 1979. Pesticide application methods. 1st ed. New York: Longman, Inc. 334 p.
Michelson D. 1990. Electrostatic atomization. 1st ed. New York: Adam Hilger. 150 p.
Mori Y, Fukumoto Y. 2001. Production of silica particles by electrostatic atomization. J Soc
Powder Technol 38(2):90-96.
Nawab M, Mason S. 1958. The preparation of uniform emulsions by electrical dispersion. J Coll
Sci Imp U Tok 13(2):179-187.
Nielsen S. 2010. Food analysis laboratory manual. 2nd ed. New York: Springer. 177 p.
Nieuwenhuyzen W. 1976. Lecithin production and properties. J Am Oil Chem Soc 53(6):425-427.
Nik W, Giap S, Senin H, Bulat K. 2007. Effect of shear rate and temperature on rheological
properties of vegetable based oil. AIP Conf Proc 909(1):253-260.
O'Dwyer J. 1973. The theory of electrical conduction and breakdown in solid dielectrics. Oxford:
Clarendon Press. 317 p.
O'Keefe S, Murphy P. 2010. Fat characterization. In: Nielsen SS, editor. Food analysis. 4th ed.
New York: Springer. p 239-260.
Okuda H, Kelly A. 1996. Electrostatic atomization - experiment, theory and industrial
applications. Phys Plasmas 3(5):2191-2196.
Oommen T. 2002. Vegetable oils for liquid-filled transformers. IEEE Electr Insul M 18(1):6-11.
Orthoefer F. 1998. Lecithin and health. 1st ed. Illinois: Vital Health Publishing. 83 p.
Palacios L, Wang T. 2005. Egg-yolk lipid fractionation and lecithin characterization. J Am Oil
Chem Soc 82(8):571-578.
67
Palaniappan S, Sastry S. 1991. Electrical conductivity of selected juices: influences of
temperature, solids content, applied voltage, and particle size. J Food Process Eng 14(4):247260.
Parker T, Adams D, Zhou K, Harris M, Yu L. 2003. Fatty acid composition and oxidative stability of
cold-pressed edible seed oils. J Food Sci 68(4):1240-1243.
Pearson R, Pascher I. 1979. The molecular structure of lecithin dihydrate. Nature 281(5731):499501.
Pernetti M, Van-Malssen K, Kalnin D, Floter E. 2007. Structuring edible oil with lecithin and
sorbitan tri-stearate. Food Hydrocoll 21(5-6):855-861.
Rao M. 2007. Rheological behavior of processed fluid and semisolid foods. In: Rao MA, editor.
Rheology of fluid and semisolid foods principles and applications. 2nd ed. New York: Springer. p
223-338.
Rayleigh L. 1882. On the equilibrium of liquid conducting masses charged with electricity.
Philosophical Magazine 14(87):184-186.
Sahin S, Sumnu S. 2006. Physical properties of foods. 1st ed. New York: Springer. 257 p.
Sato M, Hatori T. 1997. Experimental investigation of droplet formation mechanisms by
electrostatic dispersion in a liquid-liquid system. IEEE T Ind Appl 33(6):1527-1534.
Schmidt W. 1997. Liquid state electronics of insulating liquids. 1st ed. Boca Raton: CRC Press.
368 p.
Seddon J, Cevc G. 1993. Physical characterization. In: Cevc G, editor. Phospholipids handbook.
New York: Marcel Dekker, Inc. p 351-402.
Shchipunov Y. 1997. Self-organizing structures of lecithin. Russ Chem Rev 66(4):301-322.
Shchipunov Y. 2002. Lecithin. In: Hubbard AT, editor. Encyclopedia of surface and colloid science
volume 3. New York: Marcel Dekker. p 2997-3017.
Shinoda K, Araki M, Sadaghiani A, Khan A, Lindman B. 1991. Lecithin-based microemulsions:
phase behavior and microstructure. J Phys Chem 95(2):989-993.
Singh R, Heldman D. 2009. Fluid flow in food processing. In: Singh RP, Heldman DR, editors.
Introduction to food engineering. 4th ed. Burlington, MA: Elsevier. p 73-78.
Sumonsiri N, Barringer S. 2011. Food industry electrostatic powder coating. In: Bertrand CL,
editor. Electrostatics: theory and applications. 1st ed. Nova Science Publishers, Inc. p 159-170.
68
Suriyaphan O, Cadwallader K, Drake M. 2001. Lecithin associated off-aromas in fermented milk.
J Food Sci 66(4):517-523.
Suriyaphan O, Drake M, Cadwallader K. 1999. Identification of volatile off-flavors in reduced-fat
cheddar cheeses containing lecithin. LWT Food Sci Technol 32(5):250-254.
Syngenta. 1996. Oil sensitive paper CF1 for monitoring spray distribution. Basle, Switzerland
Publication No: 11783 /1e
Szuhaj B. 1983. Lecithin production and utilization. J Am Oil Chem Soc 60(2):306-309.
Tan C, Che Man Y, Selamat J, Yusoff M. 2002. Comparative studies of oxidative stability of edible
oils by differential scanning calorimetry and oxidative stability index methods. Food Chem
76(3):385-389.
Tang K, Gomez A. 1996. Monodisperse electrosprays of low electric conductivity liquids in the
cone-jet mode. J Colloid Interface Sci 184(2):500-511.
Tekin A, Hammond E. 1998. Factors affecting the electrical resistivity of soybean oil. J Am Oil
Chem Soc 75(6):737-740.
Tro N. 2011. Chemistry : A molecular approach. 2nd ed. Upper Saddle River, NJ: Pearson
Prentice Hall. 1227 p.
Velasco J, Dobarganes C. 2002. Oxidative stability of virgin olive oil. Eur J Lipid Sci Technol 104(910):661-676.
Viswanath D, Ghosh T, Prasad D, Dutt N, Rani K. 2007. Viscosity of liquids: Theory, estimation,
experiment, and data. The Netherlands: Springer. 660 p.
Watanabe H, Matsuyama T, Yamamoto H. 2003. Experimental study on electrostatic atomization
of highly viscous liquids. J Electrostat 57(2):183-197.
Weyland M, Hartel R. 2008. Emulsifiers in confectionary. In: Hasenhuettl GL , Hartel RW, editors.
Food emulsifiers and their applications. 2nd ed. New York: Springer. p 288-289.
Wilkerson J, Gaultney L. 1989. Electrostatic atomization of vegetable oil for pesticide spraying.
ASAE paper no 891524
Wilson J. 1982. A linear source of electrostatically charged spray. J Agric Eng Res 27(4):355-362.
Wohlfarth C. 2008. Surface tension of pure liquids and binary liquid mixtures. New York:
Springer. 96 p.
69
Yarin A, Koombhongse S, Reneker D. 2001. Taylor cone and jetting from liquid droplets in
electrospinning of nanofibers. J Appl Phys 90(9):4836-4846.
Zhu H, Salyani M, Fox R. 2011. A portable scanning system for evaluation of spray deposit
distribution. Comput Electron Agr 76(1):38-43.
70
APPENDIX: TABLES
Flow characteristics of soybean oil and soybean oil – lecithin mixture
71
72
Shear
stress
Pa
78.00
61.96
49.21
39.09
31.05
24.67
19.59
15.56
12.36
9.82
7.80
6.20
5.54
4.92
3.91
3.11
2.47
1.96
1.56
1.24
0.98
0.78
0.62
0.50
4⁰C 0% Lecithin
Shear
rate
Viscosity
1/s
Pa.s
869.80
0.08983
690.53
0.08987
549.93
0.08963
435.77
0.08988
345.83
0.08996
275.77
0.08961
219.20
0.08954
174.00
0.08962
138.63
0.08938
110.10
0.08935
86.68
0.09018
68.89
0.09007
61.66
0.08992
54.94
0.08978
43.99
0.08904
34.82
0.08941
27.76
0.08906
22.10
0.08882
17.55
0.08886
13.95
0.08879
11.16
0.08821
8.84
0.08841
7.00
0.08867
5.68
0.08829
Shear
stress
Pa
78.00
61.96
49.21
39.09
31.05
24.67
19.59
15.56
12.36
9.82
7.80
6.33
6.20
4.92
3.91
3.11
2.47
1.96
1.56
1.24
0.98
0.78
0.62
0.50
4⁰C 5% Lecithin
Shear
rate
Viscosity
1/s
Pa.s
776.70
0.10048
618.43
0.10025
490.93
0.10027
388.40
0.10070
305.73
0.10163
243.50
0.10133
192.40
0.10191
153.13
0.10168
120.93
0.10230
96.48
0.10185
76.21
0.10242
61.66
0.10273
60.34
0.10275
47.73
0.10317
37.67
0.10380
29.61
0.10490
23.37
0.10557
18.22
0.10757
14.21
0.10957
11.05
0.11190
8.48
0.11593
6.47
0.12113
4.90
0.12753
3.77
0.13370
4⁰C 10% Lecithin
Shear
Shear
stress
rate
Viscosity
Pa
1/s
Pa.s
78.00
714.53
0.10943
61.96
568.27
0.10923
49.21
450.07
0.10963
39.09
355.20
0.11043
31.05
280.40
0.11110
24.67
220.73
0.11207
19.59
175.17
0.11220
15.56
137.60
0.11347
12.36
108.34
0.11457
9.82
84.73
0.11650
7.80
66.49
0.11787
7.26
61.66
0.11831
6.20
52.20
0.11917
4.92
40.80
0.12113
3.91
31.75
0.12377
3.11
24.40
0.12803
2.47
19.04
0.13040
1.96
14.80
0.13320
1.56
11.38
0.13813
1.24
8.67
0.14507
0.98
6.77
0.14697
0.78
5.15
0.15450
0.62
3.87
0.16440
0.50
2.88
0.18263
65
4⁰C 15% Lecithin
Shear
Shear
stress
rate
Viscosity
Pa
1/s
Pa.s
78.00
641.87
0.12157
61.96
509.93
0.12153
49.21
403.90
0.12190
39.09
317.83
0.12303
31.05
250.47
0.12400
24.67
198.03
0.12460
19.59
155.83
0.12573
15.56
122.30
0.12727
12.36
96.69
0.12783
9.82
76.06
0.12913
8.04
61.66
0.13040
7.80
59.75
0.13057
6.20
46.85
0.13227
4.92
36.71
0.13407
3.91
28.64
0.13650
3.11
21.95
0.14160
2.47
16.94
0.14563
1.96
12.67
0.15480
1.56
9.07
0.17217
1.24
5.97
0.21393
0.98
3.70
0.27763
0.78
2.77
0.28233
0.62
2.05
0.32927
0.50
1.22
0.43140
22⁰C 0% Lecithin
73
Shear
stress
Pa
78.00
75.72
60.14
47.77
37.95
30.14
23.94
19.02
15.11
12.00
9.53
7.57
6.01
4.78
3.80
3.49
3.01
2.39
1.90
1.51
1.20
0.95
0.76
0.60
0.50
Shear
rate
1/s
Viscosity
Pa.s
1385.62
1345.33
1062.33
849.17
670.60
533.90
424.93
336.20
266.23
211.83
168.67
134.40
106.07
84.55
67.12
61.66
53.17
42.29
33.62
26.59
21.13
16.73
13.36
10.51
8.76
0.05635
0.05634
0.05664
0.05629
0.05660
0.05649
0.05637
0.05659
0.05677
0.05667
0.05653
0.05635
0.05670
0.05652
0.05654
0.05661
0.05671
0.05662
0.05657
0.05684
0.05682
0.05698
0.05670
0.05728
0.05708
22⁰C 5% Lecithin
Shear
stress
Pa
78.00
75.72
60.14
47.77
37.95
30.14
23.94
19.02
15.11
12.00
9.53
7.57
6.01
4.78
4.24
3.80
3.01
2.39
1.90
1.51
1.20
0.95
0.76
0.60
0.50
Shear
rate
1/s
Viscosity
Pa.s
1234.55
1198.33
952.77
749.53
594.50
470.90
372.70
293.53
230.73
182.97
144.50
114.00
89.42
70.28
61.66
54.48
42.66
32.85
25.21
19.11
14.19
10.53
7.52
5.11
3.56
0.06321
0.06322
0.06316
0.06377
0.06387
0.06404
0.06426
0.06483
0.06550
0.06562
0.06599
0.06645
0.06728
0.06804
0.06895
0.06972
0.07075
0.07298
0.07570
0.07952
0.08556
0.09193
0.10382
0.12608
0.15702
22⁰C 10% Lecithin
Shear
stress
Pa
78.00
75.72
60.14
47.77
37.95
30.14
23.94
19.02
15.11
12.00
9.53
7.57
6.01
4.78
4.57
3.80
3.01
2.39
1.90
1.51
1.20
0.95
0.76
0.60
0.50
66
Shear
rate
1/s
Viscosity
Pa.s
1156.29
1121.67
890.50
710.47
559.23
445.00
351.97
279.33
219.73
172.90
136.20
107.07
83.60
64.79
61.66
50.26
38.55
29.06
21.69
15.93
11.33
7.80
5.16
3.10
1.64
0.06746
0.06750
0.06754
0.06725
0.06786
0.06775
0.06803
0.06810
0.06875
0.06940
0.06999
0.07071
0.07195
0.07375
0.07413
0.07552
0.07820
0.08240
0.08773
0.09488
0.10600
0.12247
0.14683
0.19450
0.30657
22⁰C 15% Lecithin
Shear
stress
Pa
78.00
75.72
60.14
47.77
37.95
30.14
23.94
19.02
15.11
12.00
9.53
7.57
6.01
4.88
4.78
3.80
3.01
2.39
1.90
1.51
1.20
0.95
0.76
0.60
0.50
Shear
rate
1/s
Viscosity
Pa.s
1110.49
1078.00
849.40
673.13
530.40
418.37
331.47
261.77
206.53
163.33
128.60
100.19
78.24
61.66
60.18
45.84
35.01
26.12
19.39
14.01
10.15
7.12
4.81
3.06
1.86
0.07036
0.07036
0.07094
0.07114
0.07166
0.07220
0.07236
0.07280
0.07332
0.07368
0.07433
0.07586
0.07704
0.07936
0.07957
0.08291
0.08617
0.09171
0.09809
0.10787
0.11823
0.13380
0.15763
0.19683
0.26910
74
47⁰C 0% Lecithin
Shear
Shear
stress
rate
Viscosity
Pa
1/s
Pa.s
78.00
2582.67
0.03047
61.96
2357.67
0.02631
49.21
1889.00
0.02609
39.09
1498.67
0.02611
31.05
1196.33
0.02598
24.67
950.83
0.02598
19.59
753.90
0.02603
15.56
603.33
0.02583
12.36
476.60
0.02598
9.82
378.83
0.02595
7.80
301.27
0.02592
6.20
239.93
0.02585
4.92
190.90
0.02582
3.91
151.53
0.02584
3.11
120.30
0.02583
2.47
95.67
0.02581
1.96
75.82
0.02587
1.59
61.66
0.02575
1.56
60.53
0.02574
1.24
47.93
0.02583
0.98
38.24
0.02572
0.78
30.17
0.02591
0.62
24.18
0.02567
0.50
19.47
0.02573
47⁰C 5% Lecithin
Shear
Shear
stress
rate
Viscosity
Pa
1/s
Pa.s
78.00
2740.67
0.02846
61.96
2206.00
0.02809
49.21
1743.67
0.02823
39.09
1397.67
0.02797
31.05
1104.33
0.02812
24.67
882.13
0.02796
19.59
698.10
0.02807
15.56
558.53
0.02786
12.36
442.03
0.02797
9.82
352.13
0.02789
7.80
281.33
0.02773
6.20
221.17
0.02801
4.92
176.43
0.02790
3.91
140.37
0.02785
3.11
110.70
0.02807
2.47
88.56
0.02785
1.96
70.58
0.02776
1.71
61.66
0.02775
1.56
56.09
0.02775
1.24
44.14
0.02801
0.98
35.29
0.02782
0.78
28.38
0.02750
0.62
22.58
0.02744
0.50
18.17
0.02753
47⁰C 10% Lecithin
Shear
Shear
stress
rate
Viscosity
Pa
1/s
Pa.s
78.00
2692.67
0.02898
61.96
2162.67
0.02866
49.21
1716.00
0.02870
39.09
1379.00
0.02836
31.05
1089.33
0.02853
24.67
866.30
0.02848
19.59
685.73
0.02858
15.56
550.63
0.02828
12.36
436.93
0.02831
9.82
345.63
0.02842
7.80
274.30
0.02846
6.20
218.57
0.02836
4.92
173.33
0.02841
3.91
138.80
0.02817
3.11
110.23
0.02819
2.47
87.60
0.02817
1.96
69.42
0.02824
1.74
61.66
0.02822
1.56
55.21
0.02820
1.24
43.70
0.02831
0.98
34.77
0.02825
0.78
27.56
0.02831
0.62
21.96
0.02823
0.50
17.76
0.02817
67
47⁰C 15% Lecithin
Shear
Shear
stress
rate
Viscosity
Pa
1/s
Pa.s
78.00
2569.33
0.03036
61.96
2053.33
0.03018
49.21
1637.00
0.03007
39.09
1298.67
0.03010
31.05
1039.67
0.02987
24.67
826.10
0.02986
19.59
655.07
0.02991
15.56
520.23
0.02992
12.36
413.93
0.02987
9.82
329.00
0.02984
7.80
260.20
0.02998
6.20
207.67
0.02984
4.92
165.13
0.02980
3.91
130.40
0.02998
3.11
104.07
0.02984
2.47
82.56
0.02988
1.96
65.83
0.02977
1.84
61.66
0.02987
1.56
51.72
0.03009
1.24
41.20
0.03001
0.98
32.98
0.02978
0.78
25.98
0.03003
0.62
20.67
0.02997
0.50
16.60
0.03011