Surface tension of water with various kinds of salt
under microwave radiation
Yushin Kanazawa1, Harisinh Parmar2, Yusuke Asakuma1, Vishnu Pareek2, Chi M. Phan2 and Geoffrey Evans3
1
Department of Mechanical and Systems Engineering
University of Hyogo, Himeji, Hyogo, Japan
Email: [email protected]
2
Centre for Process Systems Computations, Curtin University, Perth, WA, Australia
Email: [email protected]
3
Department of Chemical Engineering, University of Newcastle, Callaghan NSW, Australia
Abstract— Surface tension of any fluid is crucial for multiphase
systems and is often manipulated during industrial processes by
introducing surfactants. Normally, water used in an industry
includes impurity such as salt. In this study, effect of salt kinds
and concentrations on surface tension of water was investigated
under microwave irradiation. It has found that surface tension
decreased quickly, while temperature increased during
microwave. Once the radiation is turned off, the temperature
returns rapidly as expected. However, surface tension recovery
was much slower than temperature. Moreover, the recovery and
reduction amounts by non-thermal effect depend on salt kinds
and concentrations.
Keywords-component; Microwave; surface tension; nonthermal effect
I.
INTRODUCTION
Application of microwave irradiation for chemical
processes, such as emulsification and polymerization, has been
reported [1,2]. Surfactant free emulsion can be produced with
the help of microwave irradiation. Surface tension is an
important property for the industrial process such as
foaming/defoaming,
wetting/dewetting
and
flotation.
Similarly, the interfacial tension plays crucial role in
separation and mixing process of two immiscible liquids,
which are important unit operations of the fundamental
chemical engineering. In practice, surface and interfacial
tensions are often altered by introducing surfactants.
In our previous research [3], specific property for surface
tension of water droplet under microwave radiation was found.
For example, lower surface tension after the radiation was
measured. The formation of nano-bubble will explain this
behavior. However, the exact mechanism is not clear. This
study investigates the influence of microwave on surface
tension of water with different kinds of salt.
II.
EXPERIMENTAL
Figure 1 is the schematic diagram of the microwave reactor
(designed and build by Shikoku Instrumentation Co., Inc.)
from side view and top view respectively. Pendant method
was employed for the measurement of surface tension of
water. Since metal is not microwave friendly, Teflon pipe with
the dimension of 1 mm inside and 2 mm outside diameter (and
1.4 cm long) was chosen for the experiments. Droplet with salt
was produced via injection syringe with exactly 20 micro liter
of distilled water. Temperature was measured with the help of
Temperature measurement equipment, designed by Anritsu
meter Co., LTD (device model: FL-2000 Optical fiber: FS100M), which was inserted from the top of the reactor to check
the temperature inside the droplet during microwave
irradiation. Two types of experiment for different
concentrations and kinds are conducted and the experimental
condition of salt is listed in Table 1.
Care was taken while inserting the temperature probe inside
the droplet to ensure uniform shape of the droplet for all
experiments. Light source was used from one end of the
microwave reactor and high speed camera (Sigma Koki Co.,
LTD Model SK-TC202USB-AT) was employed from the other
end to capture the shape of the droplet. Experiments analysis
was divided in three parts.
1. Initial capturing of the image before microwave
irradiation to measure initial surface tension of the
water droplet with salt.
2. Capturing of the images during microwave
irradiation. 600 w was used as microwave power.
The microwave was turned off at 120 seconds for
all experiments.
3. Capturing of the images after microwave
irradiation for 180 sec.
Axisymmetric drop shape analysis (ADSA) in Fig. 2 was
employed to measure initial surface tension of the droplet and
droplet images captured during and after microwave
irradiation [4,5].
Table 1 Experimental conditions
No.
Salt
Salt conc.
[mol/L]
1
NaCl
0
0.01
0.025
0.1
1
2
NaCl
0.1
KCl
MgCl2
NH4Cl
NaBr
Power
[w]
600
Radiation time
[sec.]
120
600
120
(a) Drop profile
(b) Smoothed edges
Figure 2 ADSA method f or final surface tension measurement
III.
(a) Side view
(b) 3D view
Figure 1. Experimental apparatus
RESULTS AND DISCUSSIONS
A. Effect of salt concentration
Figure 3 is surface tension of water droplet of NaCl
solution as a function of time and temperature. Surface tension
of water droplet with different concentration of NaCl was
plotted. During the microwave radiation, the temperature
increased at expected. The surface tension decreased
significantly as well [6,7]. The changes are proportional to
NaCl concentration. Once the microwave is turned off (at 120
s), the temperature dropped quickly to the room temperature,
whereas the surface tension did not recover so much. When
NaCl concentration is higher, surface tension can retain at the
lowest value after the radiation.
B. Effect of salt kinds
Figure 4 is surface tension of droplet with different salts as
a function of time and temperature. Surface tension of water
droplet with different salt was plotted. When microwave is
turned off, surface tension can retain the lowest value. The
reduction amount depends on salt kinds. The results indicated
that the surface tension reduction is not due to thermal effect
solely. The data after microwave radiation clearly
demonstrated the non-thermal behavior of surface tension.
However, it can not be explained by properties such as ion
strength, size and electrical charge.
Pure water MW
0.01mol/L MW
0.025mol/L MW
0.1mol/L MW
1mol/L MW
Pure water after MW
0.01mol/L after MW
0.025mol/L afterMW
0.1mol/L after MW
1 mol/L after MW
80
80
NaCl after MW
KCl after MW
MgCl2 after MW
NH4Cl after MW
NaBr after MW
75
Surface tention (mN/m)
75
Surface tention (mN/m)
NaCl MW
KCl MW
MgCl2 MW
NH4Cl MW
NaBr MW
70
65
60
55
50
45
70
65
60
55
50
45
40
0
50
100
150
40
200
0
50
100
Time (s)
150
200
Time (s)
(a) Time
(a) Time
Pure water MW
0.01mol/L MW
0.025mol/L MW
0.1mol/L MW
1mol/L MW
Pure water ref.
80
pure water after MW
0.01mol/L after MW
0.025mol/L after MW
0.1mol/L after MW
1mol/L after MW
NaCl MW
KCl MW
MgCl2 MW
NH4Cl MW
NaBr MW
80
NaCl after MW
KCl after MW
MgCl2 after MW
NH4Cl after MW
NaBr after MW
75
Surface tention (mN/m)
Surface tension (mN/m)
75
70
65
60
55
50
70
65
60
55
50
45
45
40
40
0
20
40
60
80
100
120
0
Temperature (oC)
(b) Temperature
Figure 3. Surface tension of NaCl solution droplet as a function of time
and temperature (solid symbols: during MW, unfilled symbols: after MW)
20
40
60
80
100
120
Temperature (oC)
(b) Temperature
Figure 4. Surface tension of water droplet with different salts as a function
of time and temperature(solid symbols: during MW, unfilled symbols: after
MW)
The data clearly indicated a non-thermal effect of
microwave of surface tension of water. It should be noted that
the reduction of surface tension occurs in case of higher
concentration of salts. It’s also noteworthy that microwave can
only penetrate 1.4 cm deep into water and surface water should
be influenced strongly by microwave. Accordingly, it can be
explained by the formation of nano-bubbles inside the water
droplet due to homogeneous heating [8-10]. Nano-bubble is
normally produced by high shear flow, ultrasonic because
network of water molecule is organized at molecular level. It is
similarly caused by microwave because water molecule is
strongly rotated as well. It might reduce surface tension. It is
kinds of ”impurity effect” and generally surface tension is
influenced for impurity such as surfactant, alcohol and salt etc.
These nano-bubbles are negatively charged and metal ion is
adsorbed on the surface of nano-bubble. Consequently, nanobubble is more stable and the surface tension after the radiation
remains the lower value for longer time. Moreover, water,
which include metal ion, absorbs microwave easily because of
the higher electric constant. Microwave effect becomes larger
in the case of electrolyte aqueous solution.
CONCLUSIONS
The surface tension and temperature of water droplet with
different salt was measured directly under microwave
irradiation. It has found that surface tension decreased quickly
during microwave. However, surface tension recovery was
much slower than temperature after radiation is off. For the
highest concentration of NaCl, the surface tension remains at
lower values. It’s conclusive that the surface tension reduction
is not due to thermal heating and water with different salts
shows different thermal effect. The results shown that retains
some “memory” after microwave, which is might be caused by
nano-bubble with negative charge. Moreover, water with salt
was changed structurally at molecular level during the
microwave and cation will be adsorbed on the surface of nanobubble. Accordingly, this ‘impurity effect’ becomes more
stable and the surface tension can keep lower values for longer
time. This presents a better physical method to manipulate the
interface layer of water.
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