S
As appeared in October 2015 PBE
ome very small interparticle
forces can create some very big
powder and bulk solids
headaches. These forces can cause high
pressure drops and plug our pneumatic
conveying lines.1 When polyolefin
(polyethylene and polypropylene)
powders collect into sheets and crash
down into our fluidized bed reactors, or
agglomerates and caking make our
granulation and tabulation more
difficult, or cohesion inhibits hopper
emptying, these forces are at work.
Yet, these interparticle forces aren’t
always evil. They’re useful when we
need to powder coat our automobiles
or deposit toner on our printed copies.
Indeed, these forces are even
responsible for the early stages of
planet formation. In this column, we’ll
take a look at the nature and constraints
of interparticle forces to better
understand how they can plague or
enhance your process. [Editor’s note:
For information on obtaining previous
“Particle Professor” columns, see “For
further reading” later in this article.]
The most common types of
interparticle forces in particle
technology are: van der Waals,
coulombic, and capillary forces.
These forces are very different from
each other in terms of strength, range
of interaction, and sensitivity to the
composition material.
Van der Waals forces
Named after the Dutch scientist
Johannes Diderik van der Waals, this
force was at one time attributed to any
repulsive or attractive force that
couldn’t be explained by covalent
bonds, hydrogen bonds, coulombic
forces, or capillary forces. Later, this
definition was more formalized as the
force between two permanent,
instantaneous or induced dipoles
(including Keesom, Debye, and
London forces). The van der Waals
force is weaker than covalent bonding,
has no directional dependency, always
exists, and can’t be saturated or
diminished.2
A gecko’s toes provide a good
example of the van der Waals force. A
Copyright CSC Publishing
www.powderbulk.com
Ray Cocco
Interparticle forces in
powder technology
g ec k o c a n c l i m b s m o o t h g l a s s
surfaces because its toes have a large
surface area to maximize the contact
between the gecko and the glass
surface.3 Originally, capillary forces
were thought to be at play here, but
that hypothesis was discounted a few
years ago.4 The large surface area of
the gecko’s toes provide abundant
local curvatures that provide the
induced dipoles needed for the van
der Waals force.
The van der Waals force’s attractive
or repulsive range is very short, just a
couple nanometers for particles.5 As a
result, particle-to-particle attraction
due to the van der Waals force is only
possible when the particles are in
close proximity to each other.
Furthermore, the strength of this
interaction is relatively weak for
particle systems, so particles larger
than 100 microns are more likely to
be controlled by gravitational or
hydrodynamic forces than by van der
Waals forces.6 Even particles around
10 microns in size tend not to exhibit
strong van der Waals forces. This is
because gravitational force is
proportional to the cube of the particle
diameter, and van der Waals forces
are linear with the particle diameter.
N a n o p a r t i c l e s , h o w e v e r, a r e
susceptible to van der Waals forces on a
level that could impact your process.
These forces initiate clustering in
carbon black and fume silica. As a
result, both materials tend to be easy to
fluidize, convey, and classify even
though the particles themselves are
small and cohesive. With van der Waals
forces, these small particles cluster
together without creating agglomerates
or sinter into a more manageable size. If
van der Waals forces weren’t limited to
a very short range, these materials
would cake instead of forming
manageable particles.
Managing van der Waals forces can be
challenging. They’re omnidirectional,
can’t be saturated or diminished, and
always exist. Particle properties,
rather than the environment, need to
be managed, and changing the particle
size or surface morphology is a good
first step.
Coulombic forces
Coulombic (or electrostatic) forces
involve the exchange of electrons or
ions from one surface to another. For
small particles in close proximity, van
der Waals forces can be stronger than
c o u l o m b i c f o r c e s . H o w e v e r,
coulombic forces are much more farreaching so are more prevalent and
problematic than van der Waals
forces.
We’ve all experienced coulombic
forces in action. Polyethylene or
polypropylene pellets are difficult to
manage because of their electrostatic
forces. Opening and placing a trash
bag in a trash bin demonstrates this
difficulty. Electrostatic precipitators
use these same coulombic forces to
collect fine particulate over a
relatively large distance.
The nature of coulombic forces
makes sense when considering the
interaction of two different materials.
Rub a rubber rod along fur and the rod
becomes more positively charged
while the fur becomes more
negatively charged. This is because
one material is more of an insulator
than the other. Yet, why does this
happen when considering just one
material? In other words, why is a bed
of pellets of the same material subject
to coulombic forces?
Some hypotheses have stated that this
behavior is due to the combination of
asymmetry between two contacting
surfaces, which results in the
tunneling of trapped electrons.7 The
surface with more curvature can trap
more electrons. So in a bed of particles
of the same material, particle
collisions will result in smaller
particles capturing electrons and
becoming more negatively charged,
while larger particles will become
more positively charged because of
electron depletion.8 Particles of unlike
charges are now attracting each other,
leading to particle cluster formation
and possible agglomeration. After
such particle collisions, particles with
no previous attraction to equipment
may now have an affinity to stick to
the walls.
Surface morphology plays an
important role here. If particles are
rough enough and have lots of surface
curvatures, the charge disparity
remains on the particles, and the
electrons are less likely to move to the
smaller particles. This was
demonstrated and explained by
Professor Heinrich Jaegar and a
group of researchers at the University
of Chicago in the journal Nature.9 The
group performed powder drop
experiments and found that similarly
sized glass beads formed clusters
while copper ones didn’t. Artificially
roughing the surface of the glass
beads with fumed silica resulted in the
glass beads no longer clustering.
The group also found that electron
charge transfer alone doesn’t always
explain the attractive and repulsive
behavior in particle systems. The
exchange of ions also can be a
Copyright CSC Publishing
significant factor.8 The most common
ions are hydroxyl groups on particle
surfaces, usually due to the presence
of water. Water can quickly coat a
particle’s entire surface as a monolayer
(or a one-molecule-thick layer) of
hydroxyl groups and should be
considered to be everywhere, so
coulombic forces can happen in the
most unexpected places.
U n l i k e v a n d e r Wa a l s f o r c e s ,
coulombic forces can be managed
through the environment. Adding
antistatics can often help reduce
particle clustering, agglomeration, and
buildup. Quaternary salts such as some
ammonia, amides, phosphate esters, or
ethoxylated amine salts provide a
conductive tunnel between particles to
dissipate electron buildup on smaller
particles. Grounding also may help but
usually only for smaller systems.
Adding small nanoparticles such as
carbon black or fumed silica may work
well for your application. As
previously noted, these materials
artificially roughen particle surfaces.
Humidification also works, provided
the correct amount is used. Too little
humidification may just add more
hydroxyl groups, depending on the
particles, and too much humidification
may cause capillary forces to take hold.
Capillary forces
Capillary forces are those where
surface tension and bonding, or both,
(hydrogen bonding for instance),
result in particle-to-particle attraction.
Unlike van der Waals and coulombic
forces, capillary forces are only
attractive forces. This is the attractive
force that allows liquid to move
against gravity up a small straw or
wet a surface above the liquid layer
the surface is immersed in. Trees and
plants use capillary forces to move
water from the ground up to the
leaves.
Capillary forces can be significant
and stronger than van der Waals or
coulombic forces. However, capillary
forces aren’t as far-reaching as
coulombic forces, and particles
typically need to be within close
proximity — if not touching —
before capillary forces take hold.
Traditionally, capillary forces are
thought of as liquid bridging between
two surfaces. That’s certainly the case
when caking is an issue. However,
capillary forces can happen with a lot
less liquid than what’s needed for
bridging. Water molecules only a
monolayer thick on a surface can be
the attractive conduit between two
particles through hydrogen bonding.
Recent work from MIT suggests that
an additional intermolecular force
may be at play with liquid–solid
interfaces10. This additional force,
although miniscule, can affect the
hydrodynamics when a large number
of liquid–solid interfaces are
involved.
Capillary forces operate on a larger
s c a l e t h a n v a n d e r Wa a l s a n d
coulombic forces and can be adjusted
by the amount of available “liquid”
(including monolayers) and that
liquid’s properties, such as surface
tension and viscosity. Additives that
decrease the surface tension or
viscosity could reduce caking or
agglomeration. Such additives for a
wet particulate system could be an
alcohol or a surfactant. For ionic
liquids the pH adjustment could have
an impact.
Solutions
If interparticle forces are causing too
much or not enough clustering,
agglomeration, sintering, sheeting, or
caking in your process, understanding
the force, or forces, responsible for
such behavior may help you find a
solution. However, this isn’t always
easy. More often than not one must
resort to trial-and-error. Using the
scientific method, try the following
experiments to see what’s best for
managing your interparticle forces:
• Add smaller particles of the same
material,
• Add nanoparticles of a different
material such as carbon black or
fumed silica,
• Narrow or broaden the particle size
distribution.
• Roughen particle surfaces using a
chemical etching agent.
• Add an antistatic agent such as a
quaternary salt.
• Try humidification. (Relative
humidities of 40 to 60 percent are
typical.)
• Add a surfactant or polar solvent or
change the material’s pH.
PBE
For further reading
Find more information on this topic in
articles listed under “Particle
analysis” in Powder and Bulk
Engineering’s article index in the
December 2014 issue and in the
Article Archive on PBE’s website.
(All articles in the archive are
available for free download to
registered users.)
References
1. P. Bunchatheeravate, J. Curtis, Y. Fujii, S.
Matsusaka, “Prediction of particle charging
in a dilute pneumatic conveying system,”
AIChE Journal. 59 (2013) pages 2308–2316.
2. Van Oss, C.J.; Absolom, D.R.; Neumann,
A.W. (1980). “Applications of net repulsive
van der Waals forces between different
particles, macromolecules, or biological
cells in liquids.” Colloids and Surfaces 1
(1): pages 45–56.
3. Russell, A. P.; Higham, T. E. (2009). “A
new angle on clinging in geckos: incline,
not substrate, triggers the deployment of the
adhesive system.” Proceedings of the Royal
Society B: Biological Sciences 276 (1673):
pages 3705–3709.
4. Chen, B.; Gao, H. (2010). “An alternative
explanation of the effect of humidity in
gecko adhesion: stiffness reduction
enhances adhesion on a rough surface.” Int
J Appl Mech 2: pages 1–9.
5. Visser, J., “Van der Waals and Other
Cohesive Forces Affecting Powder
Fluidization,” Powder Tech., (1998) 58,
pages 1-10.
6. Seville, J.P.K., Willett, C.D., Knight, P.C.
“Interparticle forces in fluidization: a
review,” Powder Tech., (2000) pages 1–8.
7. Lowell, J., Truscott, W., J. Phys. D., Appl.
Phys. (1986) 19, 1273.
8. Waitukaitis, S.R., V. Lee, J.M. Pierson, S.L.
Forman, H.M. Jaeger, “Size-Dependent
Same-Material Tribocharging in Insulating
Grains,” Physical Review Letters. page 112
(2014).
9. Royer, J.R., D.J. Evans, L. Oyarte, Q. Guo,
E. Kapit, M.E. Möbius, et al., “High-speed
tracking of rupture and clustering in freely
falling granular streams,” Nature. 459
(2009) pages 1110–1113.
10. http://www.techtimes.com/articles/67775
/20150712/physicists-unravel-mechanicsof-why-puddles-dont-spread.htm
Copyright CSC Publishing
Ray Cocco is president of Particulate
Solid Research Inc. (773-523-7227,
[email protected]) and
holds a PhD in chemical engineering
from Auburn University in Auburn,
Ala. He has more than 20 years
experience in particle technology,
holds several patents, and has
published numerous technical
articles on particle technology topics.
Particulate Solid Research Inc.
Chicago, IL
www.psrichicago.com