Chemical Engineering January 2015 - 38

Cover Story
Sharp (lower convexity)
particles interact more at
lower deformations
Under shear
Smooth particles can
flow easily over each
other under these low
deformations
At rest
Shear rate
Shear rate
Figure 4. Sharper particles (those with lower convexity) give rise
to suspensions with higher viscosity, all other factors being equal
ity of a suspension can often be
reduced by using smoother, more
regular particles. Although once
again, any impact on viscosity is
usually more marked at low shear
rates and high volume fractions.
At low shear forces, elongated
particles produce higher viscosity
than spherical ones, although
the converse is true at high shear,
where elongated particles are associated
with lower viscosity than
spherical analogs.
Elongated particles produce a
different suspension viscosity profile
than spherical analogs (Figure
5). Particle-particle interactions between
spherical particles typically
break down as shear increases, and
this gives rise to shear-thinning
behavior. Elongated particles are
generally randomly oriented at low
shear, thus occupying larger volumes,
but at high shear, will tend
to orient in way that is parallel to
the direction of flow, resulting in
more efficient packing. As a result,
suspensions with elongated particles
also shear-thin, but far more
markedly that those containing
spherical particles.
At low shear forces, a suspension
that contains particles with elongated
shapes typically has higher
viscosity than one that contains
spherical particles of equivalent
size. At high shear, however, this
situation is reversed, and the suspension
with spherical particles
will have higher viscosity than
those with elongated particles of
equivalent size.
Figure 5. As a result of orientation at high shear rates,
the viscosity of suspensions with elongated particles has
a different profile compared to that of a suspension containing
spherical particles of equivalent size
Electrical charge zeta potential
The preceding analysis of how particle
size and shape impact viscosity
has shown that these physical
characteristics influence how easily
particles within a suspension can
move, relative to one another. One
further parameter that can also
influence particle movement, and
consequently viscosity and stability,
is the electrical charge on particles
within a system. One way of measuring
this charge is to quantify
zeta potential.
Zeta potential quantifies the
magnitude of the electrostatic repulsion
or attraction in a system.
It is a measure of particle charge at
the edge of the slipping plane between
the particle and associated
double layer, and the surrounding
solvent (Figure 6). The fact that
zeta potential is not the charge
on the surface of the particle, but
rather at the edge of the particle's
" sphere of influence, " makes it
highly relevant to suspension behavior.
If a suspension has a large
negative or positive zeta potential,
then the particles within it will
tend to successfully repel each
other. Low zeta-potential values
increase the likelihood of flocculation
or agglomeration (the joining
together of discrete particles).
In sub-micron colloidal systems,
particles tend to move under Brownian
motion and the influence of
gravity is low because particle mass
is small. Here, increasing the zeta
potential causes particles to repel
one another, inhibiting movement
and increasing viscosity, especially
at low shear rates (see Figure 7),
when the particles exhibit a larger
effective hydrodynamic volume. A
zeta potential of sufficient magnitude
(around ±30 mV) will maintain
particle separation. In these
systems, gravitational forces only
become large enough to cause sedimentation
if the particles grow in
mass by a process of agglomeration.
Keeping particles separate therefore
ensures stability.
In a suspension with larger particles,
zeta potential may be controlled
in order to deliver a quite different
effect. For particles of significant
mass, gravitational forces are sufficient
to induce sedimentation. Here,
engineering a high zeta potential to
maintain the particles as discrete
entities does not produce a stable
suspension. Counterintuitively, the
opposite can work. In certain systems
reducing the zeta potential induces
partial agglomeration of the
particles, creating a networked gel
that can give appreciable viscosity
to a suspension. Reduction in zeta
potential also induces an apparent
yield stress in these cases.
Rheology measurement
Clearly, a number of strategies can
be used to refine rheological properties
to meet performance targets
for a specific suspension. However,
the driver for such formulation optimization
is analytical data. Laser
diffraction and dynamic light scattering
(DLS) are well-established
techniques for measuring particle
ChemiCal engineering www.Chemengonline.Com JanUarY 2015 37
Viscosity
Viscosity
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Chemical Engineering January 2015

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