control rheology by changing particle properties like size zeta potential

Inform is a series of white papers designed to provide advice on material characterization issues

10 ways to...Control rheology by changing particle properties (Size, Zeta potential and Shape)

MRK1236-02_Ten ways to Control Rheology by changing particle.indd 1 08/01/2010 11:01:38

Many materials in use today are disperse systems where one substance

(often particulate) is dispersed in another phase. These material types

include adhesives, agrochemicals, cement, ceramics, colloids, cosmetics

and personal care formulations, food and drink, mining and mineral

slurries, paints, inks and surface coatings, pharmaceuticals and polymer

systems.

For example:-

In the inks industry, the understanding of rheology and particle properties

allows solid pigment content to be changed in different formulations whilst

maintaining the critical rheological characteristics required for optimised

printing.

In the cement industry, the understanding of rheology and particle

properties, such as the aggregate morphology, allows fl ow behaviour

during processing and application to be controlled.

In the Cosmetics and Personal Care industries, it is essential to understand

the relationship between rheology and particle properties to provide the

optimum balance in terms of formulation, consumer acceptance and

application performance.

The physical properties of the dispersed particles, such as the average

particle size, the size distribution, the zeta potential or charge on the

particles and even the shape of the particles all help infl uence the overall

(bulk) materials properties such as the rheology.

This “Ten Ways” guides you through some of the fundamental properties of

the dispersed system, and demonstrates how these affect the rheological

properties. Whereas it in interesting to understand the bulk material

properties, such as rheology, which are associated with the changes

in the particle size, shape and zeta potential, these examples will also

demonstrate that this understanding allows the rheology of the material to

be controlled.

For a constant volume fraction, when the particle size is decreased, the

number of particles increases. Therefore, the number of particle-particle

interactions increases, so the viscosity of a sample typically increases. As

particle-particle interactions are weak forces, the effect is seen more at low

shear rates.

Decrease the particle size and the viscosity generally increases.1

Ten Ways to…..Control Rheology by Changing Particle Properties


Conversely, if the particle size is increased, this leads to a smaller number

of particle-particle interactions. Again, due to the weak nature of this

association, the effect is seen most dramatically at low shears.

Increase the particle size and the viscosity generally decreases. 2


Particles which have a wide span / distribution (large polydisperity) tend

to pack better than a system of particles of all the same size (a narrow

distribution). This basically means that a wide distribution of particles has

more free space to move around, which therefore means it is easier for

the sample to fl ow, i.e. a lower viscosity. So, tightening up the particle

distribution can increase the stability of a system.

For example, on keeping the volume fraction the same, a sample of

relatively large particles with a small proportion of small particles will have a

viscosity lower than either the small particles or large particles alone.

This is basically due to the two competing effects of changing the number

of particle-particle interactions on changing the size and also changing the

polydispersity. Both affect the viscosity, however, in this case, the effect of

polydispersity dominates at one particular ratio.

The effect on the viscosity of particle size and particle size distribution can be used in combination for some interesting effects. 4

Increase the particle size distribution (span) and the viscosity decreases.3


Ensuring the particle size is constant, where more and more particles are

introduced the fl ow behaviour will generally go from Newtonian (so few

particles that they do not interact with each other), to shear thin (now the

particles can interact, but the forces are so small that this interaction can

be broken down with an increasing shear rate, therefore a shear thinning

property), to shear thickening (where there are so many particles that on

increasing the shear rate the particles now start to physically collide with

each other which causes a shear thickening effect).

Increasing the number of particles in a system to change the fl ow behaviour. 5


As the zeta potential is increased, the particles are forced to stay away

from each other. This is basically preventing the particles from fl owing

freely, hence the viscosity increases. The effect is seen more at lower shear

rates as this is a small force.

For larger particles, the settling force of gravity on the particles will

overcome any repulsion of the particles due to the electrostatic charge

/ zeta potential. However, as these large particles can no longer fully

aggregate (hydration layer), this close range, yet strong, Van der Waals

attractive force can increase the low shear viscosity.

For particles greater than a micron, i.e. dispersions, (where gravity now has a signifi cant effect) at high concentration, decreasing the zeta potential towards the iso-electric point to take advantage of the secondary minimum can create a self supporting gel system to introduce a yield stress to the system.

7

For particles smaller than a micron, i.e. colloids, increasing the magnitude of the zeta potential (either +ive or –ive) increases the lower shear viscosity. 6


Smooth particles have a resistance to move as there is typically an

association between the different particles, which is often a chemical

interaction. However, with non-smooth particles there will also be

a mechanical resistance, and also the chemical association can be

increased. Therefore, non-smooth particles have higher low shear viscosity

and a higher yield stress.

Smoother particles have a lower low shear viscosity than those which are sharp/non-smooth (i.e. a lower convexity). 8


With spherical particles, there are typically particle-particle interactions

which break down under shear to give a shear thinning behaviour.

However, with elongated particles the random orientation leads to a higher

barrier to start fl ow; an increase in low shear viscosity. However, under

shear, these elongated particles can orient themselves to be streamlined

with the direction of fl ow. They are therefore easier to fl ow, resulting in a

lower shear viscosity than the spherical same size equivalent.

With soft particles, a forced shear can change the shape of the particle.

This can lead to the particles elongating and aligning under shear resulting

in a more shear thinning system.

Particles which are elongated tend to have a higher low shear viscosity but a lower high shear viscosity than their more spherical size equivalents.9

With particles of the same size, soft/deformable particles tend to have a more shear thinning behaviour than their hard/rigid equivalents.10


© 2009 MRK1236-02

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We hope you find ‘Ten Ways to…..

Control Rheology by Changing Particle

Properties’ useful.

This is one in a series of publications

designed to help decision makers in your

industry make more informed choices.


control rheology by changing particle properties like size zeta potential

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