Rheological testing refers to the measuring of the deformation of matter under the influence of imposed stress and analyzing the internal response of materials to forces. This article describes the general techniques of designing and interpreting rheological tests to improve the stability of samples.
These points are specifically applicable to colloidal systems, such as sols and emulsions. Sols are solid-in- liquid systems used in paints and coatings, inks, food and drink, pharmaceutical formulations, cosmetics and personal care. Emulsions are liquid-in-liquid systems typically used in paints and coatings, adhesives, foodstuffs, cosmetics, personal care, agrochemicals and pharmaceutical formulations. Even standard non-colloidal systems such as large particle dispersions, cement and ceramics, and mining and mineral slurries can also be assessed using the same techniques. Each test concentrates on a single property of the material and discusses ways to improve sample stability.
Zero Shear Viscosity of Material
Figure 1. Zero shear viscosity.
The term zero shear viscosity (Figure 1) refers to a material’s viscosity at a shear rate of zero. This means, the viscosity at rest which is normally the conditions to use when assessing stability. Apart from that of gravity, the material is not undergoing any major deformation forces. When the viscosity is higher at lower shear rates, the resistance of suspended particles to flow to the bottom of the sample will also be higher.
Figure 2. Introducing a yield stress type behavior to the material.
A yield stress (Figure 2) generally indicates a solid like behavior at rest which can prevent settling. Nevertheless, the real yield stress value itself can have a variety of values. When the value is higher, the structure will have more resistance to settling. Therefore, a suspension with a higher value of yield stress will be more stable. When a yield stress is introduced to a material, it will make the material to behave like a solid at rest and resist any suspended material from settling.
Minimizing Sample's Thixotropy
Most materials are normally moved from one place to another. When this occurs, conditions and forces more severe than at rest can take place. Traditional dispersion systems are shear thinning and hence the viscosity will tend to reduce under these conditions. But this in turn can make the material to settle. When thixotropy is reduced, one can reduce the time the sample remains at these low viscosities.
Cohesive Energy of Samples
Figure 3. Increasing the samples’ cohesive energy.
The term cohesive energy (Figure 3) refers to a measure of the material’s elastic strength. The elastic strength is essentially a measure of the strength of the internal structure; hence when the cohesive energy is higher a system will be more stable. One can calculate the cohesive energy by recording an amplitude sweep on the material and then obtaining the strain limit of the Linear Viscoelastic Region (LVR). The obtained values must be squared and then multiplied by half of the magnitude of the storage modulus in the LVR.
Optimizing the Viscoelastic Character
When the viscoelastic spectrum of the material is recorded with a frequency sweep, the properties of the materials under different time scales can be documented. Since sedimentation and settling is an extended process, it is important to analyze what happens at reducing frequencies.
Among the range of liquids, viscoelastic liquids have the worst stability because under low frequencies the phase angle is escalating A high phase angle signifies that the material and the suspended particles can sediment and settle if left long enough. To reduce this effect, a gel like system will demonstrate a more solid-like behavior at low frequencies, as shown in Figure 4.
Figure 4. Viscoelastic character.
While a gel like sample can demonstrate enhanced stability when compared to that of a viscoelastic liquid, for some systems with large particles, this gel structure might not be adequate to stop sedimentation. In this instance, a viscoelastic solid-like response from a frequency sweep means that the phase angle approaches zero at low frequencies. A low phase angle signifies that the material behaves like a solid.
A creep test enables the resistance of a material to flow under gravity to be determined. Using this test to apply a small force is a highly sensitive measure of how the material will withstand the gravitational force. When the resultant movement (strain) is small, the material is likely to be stable.
Particle to particle interactions affect low shear viscosity. Hence, to increase the number of particle-particle interactions, one can simply increase the number of particles in a system. This can be done by making the particles smaller. Therefore, there will be more particles for the same given mass of particles.
In case it is not possible to modify the average size of the particle in the system, changing the particle size distribution can also impact the low shear viscosity, and thus the stability. This means that for a broad distribution, particles are able to move around with more free space, thus making it easier for the sample to flow.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
For more information on this source, please visit Malvern Panalytical.