A number of materials, including ceramics, cement, adhesives, paints, pharmaceuticals, inks and surface coatings, mining and mineral slurries, and cosmetics, can be considered as dispersions, wherein one substance, usually particulate, is dispersed in another phase.
Significance of Understanding Particle Properties
In the inks industry, a knowledge of rheology and particle properties helps when changing the solid pigment content in different formulations while maintaining the key rheological properties needed for optimized printing. An understanding of aggregate morphology helps to control the flow behavior during application and processing in the cement industry.
The connection between the rhelogy of cosmetics and personal care products and their particle properties is important to obtain a fine balance with respect to formulation, application performance, and consumer acceptance. The physical properties of the dispersed particles, including the size distribution, average particle size, particle shape, and zeta potential or charge on the particles, all affect the overall (bulk) material characteristics, such as their rheology.
This article lists ten fundamental properties of the dispersed system, and explains their effect on the rheological properties. The examples presented in this article also demonstrate how the rheology of the material can be controlled by changing the particle properties, such as size, shape, and zeta potential.
Decreasing Particle Size Increases Viscosity
A decrease in particle size increases the number of particles in the case of a constant volume fraction (Figure 1). This, in turn, increases the degree of interactions between the particles, especially in the case of sub-micron sized particles. The surface charge, hydration or adsorption layers surrounding these particles substantially increase their effective hydrodynamic size.
Figure 1. Decreasing particle size increases viscosity.
The impact of increasing the number of particles present is magnified by these layers. This, in turn, increases the suspension viscosity by increasing the effective volume fraction for a specific particle loading. The effect is clearly visible in this range due to dominant inter particle (colloidal) interactions at low shear rates.
Increasing Particle Size Decreases Viscosity
The viscosity of a material increases only slightly if there is an increase in its particle size, because any relative increase in the particle’s effective hydrodynamic size caused by surface charge or adsorption layers is trivial. However, the effect is clearly visible at low shear rates due to dominant inter-particle (colloidal) interactions (Figure 2).
Figure 2. The viscosity generally decreases when the particle size increases.
Viscosity Decreases with Increasing Particle Size Distribution (Span)
Particles that have a wide span/distribution (large polydispersity) can be packed in a better manner than a suspension of particles of the same size (narrow distribution). More free space is available in the case of wide distribution for individual particles to move around. As a result, the sample flows easily. This means the viscosity is lower. Therefore, the suspension’s viscosity and stability can be increased by tightening up the particle distribution (Figure 3).
Figure 3. Viscosity decreases when the particle size distribution increases.
The Effect on Viscosity of Particle Size and Particle Size Distribution
When the volume fraction remains constant, a sample consisting of relatively large particles with a small amount of small particles will exhibit a viscosity lower than that of either of large or small particles alone. This is essentially due to two competing effects. One is related to the degree of particle-particle interactions, which show an increase when the size decreases, thus increasing viscosity. The other is that viscosity is decreased by other factors related to increased polydispersity. In this case, the impact of polydispersity becomes more pronounced with the viscosity decrease caused by incorporation of the smaller particles (Figure 4).
Figure 4. The effect on the viscosity of particle size and particle size distribution can be used in combination for some interesting effects.
Increasing Number of Particles to Change Flow Behavior
When the particle size remains constant, the introduction of more and more particles will transform the flow behavior from being Newtonian, to shear thinning, to shear thickening (Figure 5). In Newtonian, there are so few particles that they do not interact with one other, whereas in shear thinning, particle interaction can happen, but an increasing shear rate can break down this interaction as the forces are very small. Conversely, in shear thickening, the presence of more particles causes them physically collide and combine with each other, resulting in a shear thickening effect.
Figure 5. Increasing the number of particles in a system changes the flow behavior.
Increasing Magnitude of Zeta Potential Increases Low Shear Viscosity
For particles with a size of less than 1 mm, such as colloids, the low shear viscosity increases with increasing zeta potential magnitude (either positive or negative) (Figure 6). The particles repel each other with increasing zeta potential, and consequently, there is an increase in their effective size, thus preventing the free flow of the particles. This, in turn, increases viscosity. The effect is more pronounced at lower shear rates, where this interaction dominates the shear forces.
Figure 6. Increasing the magnitude of the zeta potential increases the low shear viscosity.
Effect of Decreasing Zeta Potential Towards Iso-Electric Point
A self-supporting gel system can be created for dispersions at high concentrations that include particles of more than 1 mm large (at which gravity becomes significant), when the zeta potential is decreased towards the iso-electric point (Figure 7). This results in the introduction of a yield stress, causing particles to come close towards each other to form reversible flocculation through Van der Waals attractive forces. However, shorter-range repulsive forces prevent permanent aggregation.
Figure 7. Effect of decreasing zeta potential towards iso-electric point.
Smoother Particles Have Lower Low Shear Viscosity than Sharp/Non-Smooth Particles
In a suspension, the convoluted outline of particles that have low convexity increases the chance of mechanical resistance to flow. Additionally, the specific surface area of the particles with low convexity may be relatively higher than equivalently sized smooth particles, making the chemical particle-particle interactions more robust. These effects are likely to be more pronounced at high solids loadings. The liquid flow around the particles with higher roughness is greatly deviated. This effect leads to viscosity increase (Figure 8).
Figure 8. Smoother particles have a lower low shear viscosity than those which are sharp/non-smooth.
Behavior of Elongated Particles
Increasing shear forces break down particle-particle interactions for spherical particles, causing a shear thinning effect. Conversely, the random orientation of elongated particles at low shear means they occupy larger volumes. However, the orientation of these particles at high shear will be in the direction of flow, leading to more efficient packing. Consequently, suspensions consisting of elongated particle show more shear thinning effect and higher low shear viscosity compared to suspensions with spherical particles (Figure 9).
Figure 9. Particles which are elongated tend to have a higher low shear viscosity but a lower high shear viscosity than their more spherical size equivalents.
Behavior of Soft/Deformable Particles
The shear thinning effect is found to be more pronounced in soft/deformable particles than for their hard/rigid equivalents of the same size. For soft particles, the particle shape can be changed by a forced shear. As a result, the particles are elongated and aligned under shear, creating a more shear thinning system with a lower high shear viscosity (Figure 10).
Figure 10. With particles of the same size, soft/deformable particles tend to have more shear thinning behavior than their hard/rigid equivalents.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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