Many materials that are being used today are disperse systems where one substance is dissolved in another phase. These types of materials comprise agrochemicals, adhesives, ceramics, cement, colloids, food and drink, cosmetics and personal care formulations, etc.
For instance, in the cosmetics and personal care industries, it is important to understand the association between particle properties and rheology to offer the right balance in terms of formulation, application performance, and consumer acceptance.
The properties of the dispersed particles, like size distribution, the average particle size, the zeta potential and the shape of the particles affect the overall properties of materials. This article describes the basic properties of the dispersed system and shows how they control the rheological properties.
Decreased Particle Size Leads to Increased Viscosity
Figure 1. Viscosity generally increases with decreased particle size.
In a stable volume fraction, when the size of the particle is reduced (Figure 1), it leads to increased number of particles. Hence, when the number of particle-particle interactions increases, the sample’s viscosity also increases. However, due to the weak nature of these particle-particle interactions, the effect is commonly observed at low shear rates.
Increased Particle Size Leads to Decreased Viscosity
Figure 2. Viscosity decreases with increased particle size.
However, when the size of the particle is increased (Figure 2), the number of particle-particle interactions reduces. Again, owing to the weak nature of this interaction, the effect is mostly observed at low shears.
Increased Particle Size Distribution Leads to Decreased Viscosity
Figure 3. Viscosity decreases with increased the particle size distribution.
Particles having a wide distribution or span normally pack better when compared to a system of particles of the same size (Figure 3). This means that particles with a wide span have more free space to move around, thus making it easier for the sample to flow, that is, a lower viscosity. Therefore, particle distribution can be tightened up to increase the system’s stability.
Effect on Viscosity of Particle Size and Particle Size Distribution
The effect on the viscosity of particle size distribution and particle size can be combined together to obtain some remarkable effects. When the volume fraction is kept the same, a sample of large particles with a slight amount of small particles will have a viscosity that is lower than that of the large or small particles alone. This is because of the two competing effects of altering the number of particle-particle interactions on changing the size and also altering the polydispersity.
Increasing the Number of Particles Changes Flow Behavior
Figure 4. Flow behavior of particles.
Ensuring that the size of the particles is constant where more particles are introduced, the flow behavior will normally go from Newtonian to shear thin and finally to shear thickening (Figure 4). During the Newtonian stage, there are so few particles that they do not interact with one another; in the shear thin stage the particles can interact, however the forces are so small that this association can be broken down with an increasing shear rate; in the shear thickening stage, there are so many particles that on increasing the shear rate the particles collide with one another, leading to a shear thickening effect.
Increased Zeta Potential Increases Lower Shear Viscosity
When the magnitude of the zeta potential is increased, the particles stay away from each other. This prevents the particles from flowing freely and the viscosity therefore increases. This effect is commonly observed at lower shear rates.
Van der Waals Force Increases Low Shear Viscosity
In case of larger particles, the gravitational force on the particles will overpower the resistance of the particles because of the zeta potential or electrostatic charge. However, since these large particles cannot completely combine, the strong Van der Waals attractive force can boost the low shear viscosity.
Smoother Particles Have Lower Low Shear Viscosity
Smoother particles tend to exhibit a lower low shear viscosity when compared to those which are sharp or uneven. In case of sharp particles, there will be a mechanical resistance, similar to smooth particles, but here the chemical association can also be increased. Hence, sharp particles exhibit a higher yield stress and higher low shear viscosity.
Elongated Particles have Higher Low Shear Viscosity
In case of elongated particles, the random orientation results in a higher barrier to initiate flow i.e. an increase in low shear viscosity. But under shear, these particles can adjust themselves to be streamlined with the direction of flow. Hence, they are easier to flow and lead to a lower shear viscosity when compared to the round particles of same size.
Soft Particles Have More Shear Thinning Behavior
In case of soft particles, a forced shear can alter the particle shape. This can extend the shape of the particles, which align under shear and result in a more shear thinning behaviour.
The article clearly describes the ways to control rheology by modifying particle properties, such as size, shape, and zeta potential.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
For more information on this source, please visit Malvern Panalytical.