Generally, it is said that when dispersed suspensions of small particles (~ 5 micron spheres) are pressure filtered in narrow columns, the particles travel individually and form a random closed packed bed, on the other hand, when an agglomerated suspension travels the particles travel in groups (as agglomerates).
(a) Is this suspension microstructure usually pressure dependent e.g. a dispersed suspension becomes agglomerated under high pressure?
(b) Is this also generally true that dispersed suspensions can be shear thickening when the flow rate is increased after critical stage and yield after a given point?
(c) Are agglomerated suspensions always shear thinning, in nature?
Thanks
A suspension is a system in which denser particles, that are at least microscopically visible, are distributed throughout a less dense fluid and settling is hindered either by the viscosity of the fluid or the impact of its molecules on the particles. Basically, a suspension is a two phase system comprising of solid particles suspended in a fluid matrix. The behavior of the suspension is, in general, controlled by the particle-particle interaction and the particle-matrix interaction.
(a) A dispersed suspension would become agglomerated under high pressure only if the particle-particle interactions are stronger than the particle-matrix interactions.
(b) When the flow rate is increased, the probability of particle-particle contact is increased and if the particle-particle interactions are stronger than the particle-matrix interactions, the dispersed suspension would then become shear-thickening.
(c) Agglomerated suspensions are most often shear-thinning because when they are subjected to shear, de-agglomeration takes place as the particle-particle physico-chemical interactions are broken down.
More details about the rheological behavior of suspensions can be got from the following book:
Aroon V. Shenoy, Rheology of Filled Polymer Systems, Kluwer Academic Publishers, Netherlands (1999).
Dear Dr. Shenoy, Thanks for your informative reply and your book. You wrote "A dispersed suspension would become agglomerated under high pressure only if the particle-particle interactions are stronger than the particle-matrix interactions." Do you have any particular example where this happens? I am interested in hard spheres like silica in common organic solvents. I have seen flow induced shear thickening of silica in water or some solvents but can pressure alone (> 8000 psi) on a stagnant liquid cause agglomeration in highly dispersed slurries?
I do not have any example where this happens. Pressure alone on a stagnant liquid may not able to cause agglomeration in highly dispersed slurries. Only if the applied pressure causes physical movement of the particles and increases the particle-particle contact, it is then that agglomeration could occur.
One could consider the presence of gases in the liquid, which could create aglomerates with gas content (minimising the superficial tension energy). In that case, increasing the pressure could reduce the tendecy towards forming aglomerates since the gas bubbles would be reduced in volume. The gas could be inadvertendly introduced during mixing of the solid filler with the liquid matrix.
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