Free-surface flows in dense colloidal suspensions

Shah, Phalguni S.

doi:https://doi.org/10.21985/n2-se4z-3c07

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Free-surface flows in dense colloidal suspensions

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In this dissertation, I summarize my findings of the dynamics of colloidal suspensions over a large range of volume fractions in two systems: drop impact and film rupture. The existence of a deformable surface in both these systems allows me to capture the consequences of non-Newtonian flow using high-speed imaging. The silica spheres and rods used in the experiments were synthesized in our lab, and characterized via SEM. Experiments were performed using known volumes of colloidal suspensions under controlled humidity.For impacting drops, I show that the spreading behavior for a large range of volume fractions agrees surprisingly well with Newtonian models. In the dense limit, I characterize the transition between Newtonian-like spreading to complete solidification via localized and partial solidification states. I show that this behavior is a direct result of shear jamming, and the drop solidifies faster for higher applied shear rates. I characterize the resulting solid properties and its unjamming dynamics in details, and show that both depend on shear history. Additionally, for suspensions with rod-shaped particles, I ob- serve dramatically different bouncing dynamics from sphere suspensions. I hypothesize that contact-line dynamics are heavily altered by the presence of rods-shaped particles. Rupturing films behave as Newtonian viscous fluids for a wide range of volume frac- tions. However, for high volume fractions and thinner films, I report novel instabilities during spontaneous rupture, that are reproducible under controlled humidity. I hypothe- size that instabilities develop when the film thickness competes with particle lengthscale, as discrete effects start taking effect at individual particle level. My systematic experiments span the transition from Newtonian-like to highly non- Newtonian behavior, bridging the gap between the existing understanding of Newtonian fluid dynamics and colloidal suspension dynamics. In the high volume fraction limit, my work uncovers interesting behaviors that will improve our understanding of particulate suspensions dynamics. This work connects to many pertinent questions in colloidal science, the most notable being the nature of the shear jamming transition and the dynamics of suspensions in a quasi two-dimensional geometry.

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