Sheared gels form vorticity-aligned flocs depending on the shape of connected colloidal clusters
Date:
Excerpt of abstract: We investigate the rheology, structure, and flow-induced patterning of colloidal gels composed of thermosensitive poly(N-isopropylacrylamide) (pNIPAM) microgel particles. We mix these particles with a polymer to induce a depletion attraction. At room temperature, these mixtures phase separate into coexisting fluid phases. At slightly higher temperatures, these mixtures form a gel due to the increased hydrophobicity of the colloidal microgel particles. By applying shear to our samples while in the fluid-fluid demixed state, we can deform the colloid-rich domains. By then increasing the temperature, these colloid-rich domains connect together to form a sample-spanning gel. We observe the flow-induced patterns that form when we continue shearing this gel and it yields. We find that the patterns depend on how deformed, by flow, the colloid-rich domains were prior to gelation. For a range of shear rates applied to the sample while in the fluid-fluid demixed state, when the sample transitions to the gel state and is sheared we observe vorticity-aligned log-like flocs which roll between the parallel plates of our geometry. Our data suggests that the morphology of the colloidal clusters, which is dependent on the shear rate applied while in the fluid-fluid demixed state, is an important factor in determining whether or not vorticity-aligned flocs form. This work may offer routes for tuning the strength and structure of colloidal gels.