Engineering the Flow Behavior of Colloidal Materials through Surface Modification and Shape Anisotropy.

[摘要] In this dissertation, we explore the idea that colloidal materials can be designed with enhanced rheological properties by incorporating shape and surface anisotropy. This hypothesis is motivated by our finding that structural rigidity can be used to predict the nonlinear elasticity in gels that have undergone yielding; we report a power-law scaling of the elastic modulus with the stress-bearing volume fraction that is valid over a range of volume fractions and gelation conditions. We demonstrate that the predictive power of microscopic theories can be improved when both the microstructural rigidity and the dynamical heterogeneity induced by yielding are taken into account. The dynamical heterogeneity takes a bimodal form in the self-part of the van Hove correlations, indicating subpopulations of slow and fast colloids within sheared gels. Because thermal rupture forces play a critical role in yielding, we develop a model gel system in which rheological measurements can be carried out in conjunction with microscopy experiments and optical trapping. These measurements allow us to compare simultaneous measurements of viscoelasticity, microstructure, and interparticle forces. In the second part of this dissertation, we aim to test the validity of structural rigidity by synthesizing colloidal spheroids and roughened particles. We find that colloidal oblate spheroids self-assemble into tilted structures in a specific range of volume fractions and attraction strengths. These structures could pave the way for maintaining the elasticity of colloidal gels even at exceedingly low particle loading. Finally, we observe a significant decrease in the critical shear rate required for shear thickening in concentrated suspensions of rough colloids. Nevertheless, we find a negligible difference in the time-dependent translational diffusivity between smooth and rough colloids. These results collectively provide experimental support for the applicability of structural rigidity as a guiding principle in engineering the flow properties of a broad range of soft matter.

[发布机构] University of Michigan

[效力级别] Material Science

[关键词] Colloids;Material Science;Rheology;Chemical Engineering;Engineering;Chemical Engineering