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Associate ProfessorDept. of Chemical EngineeringUniversity of CA – Santa Barbara
Colloidal gels – solid-like networks of fluid-borne particles – form a large and important class of formulated chemical products, and their rheological properties impact the processing and performance of foods, consumer products, pharmaceuticals, structural composites, inks and coatings. Despite their ubiquity, design of colloidal gels with targeted properties remains largely empirical in industrial practice because of the complex out-of-equilibrium processes by which they form and age, and the resulting dearth of microscopic theories to predict their properties. To fill the need for more rational design strategies for colloidal gels, we posit that significant inspiration can be drawn from atomic and molecular solids, where the use of thermal processing to manipulate phase separation is a conserved motif for designing materials with tailored mechanical properties that spans antiquity, modern technology and the natural world. To translate these capabilities to colloidal materials, we have developed and investigated a model thermally processable colloidal system based on nanoparticles in the presence of thermoresponsive polymers that allows for fine control over their interparticle attractions and resulting colloidal behavior. Using a combination of experiment, theory and simulation, we show how this thermoreversible behavior gives access to a range of gelation mechanisms including percolation, phase separation and glass formation, whose outcome can be selected through the thermal path taken to the gelled state. Within this palette of gelation mechanisms, we explore how thermal annealing can be used to guide the kinetics and micromechanics of structural coarsening, leading to the concept of “time-temperature transformation” of gels, analogous to that in metallurgy, from which phase separated morphologies can be developed, controlled and arrested into a gel network. Ultimately, we show how the new length scales of gel structure imparted by thermal processing can be used to tailor colloidal solids (and other materials from them) with widely varying and superior mechanical properties.