Controlling Shear-Induced Migration in Colloid/Polymer Mixtures

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Mixtures of small, rigid particles in solutions of polymers are widely used in advanced technologies such as 3-D printing and energy storage. The particles impart function, such as conductivity needed to transport electrons through 3-D printed wires. The polymers are added to control the flow behavior of the mixture. Tuning the properties of these mixtures requires controlling the distribution of the particles -- conductivity, for example, requires the particles to form a connected network. One factor that affects particle distribution during flow is shear-induced migration, in which particles migrate away from the walls and accumulate near the center of a narrow channel. This project will use experiments to determine the effect of polymers in solution on the distribution of particles as they flow through narrow channels. This knowledge will provide new routes to control the structure and functional properties of particulate suspensions for applications in next-generation energy storage and transport.Suspensions of microscale particles in complex fluids offer transformative opportunities in materials engineering for energy storage and transport. Miniaturizing these technologies requires increasing the suspension conductivity, by ensuring particle connectivity, while minimizing clogging in small channels. Complex fluid additives such as polymers or surfactants needed for suspension function or stability, however, may alter particle connectivity and density. Hence controlling particle density in the presence of additives is essential for attaining the desired functional properties of confined particulate suspensions. Polymers added to particulate suspensions may alter shear-induced migration, one mechanism that generates non-uniform density profiles in confined flows, in at least two ways: by generating aggregates or by enhancing normal stresses. Systematic understanding of how to control microscale particle migration in polymer solutions is currently lacking. The objective of this project is to understand how polymers alter shear-induced migration of colloidal particles during microchannel flow. To test the hypothesis that the extent of migration reflects competition between polymer-enhanced normal stresses and polymer-driven migration, the PI will use bulk rheology and microchannel confocal imaging experiments to characterize the normal stresses and particle microstructure in strongly and weakly migrating suspensions containing either adsorbing or non-adsorbing polymers. Measurements of particle velocity and concentration profiles will be compared to existing theoretical models for shear-induced migration of hard-sphere particles in simple liquids. This plan of work will identify the relative impact of the two competing mechanisms on shear-induced migration, leading to unprecedented control over particle density in models of technologically important materials and allowing existing models for shear-induced migration to be extended to include polymer-mediated interactions. This project will thus generate new design rules for suspensions to maximize particle connectivity during complex fluid flow while minimizing clogging. The PI will disseminate results to scientists in Houston's energy and medical industries through tutorial workshops aimed at oil and gas professionals and through the Texas Soft Matter meeting, which she founded and organizes, and will expand her ongoing outreach activities to K-12 students and the general public to include new hands-on activities on complex fluid flow.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.