Spatiotemporal Dynamics of Stresses in Shear Thickening Suspensions

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Dense suspensions, comprised of a large quantity of solid particles suspended in a fluid, exhibit a wide range of surprising mechanical behavior, including a dramatic increase in viscosity under increasing flow rates. The viscosity can increase by a factor of a hundred or more, an effect that has important implications for a variety of industrial applications and natural processes, but is still not well understood on a fundamental level. In this project the researchers employ a new measurement technique to directly measure the forces at the boundary of flowing suspensions during the thickening process. These measurements are essential to identify the factors that control the thickening transition, and to determine how to control the transition by modifying the properties of the suspended particles. This work supports the development of predictive models that can significantly advance materials design and process engineering for a wide range of applications. The project also supports a pilot activity to involve a materials science graduate student in an interdisciplinary group project in science policy, and outreach to economically disadvantaged high school students through hands-on soft materials modules for use in the Georgetown University College Immersion Program.The mechanical response of dense suspensions exhibits a wide range of nonlinear phenomena including a dramatic increase in the viscosity with increasing shear stress or strain rate. The viscosity can increase by several orders of magnitude, an effect that has important implications for a variety of industrial applications and natural processes, but is still not well understood on a fundamental level. This project employs a novel technique, Boundary Stress Microscopy, developed by the PIs to directly measure with high spatial and temporal resolution the stresses at the boundary of sheared suspensions, to elucidate the nature of the shear thickening transition and its dependence on material and system properties. Measurements of boundary stresses during shear thickening have revealed that the thickening is due to dramatic, dynamic heterogeneities indicating a localized discontinuous transition to a high viscosity state. This project leverages those insights to (a) identify the factors that control the nucleation, growth, evolution, and decay of the high viscosity phase; (b) quantify the effect of the dynamic high boundary stresses on the flow field of the suspension; (c) relate these results to direct measurement of interparticle forces at the nanoscale; (d) employ oscillatory shear to probe the strain amplitudes and timescales associated with the transition to the high viscosity phase; (e) identify the mechanisms responsible for determining the characteristic size of the heterogeneous stresses. Collectively, these measurements support the development of continuum theories that can connect particle and fluid interactions at the microscale to macroscopic rheology by accurately describing the mesoscale spatiotemporal dynamics, thereby significantly advancing materials design and process engineering for a wide range of applications.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.