Title: Dynamics of Localized Stresses in Shear Thickening Suspensions

Speaker: Dr. Vikram Rathee, Georgetown University, USA

Abstract: Dense particulate suspensions exhibit a dramatic increase in average viscosity above a critical, material-dependent shear stress, a phenomenon known as shear thickening. This thickening changes from continuous to discontinuous as the concentration is increased and is a subject of both fundamental and practical importance, but the microscopic phenomena responsible for the dramatic thickening are unknown. Using direct measurements of spatially resolved surface stresses in the thickening regime, we report the existence of clearly defined dynamic localized regions of substantially increased stress that appear intermittently at stresses above the critical stress. With increasing applied stress, these regions occupy an increasing fraction of the system, and the increase accounts quantitatively for the observed shear thickening. The regions represent high-viscosity fluid phases, with a size determined by the distance between the shear surfaces and a viscosity that is nearly independent of shear rate that increases rapidly with concentration. Thus, we find that shear thickening arises from increasingly frequent localized discontinuous transitions between distinct fluid phases with widely differing viscosities.

Synthesis of colloidal rods enabled us to investigate the role of particle anisotropy and friction in shear thickening. We coated these rods with a thermo-responsive polymer and show that the increase in viscosity can be controlled precisely by tuning the temperature.