(262c) Effect of Bidispersity on Shear Thickening in Dense Suspensions

Concentrated, or “dense”, particulate suspensions often demonstrate dramatic flow behavior such as an abrupt increase in apparent viscosity, known as Discontinuous Shear Thickening (DST). Suspension material properties such as particle size and polydispersity play a crucial role in rheology of suspensions and whether a suspension will demonstrate DST behaviour. It is widely accepted that in dense suspensions, polydispersity shifts the critical shear rate to larger values, but the mechanisms that contribute to the reduction of viscosity in polydisperse suspensions have not been thoroughly studied. Prior work [1] has demonstrated that bidisperse suspensions display changes in viscosity at high shear rates for different solid volume composition. This study focuses on the effect of size ratio of particle radii and large particle fraction on shear thickening behaviour of bidisperse dense suspensions. We use a computational model for smooth spherical particles, which accounts for short-range lubrication forces, frictional interaction, and repulsion between particles to simulate bidisperse dense suspensions of neutrally buoyant particles under a broad range of shear stresses. We demonstrate that the critical shear stress can be tuned through bidispersity and that increase in shear stress has a diminished effect on dense suspensions when moving from mono- to bimodal particle size distribution. We find that under low shear stress conditions, the suspension exhibits a remarkable rheological behavior: the viscosity gradually decreases with increase of large particle fraction. We demonstrate that the low viscosity is attributed to particles undergoing ordering under shear flow where particles arranged in layers slip each other, causing the decrease in viscosity. Microstructural investigation of the simulated suspensions is presented.

[1] Pednekar, Sidhant, Jaehun Chun, and Jeffrey F. Morris. "Bidisperse and polydisperse suspension rheology at large solid fraction." Journal of Rheology 62.2 (2018): 513-526.