(519i) Role of Particle Roughness in Shear Thickening Colloidal Suspensions

The connection between roughness and friction is not straightforward despite decades of study. Here, we examine the effect of surface roughness on suspensions that undergo continuous shear thickening (CST) and discontinuous shear thickening (DST), with special emphasis on both viscosity and normal stresses. We provide direct evidence that increasing the surface roughness in model colloidal suspensions causes the CST-DST-dilatancy transition to occur at lower volume fractions (Ï?) and shear rates. Increasing the root-mean-squared roughness beyond the hydrodynamic lubrication length scale shifts the divergence in viscosity from random close packing for frictionless spheres (Ï?max ~ 0.64) to Ï?max ~ 0.54, in accordance with theory developed for frictional grains. Our results show that the forces required to push rough colloids into close contact are an order of magnitude lower than that of smooth colloids. The data also show that lubrication is dominant in concentrated suspensions of smooth colloids, but friction becomes the primary mechanism for DST when particles are in close contact at very high shear rates, Ï?, and surface roughness.