Hydrodynamic Interactions Between Rough Particles

Hydrodynamic interactions of rough particles in dense suspensions are dominated by the near-contact interactions of their surfaces. Using lubrication theory, we study these interactions between spheres with randomly generated rough surfaces close to contact. We focus on sliding motions and study the associated tangential forces by varying the average width of the roughness texture as well as the separation between the surfaces. We find that these forces diverge as the inverse of the separation distance, as opposed to the logarithmic divergence for smooth spheres. The forces are dominated by local squeeze flows between roughness asperities near the point of closest approach. We develop an analytic framework that describes the hydrodynamic forces on the particle as the result of an averaging of bump-pair interactions, and find agreement with the average result of the many configurations of rough particle geometry.