Hydrodynamic Origin of Friction Between Suspended Rough Particles

TL;DR

This paper demonstrates that the friction-like forces observed between suspended rough particles are primarily due to hydrodynamic interactions caused by surface asperities, not contact friction, with significant implications for suspension rheology.

Contribution

It provides a theoretical analysis showing that hydrodynamic forces from surface roughness dominate particle interactions, challenging the traditional contact friction perspective.

Findings

Hydrodynamic forces increase inversely with surface separation.

Surface asperities cause localized sliding forces and torques.

These forces influence particle rotation and translation constraints.

Abstract

Tangential interactions between particles play a central role in suspension rheology. We show theoretically that these interactions, often attributed to contact friction, are a direct consequence of fluid flows between rough particles in relative motion. We find that small surface asperities generically lead to localized hydrodynamic sliding forces and torques that can exceed their smooth counterparts by orders of magnitude. A fully analytic thin-film theory shows that these forces grow inversely with the surface separation, significantly more singular than the logarithmic scaling for smooth particles. The impending singularity tightly constrains the particles' rotation with their translation, recovering a crucial ingredient in dense suspension rheology. Despite their purely hydrodynamic origin, these features resemble several aspects of dry rolling and sliding friction.