Wall slip and bulk yielding in soft particle suspensions

TL;DR

This study uses simulations to explore wall slip and yielding in soft particle suspensions, revealing how wall roughness and shear stress influence slip behavior and flow dynamics.

Contribution

It introduces a comprehensive simulation method that captures elastohydrodynamic wall slip and its effects on flow curves and yielding in soft particle suspensions.

Findings

Wall slip dominated by a thin solvent layer below yield stress.

Increased wall roughness suppresses slip.

Slip alters the steady-state flow curve and affects yielding dynamics.

Abstract

We simulate a dense athermal suspension of soft particles sheared between hard walls of a prescribed roughness profile, using a method that fully accounts for the fluid mechanics of the solvent between the particles, and between the particles and the walls, as well as for the solid mechanics of changes in the particle shapes. We thus capture the widely observed phenomenon of elastohydrodynamic wall slip, in which the soft particles become deformed in shear and lift away from the wall slightly, leaving behind a thin lubricating solvent layer of high shear. For imposed stresses below the material's bulk yield stress, we show the observed wall slip to be dominated by this thin solvent layer. At higher stresses, it is augmented by an additional contribution arising from a fluidisation of the first few layers of particles near the wall. By systematically varying the roughness of the walls, we quantify a suppression of slip with increasing wall roughness. For smooth walls, slip radically changes the steady state bulk flow curve of shear stress as a function of shear rate, by conferring a branch of apparent (slip-induced) flow even for $\sigma<\sigma_y$, as seen experimentally. We also elucidate the effects of slip on the dynamics of yielding following the imposition of a constant shear stress, characterising the timescales at which bulk yielding arises, and at which slip first sets in.