Transient flows and migration in granular suspensions: key role of Reynolds-like dilatancy

TL;DR

This paper explores the transient behavior of sheared granular suspensions, emphasizing the importance of Reynolds-like dilatancy in modeling early stress responses and nonlocal effects during flow transitions.

Contribution

It introduces a two-phase model incorporating Reynolds-like dilatancy law to accurately predict suspension dilation, compaction, and stress response during transient flows below jamming.

Findings

Reynolds-like dilatancy law captures suspension dilation across parameters.

Nonlocal stress response with a length scale diverging at jamming.

Dilation rate influences early stress via Darcy flow and pore pressure gradients.

Abstract

We investigate the transient dynamics of a sheared suspension of neutrally buoyant particles under pressure-imposed conditions, subject to a sudden change in shear rate or external pressure. Discrete Element Method simulations show that, depending on the flow parameters (particle and system size, initial volume fraction), the early stress response of the suspension may strongly differ from the prediction of the Suspension Balance Model based on the steady-state rheology. We show that a two-phase model incorporating the Reynolds-like dilatancy law of Pailha & Pouliquen (2009), which prescribes the dilation rate of the suspension over a strain scale $\gamma_0$, quantitatively captures the suspension dilation/compaction over the whole range of parameters investigated. Together with the Darcy flow induced by the pore pressure gradient during dilation or compaction, this Reynolds-like dilatancy implies that the early stress response of the suspension is nonlocal, with a nonlocal length scale $\ell$ which scales with the particle size and diverges algebraically at jamming. In regions affected by $\ell$, the stress level is fixed, not by the steady-state rheology, but by the Darcy fluid pressure gradient resulting from the dilation/compaction rate. Our results extend the validity of the Reynolds-like dilatancy flow rule, initially proposed for jammed suspensions, to flowing suspension below $\phi_\mathrm{c}$, thereby providing a unified framework to describe dilation and shear-induced migration. They pave the way for understanding more complex unsteady flows of dense suspensions, such as impacts, transient avalanches or the impulsive response of shear-thickening suspensions.