Towards a general(ized) shear thickening rheology of wet granular materials under small pressure

TL;DR

This study develops a generalized rheological model for wet and dry granular materials under small pressure, revealing how various physical time scales influence flow behavior and shear thickening phenomena.

Contribution

It introduces a four-parameter dimensionless framework that captures the combined effects of pressure, shear rate, stiffness, gravity, and cohesion on granular rheology, including wet cohesion effects.

Findings

Rheology is governed by multiplicative correction functions of key time scales.

Shear thinning occurs in the bulk, while shear thickening appears near the free surface with increased cohesion.

Wet cohesion significantly alters flow behavior, especially at the free surface.

Abstract

We study the rheology of dry and wet granular materials in the steady quasistatic regime using the Discrete Element Method (DEM) in a split-bottom ring shear cell with focus on the macroscopic friction. The aim of our study is to understand the local rheology of bulk flow at various positions in the shear band, where the system is in critical state. The general(ized) rheology has four dimensionless control parameters that relate the time scales of five significant phenomena, namely, the time scales related to confining pressure $t_p$, shear rate $t_{\dot{\gamma}}$, particle stiffness $t_k$, gravity $t_g$ and cohesion $t_c$, respectively. We show that those phenomena collectively contribute to the rheology as multiplicative correction functions. While $t_{\dot{\gamma}}$ is large and thus little important for most of the data studied, it can increase the friction of flow in critical state, where the shear gradients are high. $t_g$ and $t_k$ are comparable to $t_p$ in the bulk, but become more or less dominant relative to $t_p$ at the extremes of the free surface and deep inside the bulk, respectively. We also measure the effect of strong wet cohesion on the flow rheology, as quantified by decreasing $t_c$. Furthermore, the proposed rheological model predicts well the shear thinning behavior both in the bulk and near the free surface; shear thinning develops to shear thickening near the free surface with increasing cohesion.