Frictional Mechanics of Wet Granular Material

TL;DR

This study experimentally investigates the frictional behavior of wet granular layers under shear at low normal stress, revealing dilation dynamics, velocity-independent frictional force, and a transient shear stress exceeding the critical slip threshold.

Contribution

It provides new experimental insights into the frictional mechanics of wet granular materials at low stresses and proposes an empirical model capturing observed behaviors.

Findings

Dilation increases exponentially at slip onset.

Frictional force depends linearly on normal stress and dilation rate.

Layer can sustain shear stress above the critical slip threshold temporarily.

Abstract

The mechanical response of a wet granular layer to imposed shear is studied experimentally at low applied normal stress. The granular material is immersed in water and the shear is applied by sliding a plate resting on the upper surface of the layer. We monitor simultaneously the horizontal and the vertical displacements of the plate to submicron accuracy with millisecond time resolution. The relations between the plate displacement, the dilation of the layer and the measured frictional force are analyzed in detail. When slip begins, the dilation increases exponentially over a slip distance comparable to the particle radius. We find that the total dilation and the steady state frictional force do not depend on the driving velocity, but do depend linearly on the applied normal stress. The frictional force also depends linearly on the dilation rate (rather than the dilation itself), and reaches a maximum value during the transient acceleration. We find that the layer can temporarily sustain a shear stress that is in excess of the critical value that will eventually lead to slip. We describe an empirical model that describes much of what we observe. This model differs in some respects from those used previously at stresses 10^6 times larger.