Frictional mechanics of wet granular material

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. Surprisingly, 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 friction 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.

On the question of whether lubricants fluidize in stick–slip friction

“…Clues may also be extracted from stick-slip motion of confined granular systems under shear, where numerical simulations (33,34) and some experiments (35)(36)(37) suggest that fluidization and dilation may play a role in the stick-slip instability. While this is suggestive, differences between granular layers and lubricant films include not only five orders of magnitude between size of grains and of molecules but, in particular, the issue of molecular interactions, negligible in granular shear but all-important when shearing lubricants.…”

Sliding Friction on Wet and Dry Sand

“…The effect of the air humidity on sliding friction of sand has in particular been much discussed, the general consensus being that humidity leads to the condensation of water between the grains [10][11][12][13]. The breaking up of the water bridges during sliding is then believed to significantly increase the friction coefficient.…”

“…If adding water to sand has an effect on friction, this should have consequential repercussions for, e.g., sand transport through pipes [6]. This is an important issue, since the transport and handling of granular materials is responsible for around 10% of the world energy consumption [7], and optimizing granular transport ultimately relies on understanding the friction between the granular system and the walls [8][9][10].…”

Anomalous acoustic reflection on a sliding interface or a shear band

“…Laboratory experiments on the frictional behavior of layers of granular rock confined between granite plates [5] [6], and of glass bead assemblies [21] have shown that sheared confined granular media obey standard friction lawsnamely, beyond a static threshold, τ 12 = −f τ 22 . However, the velocity dependence of the dynamic friction coefficient is not yet very clearly established.…”