The Role of Contact Angle Hysteresis for Fluid Transport in Wet Granular Matter

TL;DR

This paper demonstrates that contact angle hysteresis significantly influences fluid distribution and mechanical properties in wet granular matter, challenging previous assumptions about geometrical disorder's role.

Contribution

It introduces a novel dynamical model incorporating contact line pinning and hysteresis, revealing their impact on fluid distribution and granular material stability.

Findings

Contact angle hysteresis governs equilibrium bridge volume distribution.

Late-time capillary pressure variance follows a power-law decay.

Final bridge volume spread correlates with contact angle hysteresis magnitude.

Abstract

The stability of sand castles is determined by the structure of wet granulates. Experimental data about the size distribution of fluid pockets are ambiguous about their origin. We discovered that contact angle hysteresis plays a fundamental role in the equilibrium distribution of bridge volumes, and not geometrical disorder as commonly conjectured, which has substantial consequences on the mechanical properties of wet granular beds, including a history dependent rheology and lowered strength. Our findings are obtained using a novel model where the Laplace pressures, bridge volumes and contact angles are dynamical variables associated to the contact points. While accounting for contact line pinning, we track the temporal evolution of each bridge. We observe a cross-over to a power-law decay of the variance of capillary pressures at late times and a saturation of the variance of bridge volumes to a finite value connected to contact line pinning. Large scale simulations of liquid transport in the bridge network reveal that the equilibration dynamics at early times is well described by a mean field model. The spread of final bridge volumes can be directly related to the magnitude of contact angle hysteresis.