Wicking in a powder

TL;DR

This paper studies the conditions under which liquids spontaneously impregnate granular media, identifying a critical contact angle and examining effects of polydispersity and pressure on wetting transitions.

Contribution

It introduces a detailed analysis of the critical contact angle for wicking in granular layers, highlighting differences from tube assemblies and effects of polydispersity and pressure.

Findings

Critical contact angle around 55° for monodisperse layers

Impregnating grains requires a smaller angle than tubes

Polydispersity and pressure shift the wetting transition

Abstract

We investigate the wicking in granular media by considering layers of grains at the surface of a liquid, and discuss the critical contact angle below which spontaneous impregnation takes place. This angle is found to be on the order of 55{\deg} for monodisperse layers, significantly smaller than 90{\deg}, the threshold value for penetrating assemblies of tubes: owing to geometry, impregnating grains is more demanding than impregnating tubes. We also consider the additional effects of polydispersity and pressure on this wetting transition and discuss the corresponding shift observed on the critical contact angle.