Couette Flow of Two-Dimensional Foams

TL;DR

This study experimentally examines the flow behavior of quasi two-dimensional disordered foams in Couette geometries, revealing the influence of boundary conditions and nonlocal effects on rheology.

Contribution

It demonstrates the applicability of the nonlocal fluidity model to describe foam flow in different geometries and boundary conditions.

Findings

Flow is localized and rate-dependent with a top plate.

Flow becomes rate-independent for freely floating foams.

Nonlocal effects explain flow in regions below the yield stress.

Abstract

We experimentally investigate flow of quasi two-dimensional disordered foams in Couette geometries, both for foams squeezed below a top plate and for freely floating foams. With the top-plate, the flows are strongly localized and rate dependent. For the freely floating foams the flow profiles become essentially rate-independent, the local and global rheology do not match, and in particular the foam flows in regions where the stress is below the global yield stress. We attribute this to nonlocal effects and show that the "fluidity" model recently introduced by Goyon {\em et al.} ({\em Nature}, {\bf 454} (2008)) captures the essential features of flow both with and without a top plate.