Impact of boundaries on velocity profiles in bubble rafts

TL;DR

This study investigates how confining boundaries influence velocity profiles and rearrangements in bubble rafts under slow shear, revealing that top boundary conditions significantly affect flow behavior even in quasi-static regimes.

Contribution

It demonstrates that boundary conditions, especially the top plate, significantly impact shear localization and flow profiles in bubble rafts at slow strain rates, challenging previous assumptions.

Findings

Top plate alters velocity profiles even at slow shear rates.

Boundary effects are significant in quasi-static shear conditions.

Flow behavior depends strongly on boundary confinement.

Abstract

Under conditions of sufficiently slow flow, foams, colloids, granular matter, and various pastes have been observed to exhibit shear localization, i.e. regions of flow coexisting with regions of solid-like behavior. The details of such shear localization can vary depending on the system being studied. A number of the systems of interest are confined so as to be quasi-two dimensional, and an important issue in these systems is the role of the confining boundaries. For foams, three basic systems have been studied with very different boundary conditions: Hele-Shaw cells (bubbles confined between two solid plates); bubble rafts (a single layer of bubbles freely floating on a surface of water); and confined bubble rafts (bubbles confined between the surface of water below and a glass plate on top). Often, it is assumed that the impact of the boundaries is not significant in the ``quasi-static limit'', i.e. when externally imposed rates of strain are sufficiently smaller than internal kinematic relaxation times. In this paper, we directly test this assumption for rates of strain ranging from $10^{-3}$ to $10^{-2} {\rm s^{-1}}$. This corresponds to the quoted quasi-static limit in a number of previous experiments. It is found that the top plate dramatically alters both the velocity profile and the distribution of nonlinear rearrangements, even at these slow rates of strain.