Comparison between step strains and slow steady shear in a bubble raft

TL;DR

This study compares stress relaxation behaviors in a bubble raft under step strains and slow steady shear, finding general agreement but no divergence in stress drops as seen in some simulations.

Contribution

It provides experimental insights into stress drop distributions in foam, clarifying differences with quasistatic simulation predictions.

Findings

Stress drops depend on strain rate similarly in both methods.

No evidence of divergence in average stress drop near specific foam densities.

Experimental results align with some simulation predictions but differ in divergence behavior.

Abstract

We report on a comparison between stress relaxations after an applied step strain and stress relaxations during slow, continuous strain in a bubble raft. A bubble raft serves as a model two-dimensional foam and consists of a single layer of bubbles on a water surface. For both step strains and continuous strain, one observes periods of stress increase and decrease. Our focus is on the distribution of stress decreases, or stress drops. The work is motivated by apparent disagreements between quasistatic simulations of flowing foam and simulations of continuous strain for foam. Quasistatic simulations have reported larger average stress drops than the continuous strain case. Also, there is evidence in quasistatic simulations for a general divergence of the average size of the stress drops that only appears to occur in steady strain near special values of the foam density. In this work, applied step strains are used as an approximation to quasistatic simulations. We find general agreement in the dependence of the average stress drop on rate of strain, but we do not observe evidence for a divergence of the average stress drop.