Novel Shear Banding Phenomenon Probes Soap Film Friction

TL;DR

This study introduces a new shear banding phenomenon in 2D foam, revealing how drag forces on soap films vary with lubrication layers, and provides insights into foam flow dynamics in confined geometries.

Contribution

It demonstrates a novel shear banding mechanism in foam and quantifies the drag forces on soap films in different lubrication conditions, supported by a theoretical model.

Findings

Shear band forms in foam due to air injection in a Hele-Shaw cell.

Drag force on soap film with wetting layer is 100 times larger than on free soap film.

The shear band minimizes total dissipation in the flow.

Abstract

We have generated a novel form of shear banding in a 2D foam and measured the relative magnitude of drag forces on soap films at different lubrication layers. We injected air part way along a flowing bubble field in a narrow Hele-Shaw cell. The injected air inflates bubbles as they flow by, and these bubbles form a shear band down the middle of the Hele-Shaw cell. This channel appears to select a height that minimizes the total dissipation. Fitting data to a simple theoretical model, we show that the drag force on a soap film in contact with the wetting layer on a plate of glass is two orders of magnitude larger than the drag on a soap film in contact with another free soap film.