Bulk elastic fingering instability in Hele-Shaw cells

TL;DR

This paper reports the discovery of a purely elastic fingering instability in elastomers within Hele-Shaw cells, driven by adhesion and occurring at a critical strain independent of elastic modulus.

Contribution

It reveals a new elastic instability mechanism in confined elastomers, highlighting the role of adhesion and strain thresholds in finger formation.

Findings

Fingers appear sequentially and propagate within the bulk

Fingering occurs at a critical strain independent of elastic modulus

Fingers' width depends non-linearly on plate separation

Abstract

We demonstrate experimentally the existence of a purely elastic fingering instability which arises when air penetrates into an elastomer confined in a Hele-Shaw cell. Fingers appear sequentially and propagate within the bulk of the material as soon as a critical strain, independent of the elastic modulus, is exceeded. Their width depends non-linearly on the distance between the confining glass plates. A key element in the driving force of the instability is the adhesion of layers of gels to the plates, which results in a considerable expense of elastic energy during the growth of the air bubble.