Digital instability of a confined elastic meniscus

TL;DR

This paper investigates the reversible elastic meniscus instability in thin soft elastic layers, revealing its energetic origin, characteristics, and potential applications in patterning and adhesive strength enhancement.

Contribution

It introduces a comprehensive theory, experimental validation, and numerical simulations of a novel reversible elastic instability with practical implications.

Findings

The instability is sudden and first-order.

Finger wavelength and amplitude scale with layer thickness.

Trigger separation inversely relates to in-plane dimension.

Abstract

Thin soft elastic layers serving as joints between relatively rigid bodies may function as sealants, thermal, electrical, or mechanical insulators, bearings, or adhesives. When such a joint is stressed, even though perfect adhesion is maintained, the exposed free meniscus in the thin elastic layer becomes unstable, leading to the formation of spatially periodic digits of air that invade the elastic layer, reminiscent of viscous fingering in a thin fluid layer. How- ever, the elastic instability is reversible and rate-independent, dis- appearing when the joint is unstressed. We use theory, experiments, and numerical simulations to show that the transition to the digital state is sudden (first-order), the wavelength and amplitude of the fingers are proportional to the thickness of the elastic layer, and the required separation to trigger the instability is inversely proportional to the in-plane dimension of the layer. Our study reveals the energetic origin of this instability and has implications for the strength of polymeric adhesives; it also suggests a method for patterning thin films reversibly with any arrangement of localized fingers in a digital elastic memory, which we confirm experimentally.