Pinning-induced folding-unfolding asymmetry in adhesive creases

TL;DR

This paper investigates how contact line pinning causes asymmetry in the folding and unfolding of creases in soft elastic and biological tissues, revealing a memory effect due to surface tension and pinning.

Contribution

It demonstrates that contact line pinning induces folding-unfolding asymmetry in creases, providing new insights into crease morphology and surface memory effects.

Findings

Surface tension induces a second fold at the contact edge.

Pinning inhibits complete unfolding, creating a folding memory.

Crease profiles exhibit self-similar morphologies with asymmetry.

Abstract

The compression of soft elastic matter and biological tissue can lead to creasing, an instability where a surface folds sharply into periodic self-contacts. Intriguingly, the unfolding of the surface upon releasing the strain is usually not perfect: small scars remain that serve as nuclei for creases during repeated compressions. Here we present creasing experiments with sticky polymer surfaces, using confocal microscopy, which resolve the contact line region where folding and unfolding occurs. It is found that surface tension induces a second fold, at the edge of the self-contact, which leads to a singular elastic stress and self-similar crease morphologies. However, these profiles exhibit an intrinsic folding-unfolding asymmetry that is caused by contact line pinning, in a way that resembles wetting of liquids on imperfect solids. Contact line pinning is therefore a key element of creasing: it inhibits complete unfolding and gives soft surfaces a folding memory.