Pattern Formation in Crumpled Hydrogel upon Rapid Dehydration with Acetone

It has been previously observed that when polymer gels (e.g. hydrogels) undergo rapid extensive swelling, a transient crumpling instability can form on the surface of the gel. This instability has a geometry characterised by many line segments of cusps patterning the gel's surface into an array of bumps that arise from shear bending of a homogeneously swollen gel surface. Here, we present experiments showing that when a hydrogel sheet whose upper surface exhibits such an instability is rapidly dehydrated (e.g. through rapid immersion in acetone), the instability is locked in place, becoming more visible as the bumps undergo a phase transition and change colour from transparent to white. We explore the different facets of this rich behaviour, comparing the acetone-induced pattern with the initial crumpling instability. In particular, we demonstrate that the pattern that emerges is independent of the total size of the blister: it is just a function of the time before immersion in acetone. Furthermore, by controlling the time before immersion we can control reliably the wavelength of the instability. We close by discussing potential next steps to be taken in investigating this fascinating phenomena.