In many constraint systems, from paintings to muddy soils, the evaporation of a solvent leads to the formation of complex drying patterns. Commonly these patterns are characterized by brittle straight cracks. Here, we study very different structures that emerge during the drying of thin films of hyperelastic hydrogels confined between two glass plates. When water evaporates, the volume lost is accommodated by an inward displacement of the air-hydrogel interface that induces an elastic deformation of the hydrogel. Once a critical front displacement is reached, we observe intermittent fracture events initiated by a geometric instability resulting in localized bursts at the interface. These bursts relax the stresses and irreversibly form air cavities that lead to cellular networks. We show that the spatial extent of the stress field prior to a burst, influenced by the air-hydrogel interfacial tension and the confinement of the gel, determines the characteristic size of the cavities.

Evaporation-driven cellular patterns in confined hyperelastic hydrogels. Baudouin Saintyves, R. Pic, L. Mahadevan, and I. Bischofberger. Physical Review Letters (2023).