Reversal of Solvent Migration in Poroelastic Folds

Polymer networks and biological tissues are often swollen by a solvent such that their properties emerge from a coupling between swelling and elastic stress. This poroelastic coupling becomes particularly intricate in wetting, adhesion, and creasing, for which sharp folds appear that can even lead to phase separation. Here, we resolve the singular nature of poroelastic surface folds and determine the solvent distribution in the vicinity of the fold tip. Surprisingly, two opposite scenarios emerge depending on the angle of the fold. In obtuse folds such as creases, it is found that the solvent is completely expelled near the crease tip, according to a nontrivial spatial distribution. For wetting ridges with acute fold angles, the solvent migration is reversed as compared to creasing, and the degree of swelling is maximal at the fold tip. We discuss how our poroelastic fold analysis offers an explanation for phase separation, fracture, and contact angle hysteresis.