  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.
