Crease in a ring of a pH-sensitive hydrogel swelling under constraint

When a hydrogel swells, its surface often forms creases, a type of localized instability that nucleates and propagates in the form of self-contact. Motivated by recent applications of pH-sensitive hydrogels as actuators in adaptive lenses, here we analyze creases induced by constrained swelling. A ring of a gel, bonded between two rigid plates, swells by absorbing a solution from its external wall. The amount of swelling is adaptive in response to the change of the pH of the external solution. We analyze the large and inhomogeneous deformation in the gel by using a previously developed nonlinear field theory and finite element method. We show that, as the pH in the external solution increases, a short ring swells smoothly, but a tall ring forms a crease. We further show that the large deformation and instability can significantly affect the functionality of the adaptive lenses.