Nucleation and growth of creases on swelled polymer gel surfaces

We have studied the processes by which surface creases form, evolve, and disappear using thin temperature-responsive poly(N-isopropylacrylamide) copolymer hydrogels. For shallow quenches beyond the critical level of compression, creases nucleate and growth, and thus the observed onset and morphology are typically dominated by heterogeneous defects. Measurements of crease growth velocities at different quench depths are used to precisely determine the compression at which the surface first becomes unstable, and this critical strain is found to be slightly elevated as film thickness is reduced. This behavior is captured by a simple model for the energy change upon forming a fold, with a nucleation barrier to fold formation provided by the gel/water surface-energy. While hysteresis between onset and disappearance is often observed in experiments, this reflects predominantly the degree of ``undercooling'' necessary to yield nucleation and growth of creases.