Nucleation, growth, and hysteresis of surface creases on swelled polymer gels

The formation and disappearance of surface creases on gel layers subjected to compressive swelling stresses due to attachment to a rigid substrate are investigated using thin (4-126 μm thick) temperature-responsive poly(N-isopropylacrylamide) copolymer hydrogels. For shallow quenches beyond the critical level of compression to trigger the elastic instability, creases form by nucleation and growth, and we show that the observed onset, morphology and cycle-to-cycle memory are therefore typically dominated by heterogeneous defects. Observations of isothermal crease growth velocities at different quench depths provide precise measurements of the level of effective compressive strain at which the surface first becomes unstable, and this critical strain is found to be slightly elevated as film thickness is reduced. These observations are consistent with a simple model for the energy change upon forming a fold, with the inclusion of a gel/water surface-energy term that provides a nucleation barrier. While significant hysteresis between the onset and disappearance temperatures is often observed in experiments, this is found to reflect predominantly the finite quench depths necessary to yield nucleation and growth of creases on experimental time-scales.