Heterogeneous nucleation of creases in swelling polymer gels

Surface creasing is a common occurrence in gels under strong enough compression. The transition from smooth to creased surface has been well studied in equilibrium conditions and applied to achieve stimuli-responsive properties. Classical predictions of the creased state, assuming the gel is at equilibrium and homogeneous, are generally satisfactory, while the transient behavior in swelling gels is often far from equilibrium and is commonly heterogeneous. The short-time response is essential for materials in dynamic environments, but it remains unreported and largely unknown due to the limited temporal resolution of the techniques used so far. Here, we use spatially resolved multispeckle diffusing wave spectroscopy (MSDWS) with submicrosecond time resolution to measure the spatially dependent swelling and creasing of a constrained poly (vinyl alcohol) chemical gel in borax solutions of varying concentrations. Our high-speed imaging by MSDWS shows that the swelling behavior and mechanical response at the microscopic level can be highly heterogeneous in time and space, and is detectable hundreds of seconds before the corresponding macroscopic creasing transition. This unprecedented visualization of the heterogeneous and time-dependent behavior beyond equilibrium morphological changes unveils the full complexity of the transient material response after exposure to external stimuli and sheds light on the formation mechanism of metastable states in transient processes.