Pattern Formation During Swelling in Aqueous Glycerin Solutions of Hydrogel Beads

We investigated the surface instabilities that develop during the swelling of hydrogel beads immersed in aqueous glycerin solutions of varying concentrations. Using a simple laser imaging technique, we tracked the evolution of a dry inner core and a hydrated shell along with the transient formation of surface patterns. The experiments show a consistent transition from wrinkles to a labyrinth morphology, as both the number of lobules decreases and their characteristic wavelength increases. We find that the nondimensional wavelength of the surface patterns scales with the nondimensional thickness of the hydrated shell, independent of glycerin concentration. Additionally, when time is rescaled with a characteristic poroelastic time, the evolution of both the number of lobules as well as their wavelength from experiments with all of the different glycerin concentrations collapse onto a universal curve, highlighting that increasing the solution viscosity primarily delays the onset of the instability without altering its geometric nature.