Freezing-Induced Deformation in Water and Hydrogel Droplets.

We performed experiments to directionally freeze agar hydrogel drops on a copper substrate maintained at low temperature. Unlike the water droplets studied in the literature, where the liquid part can reorganize during freezing, these droplets present a strictly vertical expansion. This is due to the presence of the viscoelastic solid polymer matrix, which prevents deformation of the unfrozen part, leading mainly to an elongation in the direction of the temperature gradient. We can then derive two distinct models to describe the deformation upon freezing for 2D water droplets and hydrogels, respectively. They are both in good agreement with the experimental observations. The model is also predictive for different object shapes, as the hydrogel can be freely molded without a container.