It is well known that water expands when it freezes. At the capillary scale, this effect leads to an unusual shape change for a water droplet with the formation of a pointy tip and to a specific dynamics of the freezing front. In the first part of this talk we describe this phenomenon and model both the dynamics and the drop’s final shape. We also discuss the limitations of these models. In the second part of the presentation we compare these findings to the case of a hydrogel drop. Indeed a hydrogel is a material consisting of a polymer matrix which can absorb up to 99% water. This polymer matrix affects the overall shape of such objects during its freezing and thus how it interacts with the ice growth. We find that our hydrogel drops expand solely vertically in the direction of the temperature gradient. We propose a model to predict this shape change by assuming several conditions that we observe experimentally which differ from the freezing of a water drop. Moreover, since we can mold hydrogel freely without any container, into any kind of shape, we can then use our model to predict the final shape of any random hydrogel shape once frozen. Finally, we find that for hydrogels with a low concentration of polymer, water is expelled from the gel onto its surface during the freezing process (see picture below). We will discuss how this phenomenon may be related to the poroelastic nature of the material.

Contributors

Lila Séguy(1), Axel Huerre(2) and Suzie Protière(1)

(1)Institut Jean le Rond ∂’Alembert, CNRS UMR7190 Sorbonne Université, 75005 Paris France

(2)Laboratoire MSC, CNRS Université Paris Cité, 75013 Paris France