Actuating shape change in hydrogels via "Extreme Thermodynamics"

“Extreme Mechanics” often refers to exploiting mechanical instabilities to achieve large shape deformations. An example is provided by the swelling of patterned hydrogels. Here, we consider an alternative mechanism in which a thermodynamic instability or phase transition plays the operative role, actuating similarly large shape changes in an unpatterned hydrogel. This example of “Extreme Thermodynamics” consists of rapidly heating a swollen hydrogel torus through a first-order phase transition to its de-swollen phase. The resulting phase coexistence is characterized by large internal stresses that deform the gel and induce buckling. We will present both our experimental results and a theory qualitatively accounting for our observations.