Drops inside elastic media: cavitation through diffusion

Plants have found an elegant way to combine fluid mechanics and elasticity to generate rapid motion. By making use of the elasticity of their tissue structure and the pressure differences in their internal liquid conducts, plants can easily achieve surprisingly fast movements, useful for instance in spore dispersion.
In this work, we study this conversion from elastic to kinetic energy using a model experiment: an evaporating water drop trapped in the bulk of a poro-elastic gel (Polydimethylsiloxane). The droplet shrinks due to evaporation, which follows a slow diffusive-limited process and can be modeled accordingly. During the droplet shrinkage, the pressure in the liquid decreases as a consequence of the build up of elastic stress at its interface. This underpressure eventually leads to the birth of a cavitation bubble. Using high speed imaging, we access the dynamics of the bubble growth and subsequent oscillations. We show that these observations can be understood through a modified Rayleigh-Plesset equation to account for elasticity.