Turning Drops into Bubbles: Cavitation by Vapor Diffusion through Elastic Networks

Abstract

Some members of the vegetal kingdom can achieve surprisingly fast movements making use of a clever combination of evaporation, elasticity and cavitation. In this process, enthalpic energy is transformed into elastic energy and suddenly released in a cavitation event which produces kinetic energy. Here we study this uncommon energy transformation by a model system: a droplet in an elastic medium shrinks slowly by diffusion and eventually transforms into a bubble by a rapid cavitation event. The experiments reveal the cavity dynamics over the extremely disparate timescales of the process, spanning 9 orders of magnitude. We model the initial shrinkage as a classical diffusive process, while the sudden bubble growth and oscillations are described using an inertial-(visco)elastic model, in excellent agreement with the experiments. Such a model system could serve as a new paradigm for motile synthetic materials.

Collapse and cavitation during the drying of water-saturated PDMS sponges with closed porosity

“…For larger pores the dissolution time is longer. This may be explained with the diffusive model of the pore shrinkage proposed by Milner et al 10 and Bruning et al 12 . In this model, the diameter of the pore scales with time t as = 0 √1 − 0 2 ⁄ where d0 is the initial diameter and the kinetic factor k is mainly governed by the diffusion coefficient and the difference between the water concentration ceq in PDMS near the pore and c∞ near the sample's edge.…”

“…about 1 MPa. This explains why the individual large pores in PDMS observed by Milner et al 10 and Bruning et al 12 reopen after cavitation at pcav ≈ -1.4 MPa.…”

“…Because of the small pore size, the optical microscopy doesn't allow to determine precisely the nature of the cavitation (liquid or "solid" one). The results of Bruning et al 12 on millimeter-scale pores in PDMS demonstrate that the cavitation occurs in the water phase and the pore reopening happens about 0.1 ms after cavitation. We did not observe any correlation between different cavitation events: reopening happened some time after the complete shrinkage of a given pore, with no obvious trend.…”

“…This stage is followed by the release of the (negative) pressure inside the pore and by an expansion of the pore back to its initial size and shape. By tackling the bubble expansion velocity during this last stage, Bruning et al estimated 12 the cavitation pressure pcav ≈ -1.4 MPa, which absolute value is much lower than 20-30 MPa found for water in rigid monodisperse pores by Vincent et al 13 and synthetic trees by Wheeler et al 14 . This difference was explained by the highly hydrophobic nature of the PDMS which favors heterogeneous bubble nucleation.…”

“…The number of creases depends on the surface energy of the pore and on the high-strain mechanical properties of the elastomer. The next drying stage which takes place at a certain moment before or after creasing is cavitation of the water it contains 10,12 (i.e. nucleation of a water vapor bubble).…”