Soft cavitation in colloidal droplets.

When a pure droplet evaporates inside an elastic medium, two instabilities are typically observed. As the droplet shrinks, the elastic medium needs to deform and elastic tension builds up. At a critical strain, accumulated tension at the gel-droplet interface is released by developing creases. The droplet keeps shrinking beyond this point, pulling the elastic network and therefore decreasing the pressure in the liquid phase. This drives the liquid phase into a metastable state, and leads to the second instability: the nucleation of a vapour bubble in the liquid phase by cavitation. These instabilities consistently occur in the described order whenever a pure liquid (water, in this case) is used. The presence of colloidal particles inside droplets is common both in vitro and in natural environments, and they can change such phenomenology significantly by stimulating cavitation events before any creasing instability. In this work, we study the role of colloidal particle size and concentration on the early inception of cavitation in water droplets in an elastic medium. Our results reveal an unexpected dependence with the particle size and with the size distribution of the colloidal particles. Given the simplicity and reliability of the system and preparation, the method described here could be eventually used to measure tensile strengths of particle solutions with accuracy.

Blast wave attenuation in liquid foams: role of gas and evidence of an optimal bubble size.

Liquid foams are excellent systems to mitigate pressure waves such as acoustic or blast waves. The understanding of the underlying dissipation mechanisms however still remains an active matter of debate. In this paper, we investigate the attenuation of a weak blast wave by a liquid foam. The wave is produced with a shock tube and impacts a foam, with a cylindrical geometry. We measure the wave attenuation and velocity in the foam as a function of bubble size, liquid fraction, and the nature of the gas. We show that the attenuation depends on the nature of the gas and we experimentally evidence a maximum of dissipation for a given bubble size. All features are qualitatively captured by a model based on thermal dissipation in the gas.