Acoustically levitated granular liquid: pendant droplets, pinch-off, and impact

Investigations of the behavior of small droplets have contributed greatly to our understanding of both simple and complex fluids. When solid particles are acoustically levitated in air, they can effectively form a 2D liquid. They experience both anisotropic interparticle attraction from sound scattering and repulsive microstreaming flows. These particles form quasi-2D raft structures, and if the particles are below a certain size, they sit at a finite distance from each other instead of making contact. The strength of the sound field controls hydrodynamic fluctuations and acts as an effective temperature in this underdamped system. This results in a material whose properties can vary from those of a soft, crystalline solid to a more active liquid. Here, we perform experiments on mesoscale 2D 'droplets' of this granular fluid where they are significantly perturbed, analogous to testing on molecular liquids. These include pendant drop tests where a pinned droplet is attracted from a distance to a larger object and acoustic scattering acts as 'gravity,' as well as observations of droplets 'falling' and impacting surfaces. We examine how parameters such as effective temperature and droplet size affect the mechanical properties of this granular liquid.