Acoustically Levitated Granular Matter

From protoplanetary disks to the rings of Saturn, many astrophysical systems are composed of granular matter. Forces that are negligible in earthbound experiments --- such as electrostatic attraction and self-gravity --- play an important role in driving the assembly and dynamics of such matter. These dynamics often take place over millions of years, and while numerical models can be constructed to probe their underlying physics, there remains a need for tabletop experiments that controllably test models of granular materials in low gravity environments. In this talk, I will outline a new direction for tabletop experiments that aims to probe and model these interaction forces in astrophysical granular matter.  We levitate particles in an ultrasonic standing wave to compensate forces due to gravity, and enable substrate-free assembly and manipulation. We mimic the effects of gravitational attraction via secondary scattering, which generates controllable attractive forces between particles. In addition, the small viscosity of air allows for the exploration of underdamped dynamics, where inertial effects are crucially important. I will discuss two areas in which it provides new insights into astrophysical phenomena: granular contact charging, and the rotational disruption of rubble-pile asteroids. Our results suggest a path toward the study of “tabletop asteroids", where astrophysical processes that take millions of years, on the scale of kilometers, can be observed and controlled in minutes.