Dynamic self-assembly of microparticles in rotating drops under acoustic levitation*

Atmospheric aerosol such pollen, volcanic ash, and dessert dust critically impacts human health and the environment. One of natural process for cleaning up aerosols and air pollutants is rain. Raindrops absorb aerosols as they fall through the air, yet release aerosols back to atmosphere as they evaporate. However, it remains unclear how particle dynamics within a droplet affect the production of aerosols during evaporation. Using acoustic levitation, we experimentally investigate the collective dynamics of glass microspheres inside rotating, evaporating drops. The levitated drops can easily rotate at rates up to 30 Hz through feedback with the acoustic field. We find that microparticles exhibit distinct self-assembly dynamics, from "bands" along the equator of the axis of rotation to "sea ice" accumulated at the north and south poles, to "conveyor belts" of particles transported along the drop surface. To further understand these phenomena, we perform numerical simulations of particles in a rotating fluid with centrifugal force, Coriolis force, gravity, buoyancy, Stokes drag, and hydrostatic force due to centrifugal pressure gradient. Simulation results show that different dynamics of microspheres are determined by the angle between gravity and angular velocity, and the density difference between microparticles and water.