Collective particle dynamics in rotating drops under acoustic levitation

Atmospheric aerosol such pollen, volcanic ash, and dessert dust critically impact human health and the environment. One of the natural processes for cleaning up aerosols and air pollutants is raining. Raindrops absorb aerosols as they fall through the air, but 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 two distinct collective dynamics: they can accumulate along the equator of the axis of rotation to form a "tropical belt", or they can also accumulate near the poles to form "polar glacier". To further understand these dynamics, we developed a model of the motion of microparticles in a rotating fluid with centrifugal force, Coriolis force, buoyancy, Stokes drag, and hydrostatic force due to centrifugal pressure gradient. The model shows that different dynamics of microparticles can be determined by a simple stability criterion that compares the relative magnitudes of centrifugal acceleration and gravity, and the angle between them. We also show that the shape of the aerosols generated by drop evaporation can be influenced by these particle dynamics.