The Dynamics of Fluid Droplets in Acoustic Traps*

Acoustically levitated particles constitute wave-matter composite systems in which waves exert forces on objects, the objects scatter the waves, and the scattered waves mediate interactions that organize the objects. This interplay of influences causes solid spheres to organize themselves into floating monolayer crystals within the nodes of acoustic standing waves. The behavior of wave-matter composites becomes substantially more complex when the levitated objects are deformable, as in the case of acoustically levitated emulsion droplets. Compliant droplets couple more strongly than solid spheres to solenoidal components in a sound wave's velocity field and therefore can spin rapidly. The resulting hydrodynamic coupling contributes to new dynamical states. This talk briefly reviews the principles of acoustic radiation forces and combines acoustokinetic analysis with computer simulation to explain recent observations of anomalous behavior in acoustically levitated droplets.