Fluid Droplets in Acoustic Force Fields Probed with the Immersed Boundary Method

Acoustic trapping relies on the forces and torques exerted by sound waves to localize granular materials and transport them along three-dimensional trajectories. The composite wave-matter system can display complex emergent dynamical behavior arising from forces mediated by scattered waves. This phenomenology becomes substantially richer when the particles themselves can be deformed by acoustic forces, as is the case for viscous fluid droplets in acoustic traps. We demonstrate how to model acoustic forces at the surface of acoustically levitated drops using the immersed boundary method (IBM), and how those forces drive complex shape deformations and near-field fluid flows which modify single- and multi-droplet dynamics. These simulations cast light on such outstanding anomalies as the tendency of steadily levitated droplets to spin and spinning droplets to form chains.