Leaky wave coupling strength prescribes rigid kinematics and acoustic streaming in superhydrophobically stabilized droplets.

Acoustofluidics at micro- to nano-scales opens new doors to lab-on-a-chip technologies as it enables the introduction of chaotic advection and turbulence into fluidics systems in which low Reynolds number flows dominate. This technology relies on surface acoustic waves (SAWs), generated on piezoelectric substrates like lithium niobate, propagating along the surface and emitting energy into the fluid domains they encounter. However, the underlying physics is often obscured as existing theories fail to explain the complex, enigmatic behaviors observed at these extreme spatiotemporal scales. In this work, we demonstrate an easy-to-replicate system involving traveling SAWs introduced into a well-understood geometry—a spherical droplet stabilized on a superhydrophobic surface—through a microscopic circular defect mediating the transduction of the acoustic waves at the fluid-substrate interface: from internal streaming to bulk kinematic motion of the droplet, with interpolated behaviors marrying these extremes. We show that this continuously varying behavior is a function of specific system geometry and is independent of input power. Furthermore, we perform numerical simulations of the droplet's acoustic field to complement analysis and experiment, providing additional information to help understand the resulting hydrodynamics.