Acoustically induced droplet rotation via wave–fluid-structure interaction for tunable microscale manipulation

Acoustic waves not only propagate through fluids but also actively interact with them, inducing dynamics through wave–fluid–structure coupling. In this work, we present our recent findings on how acoustic waves applied to sessile droplets can activate rotational fluid motion and direct particle transport through coupled acoustic–fluid–particle interactions. By launching surface acoustic waves (SAWs) asymmetrically at the droplet base, we generate a rotational Stokes flow that drives the entire droplet into a controlled spinning state. This acoustically induced rotation gives rise to helical particle trajectories, enabling robust and tunable concentration, sorting, and patterning of suspended micro- and nano- particles. Through a combination of experimental measurements and numerical simulations, we characterize how key parameters, including wave amplitude, frequency, droplet size, viscosity, and surface tension, govern the strength and pattern of the internal flow. Our findings highlight the interplay between acoustic energy input, fluid inertia, and capillary confinement in shaping droplet-scale fluid dynamics. Finally, we demonstrate the potential of this platform for on-chip applications such as rapid particle enrichment, localized biochemical assays, and droplet-based centrifugation. This work expands the understanding of acoustofluidic control and offers new routes for programmable fluid and particle manipulation in open microscale systems.