Acoustofluidic spin control for 3D particle manipulation in droplets

The rotation of objects and corresponding dynamic systems plays a critical role in applications ranging from microscale droplet-based biochemical assays to nanoscale fluid transport and targeted drug delivery. However, directly observing and controlling these rotational phenomena across these different scales remains a challenge. Here, we introduce an acoustofluidic spinning control method that dynamically guides particles into three-dimensional, periodic spatial patterns within a droplet. Using surface acoustic waves, we induce internal streaming that generates centrifugal forces counteracted by surface tension, leading to the formation of rotating Stokes waves along the droplet's equator. We show that fluid motion inside the droplet couples with these rotating waves, giving rise to a controllable superimposed helical particle orbit. These findings provide a platform for controlled rotational flows with potential applications in droplet-based microfluidics, biochemical processing, and tunable particle transport in lab-on-a-chip systems.