Tunable microscale assembly with ultrasound vortex tweezers

Ultrasound tweezers provide a powerful approach for the contactless manipulation of microscale objects, yet achieving structurally simple platforms capable of tunable particle assembly remains challenging. Here we introduce ultrasound vortex tweezers based on a Fresnel-spiral diffraction grating (FSDG) that converts an incident ultrasonic plane wave into a vortex beam carrying orbital angular momentum. The passive grating structure enables the generation of ultrasound vortex fields with tunable topological charge, producing ring-shaped trapping regions with adjustable spatial scales. Operating at 10 MHz, the system provides improved spatial confinement compared with conventional low-frequency ultrasound tweezers. Using this platform, we demonstrate controllable self-rotation of individual microparticles, tunable aggregation of multiple particles, and core-shell encapsulation in bidisperse particle systems. The rotational speed of trapped particles shows an overall increasing trend with increasing vortex order, while the aggregation and encapsulation processes can be regulated through vortex order, driving voltage, and actuation time. These results establish ultrasound vortex tweezers as a compact and versatile strategy for tunable microscale assembly, offering new opportunities for high-precision ultrasound manipulation in materials engineering and biofabrication.