Single-transducer tweezers based on bulk acoustic waves without standing waves

This work presents a physics-guided design, optimization, fabrication, and experimental validation of Fresnel-ring-based single-transducer acoustic tweezers capable of precise three-dimensional manipulation of microparticles in liquid without reliance on (or being hampered by) standing waves. A planar piezoelectric transducer patterned with concentric half-wavelength annular electrodes is engineered to generate three axially separated focal points whose constructive interference forms a spatially enclosed radiation-force potential well. By systematically optimizing the inter-focal spacing using Finite Element Modeling (FEM), we establish quantitative design rules that link the primary focal length, inter-focal distance, trapping-zone geometry, and particle-size selectivity. Hydrophone-based three-dimensional pressure mapping validates the simulated acoustic field and confirms the formation of a well-defined low-pressure region bounded by surrounding high-pressure lobes. Experimental results demonstrate robust, size-selective confinement of microspheres and stable trapping of large biological specimens, including a 700 μm zebrafish embryo, at user-defined axial distances from the transducer surface. The observed trapping behavior agrees with theoretical predictions based on acoustic radiation force scaling and streaming-induced drag. By explicitly correlating focal configuration, radiation potential landscape, and particle-size-dependent stability, this study establishes a scalable, microfabrication-compatible framework for single-transducer acoustic tweezers. The technology based on multi-focal acoustic tweezers enables programmable trapping size, trapping position, selective manipulation, and controlled particle lifting and ejection, providing a versatile platform for advanced BioMEMS applications and next-generation contactless micromanipulation systems.