Experimental Validation of Lateral Pattern Interference Radiation Force Using a Dual-Focus CMUT for Microparticle Manipulation

A dual-focus annular capacitive micromachined ultrasonic transducer (CMUT) array was developed to experimentally validate lateral pattern interference radiation force (PIRF) for microparticle manipulation. The array emulates two interfering sources within a single device by applying phase inversion across the central axis of its multi-channel aperture. The CMUT was fabricated via micro-electro-mechanical systems (MEMS) processing with thermal oxidation and silicon direct bonding (SDB), and electrical/acoustic characterizations demonstrated uniform, phase-programmable focusing. A numerical framework combining the Rayleigh–Sommerfeld diffraction integral with the Gor’kov potential—while modeling fluid–air reflection using a virtual source—predicted lateral acoustic radiation force (ARF) distributions for single- and dual-focus operation. Experiments using floating hollow glass microspheres verified lateral PIRF: particles concentrated at the single focus and were simultaneously trapped at two symmetric focal spots in the dual-focus mode, with measured inter-spot spacing (~800 µm) consistent with predictions. These results demonstrate that lateral PIRF can be realized and anticipated a priori within a single CMUT platform, enabling compact, programmable, and non-contact particle manipulation.