Microparticle trapping at the focal spot using a dual-focus annular CMUT array on a free surface

Abstract

This paper presents a dual-focus annular capacitive micromachined ultrasonic transducer (CMUT) array designed for acoustic manipulation of microparticles floating on a fluid-air interface. The CMUT array was fabricated using a micro-electro-mechanical systems (MEMS) process incorporating double thermal oxidation and silicon direct bonding to ensure structural uniformity and high acoustic performance. Electrical and acoustic characterizations present consistent channel behavior, with a center frequency of 3.6 MHz and a fractional bandwidth of 47.3%. The array's annular configuration enables two distinct focusing modes through phase delay control: a single-focus mode and a dual-focus mode with symmetric foci. The acoustic radiation force (ARF) acting on particles was simulated using the Rayleigh-Sommerfeld integral and Gor'kov potential including surface reflection effects. Experimental results showed that the ARF field in both modes enabled stable trapping of hollow glass microspheres (106-125 mu m) at the focal region. Furthermore, dynamic phase delay adjustments laterally shifted the focal position, enabling directional manipulation of trapped particles. Two manipulation schemes were demonstrated: one with round-trip particle motion via symmetric phase modulation, and the other with one-way transport via progressive phase shift. The measured particle trajectories exhibited an agreement with the simulated focal movements, highlighting the system's capability for precise and reconfigurable particle control. These findings suggest that CMUT-based dual-focus arrays provide a viable approach for contactless and programmable manipulation of surface-floating particles, offering potential for applications in cell patterning, soft microrobotics, and surface-based microfluidic control.