Electrical Tuning of Focal Size with Single Focused Ultrasonic Transducer

This paper describes a single-element planar focusing ultrasonic transducer with electrically tunable focal size, through modifying the design of a self-focusing acoustic transducer (SFAT). The transudcer is built on a 1-mm-thlck lead zirconate titanate (PZT) with (1) Fresnel acoustic lens formed with annular rings of air cavities on the top and (2) patterned annular ring electrodes on the bottom. By controlling the number of Fresnel rings being driven from the center, we were able to tune the focal size between 371 and 866 μm, while keeping the focal length at 6 mm, with 2.32 MHz pulsed ultrasound. When tested as a droplet ejector, the transducer ejected water droplets with diameter between 294 and 560 μm (between 13.3 and 92.0 nL in volume), depending on which set of electrodes are actuated.