Control of a standing wave field using a line-focused transducer for two-dimensional manipulation of particles

Control of the positions of particles using acoustic radiation pressure in water was studied to develop a noncontact micromanipulation technique. In this paper a method to transport the particles two-dimensionally using an ultrasonic standing wave field between a line-focused transducer with multiple electrodes and a reflector placed at the focal line is described. When alumina suspension of mean diameter 16 mu m was poured into the standing wave field of 2.1 MHz, the particles were trapped and agglomerated at sound pressure nodes existing at half wavelength on the sound beam axis near the reflector. Changing the frequency alters the wave-length and hence the interval of agglomeration. Therefore the trapped particles were transported along the sound beam axis. When the next electrodes were driven, the standing wave field shifted laterally and the trapped particles moved to the corresponding nodal points. Thus two-dimensional transportation was realized using the line-focused transducer. A sound field generated by the line-focused transducer was discussed based on numerical calculations to design an optimum shape of a transducer for manipulation.