One-dimensional transportation of particles using an ultrasonic standing wave

One-dimensional transportation of particles using acoustic radiation pressure in water is studied in order to develop a non-contact micromanipulation technique. The radiation pressure traps particles suspended in water every half wavelength in a standing wave field and agglomeration occurs. It is hard to control the position of agglomerated particles using an ordinary plane transducer, because the trapping points spread in a plane and are sensitive to a slight change of the sound field. Using a concave transducer to generate focused ultrasound and placing a reflector at the focal point, the particles are agglomerated one-dimensionally along the sound beam axis in the standing wave field. Changing the frequency from 4 MHz to 8 MHz, trapped alumina particles of 16 μm in diameter were transported for 1.83 mm in maximum along the beam axis. Since the displacement is a result of accumulation of changes in the agglomeration interval determined by the acoustic wavelength, it depends on the location and larger displacement is attained at a farther distance from the reflector. The resolution of transportation depends on the frequency increment and submicron displacement is possible using sufficiently small frequency steps. The experiments showed that the direction of transportation was controlled using an appropriate frequency increment