Use of acoustic radiation pressure to concentrate small particles in an air flow

Experiments have shown that acoustic radiation pressure can be used to concentrate biological particles in a small section of a moving liquid, where the particles can be removed. We report measurements, on the use of acoustic radiation pressure to concentrate small particles in a moving air stream. Small water droplets, approximately 20μm in diameter, were entrained in an air stream that passed through a rectangular channel 29.72 mm wide by 5.59 mm deep. An electrostatic transducer, located on one wall of the channel, was used to excite a one-wavelength resonance in the depth dimension of the channel at a nominal frequency of 61 kHz. Sound pressure levels of 154 dB re 20μPa in the channel were achieved. Flow visualization was used to observe the motion of the droplets, and laser doppler velocimetry was used to measure the velocity of the airflow. The airflow velocity ranged from 4 to 11 cm/s. With some exception, the particle motions were observed to conform to predictions based upon a King´s law model for radiation force.