Use of radiation pressure to move small particles in an air flow-through resonator

It has been demonstrated many times in the past that particles suspended in a fluid medium can be manipulated with radiation pressure. Resonant cavities are often used to achieve large acoustic amplitudes. While most investigations have used water as the fluid medium, few have reported using a gas such as air. Little is known about the effect of openings, required for mean fluid flow through the cavity, on acoustic resonant amplification. Finally, few investigations report actual sound pressures that can be achieved, and correlate them to the movement of the particles. We present experimental measurements that quantify the ability of a flow-through resonator to move small water particles in air as the flow velocity and sound pressure levels are changed. The small water particles were generated with an ultrasonic humidifier. A commercial electrostatic transducer was employed to excite a one-wavelength longitudinal resonance of the cavity at a frequency near 60 kHz. Preliminary results show that sound pressure levels in excess of 150 dB re: 20 μPa can be attained, and significant particle movement is observed at flow velocities up to 11 cm/s. Observed particle movements will be compared to simple theoretical predictions based upon measured sound pressure levels and flow velocities.