Effects of micro bubbles oscillation for increase of acoustic streaming

The effects of the existence of micro bubbles in water upon increasing the acoustic streaming velocity are experimentally discussed The water tank for measurement is divided into hire areas: the measuring area and the bubble generating area. The acoustic streaming velocity is measured at the point 110mm away from the transducer on the sound axis by LDV (Laser Doppler Velocimeter). Acoustic streaming is generated by continuous ultrasonic sound transmitted by PZT transducers whose diameters are 15mm and resonant frequencies are 2.98MHz or 3.46Mhz. Sound pressures of ultrasonic sound are set to 80KPa, 160kPa and 240kPa at the last peak position on the sound axis. Micro bubbles are generated by extracting water from the bubble generating area with a syringe mechanism, decreasing the pressure of water down to 6.4% below static pressure. Average radius of micro bubbles is approximately 220 mu m. The experimental results clearly show that the acoustic streaming velocity is increased from 9mm/s to 135mm/s when micro bubbles exist The mechanism for increase of acoustic streaming is discussed on the point of view of the nonlinear effect of the bubble vibration.