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 two areas: the measuring area and the bubble generating area. The acoustic streaming velocity is measured at the point 110 mm 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 15 mm and resonant frequencies are 2.98 MHz and 3.46 MHz. Sound pressures of ultrasonic sound are set to 80 kPa, 160 kPa and 240 kPa 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 μm. The experimental results clearly show that the acoustic streaming velocity is increased from 9 mm/s to 13.5 mm/s when micro bubbles exist. The mechanism for increase of acoustic streaming is discussed from the point of view of the nonlinear effect of the bubble vibration