Generation and maintenance of bubbles in small tubes by low-frequency ultrasound

A system has been developed to,generate and maintain a bubble population in nonscattering fluid media within a small 1.3-mm (i.d,) transparent silicone tube by use of low-frequency ultrasound at 28.8 kHz. The system uses a sequence of three different kinds of acoustic cavitation processes: transient, stable and cyclic. In the final step of the sequence, low-duty cycle (similar to 10%) and low-pressure (similar to 30 kPa) ultrasound is used to maintain a microbubble population whose mean diameter is similar to 12 mu m, which can be used as Doppler ultrasound scatterers. To understand and measure the behavior of bubbles for three different kinds of cavitation processes in the tube, a CCD imaging system and a synchronized short duration flash were used to capture movie frame sequences. Subsequent analysis of the frames enabled the following bubble parameters to be measured. size distributions, fractional volume concentration, flew characteristics, rise velocity, dissolution time, growth rate and various bubble formation processes. In addition, the cavitation threshold of distilled water inside the tube was found to be similar to 292+/-99 kPa. Techniques were also developed to identify successful bubble initiation and excitation by either detecting the harmonic components of the bubble emission power spectra or by detecting the power level of the Doppler spectrum. (C) 1999 Acoustical Society of America. [S0001-4966(99)06411-5].