Microbubble Population Measurements Using Rectified Diffusion Caused Dispersion

Abstract

Decay times of 30 msec have been measured for microbubble populations repetitively grown in degassed water by bursts of 489-khz sound a few milliseconds long and of the order of 102 bars amplitude. The measurements were made by monitoring the differential phase shift of a pulsed low-amplitude sensing beam at two different times after the liquid through which it passes is perturbed by the 480-khz burst. The phase shifts were detected using a double balanced mixer and the sensing beam reference oscillator. The mixer output was then integrated during the time the sense burst was being received, the resulting integral was analog-to-digital converted, and then the appropriate differences taken digitally. The system was cycled 320 times for each measurement and digitally averaged. The differential velocity measurements were of the order of 2×10−2 cm/sec. The system noise level, including thermal shifts in the velocity, produced standard deviations in the values measured of less than 3×10−3 cm/sec. The resolution of the system was less than 1×10−6 cm/sec. [Work supported in part by the National Science Foundation.]