Abstract:
A technique is presented in which small concentrations of a polymer additive are used to suspend air bubbles in a ``gel'' to form stable bubble clouds. Bubble clouds with volume fractions as low as 0.04 size distributions determined by using a CCD camera and image processing software, show clouds with most of the bubbles less than 100 mm in radius and means near 60 mm. A theoretical framework for the sound speed and attenuation as a function of the bubble number density distribution, which includes the shear-thinning properties of the gel, is presented. This theory indicates that the acoustics of bubble clouds in this gel are, over a large range in frequencies, almost identical to that of bubble clouds in pure water. Image processing along with low-frequency acoustic measurements performed in a long steel tube clearly show that only small changes in volume fraction and bubble size distribution occur over a matter of days. Results are presented comparing the theory and measurements for sound speed and attenuation over a three-decade range of volume fractions. [Work supported by DARPA SBIR Contract No. DAAH01-95-C-R164.]