Ultrasonic-induced phase transitions of micrometer-size droplets

Abstract

Droplets for various applications were generated by mixing albumin (bovine) with saline and a low boiling point liquid (dodecaflouropentane). The resulting emulsion contains small droplets with a lower size range on the order of a few micrometers. The type of mixing process was found to determine the range of droplet sizes. The albumin, as a surface active agent, is assumed to build a shell around the droplets and prevents coalescence, since mixtures without albumin did not produce stable droplets. Increases in temperature of the resting host fluid showed that droplets could be superheated well above their natural boiling point. It is assumed that this property is caused by an increase in pressure within the droplet due to surface tension. The superheated state could be overcome by the application of ultrasonic pulses and/or hydrodynamical action (flow). The presence of ultrasound contrast agents allowed ultrasonic-induced phase transitions at the natural boiling point. Droplets producing bubbles for transpulmonary applications have been studied previously. The interest here was primarily in larger bubbles for therapeutic applications and to serve as point beacons for aberration correction. [Work supported in part by PHS Grant No. R01 HL54201 from the National Heart, Lung and Blood Institute.]