The role of bubbles in the atomization of liquids and tissues

Abstract

Ultrasonic atomization, or the emission of droplets from a liquid exposed to air, has been studied for many decades. The most accepted theory of atomization, the cavitation-wave hypothesis, states that droplets are emitted by a combination of capillary wave instabilities and cavitation bubble collapses. Recently, it was shown that tissues could also be atomized and that the result of atomization was surface erosion. Using a high static pressure chamber, we investigated the role of bubbles in the atomization of tissues and liquids. A 2-MHz, aluminum-lensed transducer was focused at the surface of either water or ex vivo bovine liver. In water at 1200 W/cm2 (p + = 6.8 MPa, p- = 5.3 MPa), we found that atomization ceased at an overpressure of 6.9 MPa, yet droplets were again released when the static pressure was increased to 13.8 MPa. In tissue at a linear in situ intensity of 22 000 W/cm2 (p + = 67 MPa, p- = 16 MPa), we found that a small increase in the static pressure (1.4 MPa) produced a qualitative change in atomization and caused thermal denaturation of the fractionated tissue rather than ejection from the surface. [Work supported by NIH EB007643, NIH DK043881, and NSBRI through NASA NCC 9-58.]