Explosionlike boiling of a viscous liquid in a high-intensity ultrasonic beam

The effect accompanying the propagation of high-intensity focused ultrasound in a viscous liquid, namely, medical gel, is studied. It is found that, under the action of continuous focused ultrasound (with an intensity of about 13000 W/cm2), in the area close to the waist of the caustic of the ultrasound beam, gas bubbles 10–15 mm in diameter periodically arise in an explosionlike way and then collapse with a lifetime of about 2 ms. The emergence of bubbles is accompanied by short pops with a duration of about 1 ms. An increase in the sound intensity leads to an increase in the frequency of the bubble generation (a decrease in the period from several seconds to fractions of a second) with practically the same shape of formations and the same dynamics of their development and collapse. Theoretical estimates adequately describe the size of the bubbles. The comparison of the effects produced by an intense focused ultrasonic beam on a gel and on biological tissue shows that the development of caverns (cavities) in tissues under the action of intense focused ultrasound is the manifestation of the effect of explosionlike boiling of the liquid.