Heterogeneous vapor bubble nucleation on a rough surface.

Vapor bubble nucleation on a microrough surface wetted by a volatile, incompressible liquid has been analyzed. The work of formation of a critical nucleus in a surface cavity has been evaluated. Concave sites of a surface with negative curvature can decrease the height of the activation barrier for the formation of a critical nucleus. The average density of the nucleation sites has been evaluated for a surface whose (small) deviation from a plane is specified by a Gaussian random function.

Investigation of noninertial cavitation produced by an ultrasonic horn.

This paper reports on noninertial cavitation that occurs beyond the zone close to the horn tip to which the inertial cavitation is confined. The noninertial cavitation is characterized by collating the data from a range of measurements of bubbles trapped on a solid surface in this noninertial zone. Specifically, the electrochemical measurement of mass transfer to an electrode is compared with high-speed video of the bubble oscillation. This gas bubble is shown to be a "noninertial" event by electrochemical surface erosion measurements and "ring-down" experiments showing the activity and motion of the bubble as the sound excitation was terminated. These measurements enable characterization of the complex environment produced below an operating ultrasonic horn outside of the region where inertial collapse can be detected. The extent to which solid boundaries in the liquid cause the frequencies and shapes of oscillatory modes on the bubble wall to differ from their free field values is discussed.