Christopher J. B. Vian

Multiple observations of cavitation cluster dynamics close to an ultrasonic horn tip

Bubble dynamics in water close to the tip of an ultrasonic horn (∼23 kHz, 3 mm diameter) have been studied using electrochemistry, luminescence, acoustics, light scattering, and high-speed imaging. It is found that, under the conditions employed, a large bubble cluster (∼1.5 mm radius) exists at the tip of the horn. This cluster collapses periodically every three to four cycles of the fundamental frequency of the horn. Following the collapse of the cluster, a short-lived cloud of small bubbles (each tens of microns in diameter) was observed in the solution. Large amplitude pressure emissions are also recorded, which correlate temporally with the cluster collapse. Bursts of surface erosion (measured in real time using an electrochemical technique) and multibubble sonoluminescence emission both also occur at a subharmonic of the fundamental frequency of the horn and are temporally correlated with the bubble cluster collapse and the associated pressure wave emission.

Opto-Isolation of Electrochemical Systems in Cavitation Environments

An electrochemical technique that can detect inertial cavitation within an ultrasonic reactor is reported. The technique relies on the erosion and repassivation of an oxide covered electrode (specifically aluminum). The sensitivity of the technique (<46 fg per erosion event) is significantly greater than normal weight loss measurements. A novel opto-isolation system is discussed which enables the electrochemical measurements to be undertaken within an earthed metallic container. Events detected in this manner are reported and compared to the noise in the absence of appropriate isolation. This system is combined with a multichannel analyzer to map the erosion/corrosion activity within an operating ultrasonic bath.