No abstract

A new method to detect the uncompensated resistance, the capacitance and the Faradaic current at an electrode exposed to ultrasonic cavitation is presented. The method enables these parameters to be resolved with a 2 microsecond resolution and relies on the detection of the impedance of an electrode recorded as a function of time with a suitable AC excitation signal (here 500 kHz). Data obtained from an aluminium electrode, held under potentiostatic control, is used to illustrate the technique with particular relevance to the effects of cavitation bubbles generated by ultrasound. Analysis of the data recorded shows that the cavitation bubbles form close to the surface of the electrode and collapse, causing damage to the passive film formed at the aluminium surface. The capacitance, uncompensated resistance and Faradaic signals are used to explore the dynamic processes and show expansion and collapse of bubbles prior to erosion/corrosion. The close proximity of the bubbles to the surface is deduced from the reductions in capacitance and increases in resistance prior to bubble collapse, which is then shown to trigger the onset of a Faradaic signal, thus confirming the erosion/corrosion mechanism previously assumed.

A technique that monitors the impedance of a 250 µm diameter aluminium electrode as a function of time with a 2 µs resolution as sand particles are impinged on the solid/liquid interface is reported. The detection of individual particles as they approach an electrode, before any erosion/corrosion was registered, is demonstrated. This study shows that at least two types of erosion mechanisms are possible; direct or 'primary' impact and tumbling or 'scrape' events.The primary impact events are correlated to the acoustic emission from the environment which is shown to be significant for these events, whereas scrape events appear to produce far weaker acoustic emission signatures under the conditions employed. The velocities of the particles are reported and are of the order of 6-8 m s -1 at the jet mouth. However, high-speed imaging of the particles as they strike the substrate indicates a significant deceleration prior to impact and an order of magnitude reduction in kinetic energy compared to that as it exits the jet.