A novel high-speed AC impedance method for monitoring transient cavitation

Abstract

This thesis details an investigation of bubble dynamics using a set of novel electrochemical impedance measurements with the ability to monitor the motion of bubbles during cavitation. This method was able to report dynamic information about the motion of bubbles over an electrode surface with a time resolution of 2 μs. The results produced from these impedance studies are supported by other techniques used to characterise cavitation, such as acoustic output measurements and high-speed imaging. Cavitation was generated inside a suitable electrochemical cell using a variety of sound sources. The different acoustic environments were chosen so that different types of bubble activity could be investigated.
Initial investigations were carried out inside a cylindrical resonance chamber, chosen for its high symmetry and good resonance properties. A solid reflector (aluminium or steel) inserted inside the cylinder formed a mobile solid/liquid interface. We were able to show that acoustic cavitation occurred at the reflector surface when regions of high acoustic pressure were formed at the electrode surface. Electrochemical data was recorded by an electrode embedded into the reflector surface. The data recorded was used to quantify the amount of cavitation activity as a function of reflector position and also to indicate mechanistic detail of the bubble growth and collapse under cavitation. Complementary studies were carried out using acoustic measurements as the bubbles acted as secondary sound sources. It was also shown that changing the reflector material changed the acoustic properties of the chamber.
Further investigations were performed using an ultrasonic horn as a piston-like emitter (PLE) to generate acoustic cavitation. The PLE was positioned over the surface of a highly polished electrode. This electrode was used to monitor the motion of cavitation bubbles electrochemically using the novel high-speed AC impedance technique. These investigations were able to interrogate different types of cavitation bubble motion to that seen inside the cylindrical cell. The electrochemical studies revealed details of the oscillatory motion of the bubbles under the influence of the PLE. Complementary evidence for the bubble activity observed by the impedance results was provided by high-speed imaging of the electrode surface.

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ORCID for Peter R. Birkin: orcid.org/0000-0002-6656-4074

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