Electrochemical investigations of acoustically driven gas bubbles

Abstract

A bubble driven to oscillate by a sound field can be detected electrochemically via mass transfer enhancement of species to electrode surface compared to that recorded in the bulk solution in the absence of sound irradiation. Two of the main oscillatory modes of an oscillating bubble (i.e. fundamental and subharmonic motion) are detected using a novel electrochemical technique for the first time.  Subharmonic motion is found to occur for a certain range of frequencies and pressures, and a threshold pressure amplitude exists at which this motion is detected.  It is found that the onset of Faraday waves is responsible for subharmonic motion and an acoustic model is derived to confirm this. However, fundamental oscillatory motion of the bubble can also be detected over a wider frequency range using the electrochemical methodology presented.  It is shown that the extent of mass transfer enhancement is smaller when compared to Faraday wave excitation of the bubble wall.

The effect of surfactants on the threshold pressure amplitude for the onset of Faraday waves and the mode number of an oscillating bubble is explored, and the results obtained are compared to the developed model.  Prior to achieving steady state surface wave motion there is a ring-up period.  Understanding of this ring-up behaviour is at a very early stage.  This thesis introduces an electrochemical study of this phenomenon.

Electrochemical detection of both the fundamental and Faraday wave motion of an oscillating bubble is shown to have several useful applications. One such application would be in detection and sizing bubbles in a population.  Electrochemical detection of individual bubbles in a rising stream and Faraday wave motion of an oscillating rising bubble in a rising stream and Faraday wave motion of an oscillating rising bubble is explored extensively.  Another application would in the area of bubble growth by a process known as rectified diffusion. Bubble growth rates are determined and these are compared to a theoretical model.

Finally, electrochemical detection of Faraday wave motion is employed in the detection of trace species in solution. In this sequence of experiments a novel approach to hydrodynamic voltammetry is introduced.

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