Investigation into use of surface waves on bubbles walls to measure surface tension and to conduct cleaning

Abstract

This thesis conducts two experimental investigations associated with the generation of surface waves (the most famous of which is the Faraday wave) on the surface of an underwater gas bubble in response to insonification by an appropriate sound field. The first of these is the measurement of surface tension. The surface tension of a liquid is an important parameter for estimating and analysing the processes that happen on the air/liquid interface, such as the air/sea gas exchange. Current methods of measuring surface tension concentrate on measuring its value at the top flat air/liquid interface. However, in cases where bubbles mediate oceanic processes (such as their contributions to air-to-sea transfers of mass, energy, and momentum), the value of surface tension that is needed (e.g., for placement in models of the evolution and persistence of sub-surface bubble clouds) is the instantaneous value on the bubble wall, as it moves through the ocean and potentially collects surface-active species onto the bubble wall. This thesis introduces a method of estimating the value of this in situ surface tension, by insonifying a bubble and observing the onset of Faraday waves on a bubble wall. This new method was compared with a traditional ring method in various scenarios. Whilst the threshold for the generation of the Faraday wave provides a clear indicator to measure the surface tension (as described above), once they have been established the Faraday waves induce other effects, and these were also investigated in this thesis. When a bubble undergoes surface wave motion, it generates high local shear and convection in the liquid close to the bubble wall, and so produce a cleaning effect, and alters the way material deposits onto surfaces. This effect is used by a novel ultrasonic cleaning device, which unlike the traditional ultrasonic cleaning bath that relies on cavitation and shock waves, has many advantages. The second part of this thesis presents an application of conducting cleaning by using surface waves on bubble walls. The thesis closes with consideration of improvements and future work in these two applications of the acoustic stimulation of surface waves on bubbles walls.

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ORCID for Timothy Leighton: orcid.org/0000-0002-1649-8750

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