The electrostatics of dispersed systems

Abstract

This thesis essentially records three theoretical and experimental studies into the behaviour of dispersed systems in intense electrostatic fields. The intent is to highlight common themes and key differences applicable to dispersions of solids in gases, liquids in gases and liquids in liquids. Electrostatic effects can be used to control the behaviour of such systems, to provide the energy to generate such systems or indeed to alter such systems so they lose their defining properties. This field of engineering, although specialised, has enormous commercial worth and all three studies were sponsored by companies interested in bringing novel processes to the marketplace. Thus, a high level of innovative work is covered within each study area. This studies are i) on a novel approach to electrostatically charge powder for use in powder coating applications, ii) into a device for nebulizing pharmaceutical material for direct inhalation by patients and iii) into the destabilisation of water in oil emulsions applicable to the oil industry. On the face of it, such diverse aspects of technology appear irreconcilable under the banner of a thesis for a higher degree but there are a number of relating themes which are woven throughout which tie the thesis together. Principally, there is electrostatics and more formally, the electrostatics of dielectric material. Electrostatic stresses are effective at opposing the more familiar stresses of surface tension, aerodynamic drag or indeed gravity when they are present on bodies of high surface to volume ratio. All dispersed systems are characterised by the change in nature of the system across an interface. The interface defines the boundary of each phase of the system. If this interface has a very high radius of curvature, then the electrostatic stresses that can be set up there are often of the same order as the other defining forces of the system. The electrical properties of the phases of the system are the key to defining how successfully these stresses can be applied and to what happens to the system when they are applied. For example, in trying to atomize a liquid into a gas, the conductivity of the liquid defines the dynamics of the charge migration through the bulk to the surface from where the droplets are emitted. The dielectric strength of the surrounding gas controls the maximum electrostatic stress that can be maintained and hence the effective work done by the applied electrostatic field. In this case, gas ionization acts like a safety valve, channeling the charge carriers away from the liquid surface before they can disrupt it (alas!). Contributions made to knowledge in this field include: i) experimental proof of the concept of inductively charging solid resinous powders for powder coating objects, a patent for which was granted to the author and collaborators, ii) a model which considerably aids technologists in predicting the critical parameters to control steady atomization of a liquid into a tightly defined size band, and iii) contributions to the experimental database for the droplet growth rate of the dispersed phase of a flowing water-in-oil emulsion exposed to an ac electrostatic field. The latter work helped build upon the University of Southampton's long line of research in this area and a recent patent was granted to the sponsoring companies who plan to develop commercial devices derived from the research.

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