Davies, Gareth Bowen; (1993) A study of the application of ultrasonic standing waves to the segregation of fine biological particles from liquids. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes research to evaluate the application of megahertz (1 to 10 MHz) ultrasonic standing waves to the segregation and separation of fine biological particles, in the size range of 0.1 to 10 μm, from liquids. Research has focused on the development of an alternative separation technique through the ability to selectively manipulate delicate, highly hydrated particles typical of many biological process streams where the sedimentation characteristics of the particles preclude traditional centrifugation-based separation methods and the requisite for non-invasive in line processing rules out filtration. A survey of both acoustic and ultrasonic research concentrating on the application of ultrasonic energy to processes involving biological particles has been carried out. An in-depth analysis of the theories of ultrasonics in relation to the stated aims of the work is presented in which the mechanisms controlling the migration of fine particles under the influence of a megahertz frequency standing wave field are discussed. Results of investigations to determine the feasibility of concentrating micron-sized particles in a standing wave field arc presented. These confirm that the small-scale separation of biological particles is achievable. The subsequent design of an experimental separation device and detailed experiments to elucidate the parameters of importance in determining the segregation of biological particles from liquids using this apparatus are described. Ultrasonic power input and fluid velocity were found to be the most critical process parameters and operational constraints as functions of particle size and ultrasonic frequency were identified. The design and development of a novel laser scanning technique for the monitoring of the migration of particles in an ultrasonic standing wave field is presented. Data obtained using this equipment has been used when discussing the design of large-scale continuous solid-liquid separation devices. Details of an ultrasonic system for the non-invasive, in-situ sample preparation of material for dynamic laser light scattering analysis of particle size distributions in the monitoring and control of bioprocesses are presented together with data from experimental trials. Results showed this to be a promising method for rapid and controlled sample preparation and well suited to handling process streams containing heterogeneous particle sizes. The thesis concludes by giving consideration to the necessary future work and to the application of the techniques described in the thesis to relevant biological separation problems.

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