Modelling of a microfluid ultrasonic particle separator

Abstract

Particles within an ultrasonic standing wave experience an acoustic force causing the particles to move to certain positions within the acoustic field. This phenomenon can be used to manipulate particles and so provides a means to separate, concentrate or trap particles, cells or spores. The work described is applied to a micro-engineered flow-through device for processing small samples and incorporates a fluid filled chamber of depth typically between 100 and 200?m, and therefore approaches microfluidic dimensions. The successful design and subsequent performance of such devices rely on the predictability of particle trajectories which are influenced predominantly by acoustic and fluid flow fields. Therefore, the majority of this research seeks an understanding of the nature of these fields and, in turn, reliable simulation of particle trajectories. Computational fluid dynamics (CFD) modelling is used to develop a robust 2-dimensional model of the device’s microchannels and is used to predict the presence of eddy regions, associated with the etch fabrication techniques, which are likely to disrupt the separation process. Based on a geometric study, simulations and subsequent test results on a fabricated device have revealed geometric modifications which minimise these eddy flows and promote the existence of laminar flow within the main channel of the device. Finite element analysis (FEA) provides a method to investigate the 2-dimensional characteristics of the acoustic field and reveals variations in acoustic pressure across the width of the device, giving rise to lateral radiation forces frequently reported in similar ultrasonic devices. This work investigates acoustic enclosure modes in 2 or 3-dimensions as a possible cause of these lateral variations, with modelled results matching well with experiment. A particle force model has also been developed which predicts the motion of particles through the device, and by which concentration and separation performance may be calculated. This tool is used to investigate acoustic design, operating conditions and separation performance for both the micro-engineered device and a device based on a quarter-wavelength, providing valuable insight into various trends observed. The novelty in this work is the application of macro-scale numerical techniques to microengineered ultrasonic particle manipulators and the execution of an extensive analysis of the design and operation of such devices. These analyses have demonstrated, and therefore have explained, various phenomena associated with the fluid and acoustic fields, and how they influence particle separation performance. The development of similar devices can be aided by the use of the numerical simulation methods featured throughout this thesis.

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