Abstract
AC electro-kinetics involves the manipulation of particles in non-uniform alternating electric fields. Micro-systems consisting of microelectrode arrays generate the field non-uniformities which can produce several observed phenomena, including lateral particle displacements (dielectrophoresis), particle rotation (electro-rotation) and induced fluid flow (electro-hydrodynamics). These effects are influenced primarily by the dielectric properties of the particle and the suspending medium, the frequency of the applied electric field and the electrode designs. In this thesis, novel processes involving the use of ac electro-kinetics in microsystems are developed. Using a quasi-three dimensional dot micro-system, dielectric properties of dielectrophoretically manipulated bioparticle suspensions are shown to be determined through the use of image analysis. Significant factors contributing to the speed and accuracy of the process were found to be dependent on the particle concentration and electrode dimensions. These dependencies showed a phenomenon which has hardly been used before in ac electro-kinetic particle characterisation, the process of spontaneous particle re-dispersion in the micro-system. The first integrated micro-system coupling ac electro-kinetic particle manipulation and piezoelectric mass detection simultaneously is described. The electrode design used enhances particle collection on to the surface of the electrode, through induced fluid flow, where detection occurs. The dynamic response of the system has shown that nano-particles are more suited for this system, with the rate and amplitude of detection shown to correspond to the concentration of particles. Pre-concentration of biological particles in micro-fluidic systems using dielectrophoresis is a useful upstream process which can be employed prior to characterisation or detection processes. However, exposure of biological particles to high field gradients can lead to cellular damage. A comparison of dielectrophoretic and electro-hydrodynamic forces as a means of particle retention in micro-fluidic flow has shown that particles can be trapped at different electrode regions. These regions correspond to the high and low electric field gradients in the electrode vicinity. At increased flow-rates, hydrodynamic forces are seen to have a significant influence on the trapping efficiencies using electro-hydrodynamic forces. Although, at lower flow-rates the number of viable of cells eluted from the micro-fluidic chamber is significantly greater than those exposed to conventional dielectrophoretic forces.