Dielectrophoresis of sub-micrometre particles
The aim of this PhD project was to develop the technology of dielectrophoresis on the sub-micrometre scale and to use DEP to manipulate sub-micrometre particles and measure their dielectric properties. Of particular interest was the application of DEP to viruses, the largest of which is approximately 250 nm in diameter. A system for virus characterisation, identification and separation based on DEP would be a major milestone in this field of research, as well as having beneficial medical and biotechnological uses. Particles with a diameter between 1nm and 1m are referred to as Colloidal particles and the dynamics of their movement are complicated by the effects of thermal energy and Brownian motion. High electric fields are required to dominate these effects but signals with high potentials and high frequencies are difficult to generate. Semiconductor manufacturing techniques can be used to fabricate micro-electrode structures which can produce high electric fields from relatively low potentials. Lithography based manufacturing techniques were developed to produce suitable electrodes for dielectrophoresis on a scale small enough to manipulate sub-micrometre particles. Detailed electric field patterns were numerically calculated for these electrodes, so that the dielectrophoretic force could be simulated, predicted and compared with experimental measurements of particle movement. The dielectric properties of latex spheres with diameters from 93 nm to 557 nm were determined through observation and measurement of the DEP movement; new theories were postulated to account for the results which did not conform to accepted theories. A rod shaped plant virus, Tobacco Mosaic Virus (TMV) was also studied and its dielectric properties determined from the experimental results. TMV is 300 nm long with a cylindrical radius of 9nm, a shape of particle which is very different from a sphere and one which has not been studied by this method previously. An expression for the frequency dependent dielectrophoretic force on such a particle was derived and values of the dielectrophoretic force on the particle were measured and compared with the theoretical model.