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Title: Dielectrophoresis of colloids and polyelectrolytes
Author: Bakewell, David John Guy
ISNI:       0000 0001 3440 7974
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2002
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This PhD dissertation describes experimental and theoretical investigations on the dielectrophoretic movement of colloidal particles and polyelectrolytes suspended in aqueous solution. Dielectrophoresis (DEP) is the movement of polarisable particles in non-uniform electric fields according to their induced, or effective, polarisability. The colloidal particles used in experiments were fluorescently labelled 216 nm diameter carboxyl-modified polystyrene micro-spheres (beads) and the polyelectrolyte particles were fluorescently labelled 12 kilobase pair DNA plasmids with approximately 1 gm planar diameter. The dielectrophoretic force was generated by applying electrical alternating current (AC) potentials of varying frequency to micro-fabricated electrodes covered with low conductivity aqueous suspending media. The electrodes used for quantitative particle measurements were interdigitated Ti/Pd/Au electrode arrays (10 μm width and 10 μm gap) microfabricated on glass microscope slides using standard photolithography techniques. The frequency dependent effective particle polarisability, ap, is a key parameter in governing the dielectrophoretic force. Time domain dielectric spectroscopic measurements of solutions of DNA gave values of ap at 2 to 80x 10"31 (F m2), in the frequency range 12 MHz - 140 kHz. For latex micro-spheres, the DEP cross-over technique was used to predict ap. Since the diameters of micro-spheres and plasmid DNA were up to a micron in size, their movement in an aqueous medium at room temperature was influenced by random, thermal Brownian motion. One and two-dimensional Fokker-Planck equation (FPE) models were constructed to predict DEP-driven collection of particles onto electrodes. The model comprised DEP-induced particle flux and thermally driven diffusion flux. The FPE computer model also predicted the diffusion of particles away from the electrode surfaces after the DEP force was switched off, called particle relaxation. Using the values of ap, the FPE model was used to simulate particle collections and relaxations under the action of DEP onto a planar interdigitated electrode surface for a range of applied frequencies and voltages. The collection of particles (beads and plasmid DNA) onto interdigitated electrodes was observed using epi-fluorescence microscopy together with video-recording of images. The images were processed using software written in MATLAB 5.0. The processed images yielded timedependent particle collection profiles representing particle accumulation on the electrodes, and particle relaxation profiles after the DEP potential was switched off. Theoretical predictions were used to compare DEP collection experiments of 216 nm diameter beads and DNA plasmids. Collection and relaxation profiles were measured for AC frequencies from 100 kHz to 20 MHz and applied voltages from 1 to 4.5 V (peak). The data was in broad agreement with theoretical predictions, but there were significant quantitative differences. There are a number of reasons for these discrepancies between theory and experiment. These include electrohydrodynamically induced fluid motion that can disturb particle movement, and distortion of the electric field generated by the interdigitated electrodes due to the presence of charges associated with colloidal particles and DNA. As a first approximation, these factors were not included in the FPE model.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TK Electrical engineering. Electronics Nuclear engineering