Dielectrophoretic characterisation and manipulation of sub-micron particles following surface modification
The aim of this thesis is to dielectrophoretically characterise sub-micron particles on the basis of their surface properties and to devise a DEP technique suitable for the fractionation and manipulation of particles on this scale. Polystyrene particles are modified by the attachment of biological ligands using various established localisation techniques and their DEP response observed using micro-electrodes with well defined high and low field regions, corresponding to a previously utilised design and modified in the course of this project for multiple sample handling. The results of these observations are modelled for the first time using a charge relaxation mechanism pertaining to a structured interfacial charge distribution and, through fitting the data to this model, fundamental parameters of the system - the surface conductance and electrokinetic charge - are predicted. The model viability is assessed with reference to both comparisons with alternative measurements and the technical limitations of the data fitting procedure, and corresponding surface charge transport mechanisms are discussed in the light of the DEP response following surface modification. Investigations are made into the possibility of a DEP based device suitable for the transport/fractionation of sub-micron particles. Given the essentially dissipative nature of sub-micro particle ensembles, a Brownian ratchet principle is chosen. A Brownian ratchet is a generic system wherein a net directional drive is effected by biasing Brownian diffusion on a periodically activated anisotropic structure. Without need of thermal gradients or net macroscopic forces Brownian ratchet pumps could be an interesting alternative in many microfluidic applications. Simulated fields and corresponding particle transport rates are compared for two basic electrode structures in order to assess their viability for use as DEP Brownian ratchets and a new design proposed, based on the simultaneous juxtaposition of positive and negative DEP forces. This device is built on the necessary scale using multi-layer fabrication techniques with a silicon elastomer moulded channel. The existence of stochastic transport on the device is investigated experimentally by means of processed video sequences and resulting possibilities for particle separation on the basis of size and surface properties inferred.