UC San Diego

The primary direction of this thesis is to expand and more clearly define the constraints and operating conditions surrounding high-conductance electrokinetic phenomena. Dielectrophoresis (DEP), the movement of polarizable particles in an electric field gradient, is an effective mechanism to separate biological particles, in the range of nanoparticles through cell clusters, from biological solutions, e.g. whole blood, plasma, serum, cerebrospinal fluid, urine, etc. Most research in this realm has been focused in low-conductance solutions, either from dilution or through substantial sample preparation. One major constraint defined is electrochemical corrosion of platinum electrodes in chlorine-bearing solution. While chlorine concentration is important, corrosion is exacerbated with low frequency electrical stimulus and high current densities. The latter manifests itself both as a high faradaic reaction rate and in local heating. A temperature dependence and frequency sensitivity indicate that platinum-chlorine complexes involved in passivation and de-passivation are kinetically rate-limited. Many confounding electrokinetic forces contribute to particle movement in high-conductance solutions. Joule heating and the resulting density-based fluid flow, i.e. electrothermal flow, is shown to be a major drag force able to overcome DEP forces. Certain device geometries are more sensitive to electrothermal flow; when high electric field gradients are allowed to exist in regions of locally low fluid flow, DEP forces easily dominate electrothermal. Likewise, effective DEP separation is more dependent on micron- or nanometer-sized geometric features to create large electric field gradients than on overall potential differences between electrodes. With corrosion concerns, the V2 relation for Joule heating, and marginal DEP force benefit from increased voltage, devices for high-conductance electrokinetics need to be optimized for low voltage and low current. This refined understanding may be applied to future device designs as to realize novel or improved device structures.