| Alternating (AC) electric fields have been investigated as a versatile tool for rapid particle and fluid manipulation in micro-Total Analysis Systems (muTAS). Different onchip electrode geometries and different particle suspensions were explored in this study with an aim to acquire a fundamental understanding of particle behavior under applied fields. Aqueous suspensions of particles of sizes ranging from nanoparticles to microspheres and having varied electrical properties (dielectric or conductive) were studied. For each system, detailed electrostatic simulations were carried out to identify the forces acting on the particles and fluid. Control of the particle-field, fluid-field and particle-particle interactions, by fine tuning the applied field, lead to the desired assembly of particles. Dielectrophoresis (DEP), the interaction between induced particle dipoles and the spatially non-uniform electric field, was used to assemble gold nanoparticles into microwires and for manipulating fluid droplets containing suspended particles in a novel liquid-liquid microfluidic system. AC Electrohydrodynamics (EHD) driven liquid flows were used for the transportation, redistribution and collection of suspended particles inside experimental cells.; Suspensions of metallic nanoparticles in water were assembled via DEP into wires of micrometer thickness between planar electrodes. Two modes of microwire assembly, one through the bulk of the suspension, and one as half-cylinders on the glass surface between the electrodes were identified. The operating conditions responsible for the two assembly modes were recognized. Control of the process parameters allowed making, for example, straight single connectors, or massively parallel arrays of microwires on the surface of the chip. The direction of microwire growth was guided by introducing conductive islands or particles in the suspension. The microwire assembly process was modeled using finite element electrostatic calculations. The experiments, supported by electrostatic calculations, showed that the wires grew in the direction of highest field intensity, "automatically" making electrical connections to the objects between the electrodes. The results point the way to controlled dielectrophoretic assembly of nanoparticles into on-chip electrical connectors, switches and networks.; A new EHD effect arising from the application of alternating electric fields to patterned electrode surfaces was observed. (Abstract shortened by UMI.)... |
