Electrokinetic Pumping and Spraying at Microano Level

This thesis addresses electrokinetic phenomena at microano level for potential microfluidic applications. Two kinds of electrokinetic objects: electroosmosis and electrospray under both DC and AC conditions have been explored and exploited. (1) Nonlinear AC electrokinetic flow has been observed based on electric field induced mobile charges at the interfaces of fluid/electrode. Polarization of the electrodes by charge transfer from the electrode or the bulk is used to improve a tangential Maxwell force and a surface flow along the electrodes. A prototype chip made of in-house fabricated microelectrodes is designed to study AC EO phenomena for pumping and mixing. (2) Both high-pressure and high flowrate electrokinetic micropumps have been fabricated. Electroosmosis flow model within microchannels is analyzed and verified experimentally. Thermodynamic efficiency analysis is carried out which shows the optimum efficiency can be achieved by matching double layer thickness to the channel pore size. Applications towards micro total analysis systems are also demonstrated. (3) New deformation/fission phenomena have been reported for micro-drops driven by an AC electric field at their resonant frequencies. Maxwell force can be enhanced when AC frequencies are comparable to both the drop resonant frequency and the inverse charge relaxation time of the double layer. High throughput electrospray is achieved under resonant conditions. Additionally, DC electrospray has been exploited as a novel coating method to deposit carbon nanotubesafion suspensions upon target electrodes. The deposition is achieved by using electrohydrodynamic atomization to eject monodispersed droplets into the gaseous phase followed by directing them to the target electrodes alone electric field. Electrochemical study of the deposited layer shows its advantage over conventional coating methods.