A new electro-kinetic means of transporting, metering, controlling, separating and concentrating liquid and bio-particles in a micro-fluidic device will be investigated. Unlike existing direct-current (DC) and alternating-current (AC) electro-kinetic techniques, this Faradaic AC Electro-kinetic (FACE) mechanism produces large quasi-equilibrium polarization of microfabricated electrodes via transient Faradaic electrode reactions to drive fluid and particle motions with speeds in excess of 1 cm/s in micro-channels, representingone of the most powerful nonmechanical micro-pump. Due to the high-frequency (> 100 kHz) AC field employed, the transient reactions do not produce bubbles and ionic contaminants in the micro-channels. It is also not as damaging to DNA, proteins and bio-particles as DC fields. A variety of new single-phase and multi-phase electro-kinetic phenomena, with qualitatively different flow topologies and instabilities, have been observed. The richness of the flow topologies is attributed to two fundamental Faradaic polarization mechanisms that generate a localized pressure sink and a tangential slip velocity. The proposed project will carry out a fundamental analysis of this new electro-kinetic mechanism and delineate/quantify its various flow properties. This fundamental study will allow us to optimize the design of micro-fluidic components based on this new mechanism, sometimes by harnessing the new phenomena. The studies include a matched asymptotic analysis to replace the fast (us) reaction and capacitive charging dynamics within the nm-sized double layer with time-averaged effective electrostatic and slip boundary conditions, thus simplifying the relevant multi-scale numerical studies. An analysis of tangential conduction and charge accumulation at geometric singularities, which are believed to drive some of the anomalous flows, will also be carried out to fully delineate the complex spatio-temporal electrode polarization dynamics. The high shear rate afforded by the strong flow causes many bio-particle segregation and aggregation phenomena, which will be studied in detail to produce effective particle-fluid separation strategies, bio-particle and virus for fluorescent tag detection. Microfabricated AC electro-magnets will produce transverse Lorentz forces to generate spiral flows as well as the current vortex and linear FACE flows. The proposed fundamental work will allow us to fruitfully understand and exploit this new FACE mechanism to build a functional micro-fluidic technology with optimized electrode geometries and applied electric fields.
Educational Impact: The proposed work will provide graduate students with an unusually rich educational experience. It combines fundamental scientific studies of new physical phenomena with engineering design projects to produce nearly commercializable micro-fluidic devices. It involves sophisticated theoretical and numerical analyses, as well as state-of-the-art fabrication technology. In the last four years, our laboratory has placed 5 group members in tenure-track faculty positions at major U.S. research universities, two in leading industrial research centers and two undergraduate members at top graduate programs. Four others will be seeking tenure-track positions shortly. We expect the proposed project to make a comparable educational impact.
Scientific and Technological Impact: Recent fundamental scientific research on chip-scale analytical techniques for viral assays, bacteria detection, chemical chromatography, etc. has and will continue to spur a major technological advance in the medical, environmental and national security industries. A major component that is still under development is micro-fluidics: the ability to precisely transport, mix and manipulate fluids and, more importantly, micro- and nanoscale bio-particles within the chip micro-channels. Electro-kinetics is one approach that utilizes mature micro-fabrication technology to imbed micro-electrodes to produce the desirable microfluidics. This project investigates a new electro-kinetic mechanism, first revealed by the group, that promises to be more powerful, reliable and sensitive than other micro-fluidic components. The proposed work will lay the fundamental groundwork for the first functional micro-fluidic components based on this new mechanism.
