Field-Effect Flow Control in Microfluidics
DeVoe, Don L
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Lab-on-chip (LOC) devices have miniaturized routine laboratory processes for automated, high-throughput chemical analysis. Separations of biomolecules, including protein and DNA, have been performed with high efficiencies in LOC devices, but the need for improved fluid flow control, i.e. pumping and valves, remains a significant challenge for next-generation systems. This dissertation explores the development of novel flow-control technology in polymer microfluidic networks for the realization of inexpensive, next-generation LOC devices. In the microchannels, electroosmotic flow (EOF) is used to electro-kinetically move the fluid with a longitudinal electric field. To modulate the EOF velocity, the technique of field-effect flow control (FEFC) is employed. In FEFC, a second electric field is applied through the microchannel wall to influence the surface charge at the fluid-microchannel interface for independent control of the EOF. Presented in this work is the first demonstration of FEFC in a polymer network. The microchannel walls were composed of Parylene C (1 - 2 um thick), which is an inexpensive, chemical vapor deposited polymer. In this work, FEFC modulated the EOF velocity from 240% to 60% of its original value in a microchannel that was 40 um in height, 100 um in width, and 2 cm long. The next research phase integrated FEFC technology into microfluidic networks with microchannels in the second and third dimensions. At the T-intersection of three microchannels, FEFC established different EOF pumping rates in the two main microchannels. The different flow rates induced pressure pumping in the third, field-free microchannel with ultra-low flow rate control (-2.0 nL/min to 2.0 nL/min). Moreover, adjusting the secondary electric fields enabled bi-directional flow control for the induced pumping in the 2D and 3D field-free microchannels. To improve the microfluidic networks, an electro-fluid flow model was developed to describe the induced pressure and FEFC phenomenon. The model closely predicted the experimentally obtained flow rates in the field-free microchannel. Additionally, the study of multiple gate electrodes along the same microchannel showed that FEFC has only a local effect over the EOF, but revealed that position and size of the electrodes influence the EOF control in the microfluidic network.