Microfluidic switching and electrowetting techniques.

Abstract

Microfluidic switching and electrowetting could be employed as an integral part of a low-cost, high performance microfluidics-based Fluorescence Activated Cell Sorter (micro-FACS) for rapid, reliable detection and sorting particles, cells, or cell components such as DNA. Our research group has developed a microfluidic switch, based on the actuation of a liquid stream in an air-liquid two-phase flow. The switching is accomplished through a novel application of electrowetting-on-dielectric (EWOD) for the manipulation of a continuous fluid stream. It is based on the nonlinear transition of two-phase flows between different flow patterns. The contact angle-voltage relationship is important in fluid evaluation for the EWOD switch. We consider ionic liquids as a novel class of electrowetting agents. The observed deviations from the Lippmann-Young curve suggest that the physical phenomena occurring during electrowetting are controlled and altered by the underlying chemical structures and properties of the cations and anions involved. The air-sheath aqueous liquid system is sensitive to amphiphiles or proteins that foul and change the channel surface. This challenge is addressed in our oil-sheath aqueous liquid experiments which handle protein-containing cell culture media. In this custom device, the interaction of two immiscible fluids is employed to produce surface tension dominated flow patterns. Fluid selection, channel size and geometry have a strong influence on the flow regime exhibited in a microchannel, since they affect the inertial, viscous, surface tension, and gravity forces. There are no comprehensive formulas which would account for all of the influences on microflows in order to predict the characteristics of flow regimes. Therefore, we have developed flow regime maps for biologically relevant fluids to provide empirical feedback on optimal combinations of channel material and flow speeds. These maps have predictive value for fluidic behavior, which needs to be taken into account in the development of microfluidic devices. We establish the validity of contact angle variation as a method to activate flow pattern transition in microchannels, supporting the scalability of previous millichannel findings to the microchannel domain. On a linear-linear scale, the stratified rivulet flow regime is the dominant configuration observed in the majority of cases. The polymonochloro-para-xylylene (Parylene C) channel supports more stable flows than the polydimethylsiloxane (PDMS) channel for all tested fluid combinations. That is, the PDMS channel allows a stratified rivulet flow to be converted to an intermittent flow regime more easily than the Parylene C channel. Some oil-water flow patterns in our experiments give rise to uniform aqueous droplets which can serve as encapsulation compartments. These vesicles in continuous oil phase can be utilized in transport and manipulation of small liquid volumes for biological and chemical reactions. We demonstrate a droplet generation and consistency monitoring system with laser optics excitation and detection. This introduces rigorous engineering methodology and high-precision quantitative analysis to the droplet-based microfluidics discipline. For a fixed oil flow rate, we observe that plug length is directly proportional to the 1.59th root of water flow rate, indicating that the plug expands preferentially in the direction of flow relative to any possible transverse expansion direction. The detection of green fluorescent protein (GFP) labelled cells within aqueous droplets is demonstrated with the laser-based excitation setup. Our computations on the vibrations of GFP yield information about thermal fluctuations and rigidity, which is crucial for the emission of bright fluorescence. This protein fluorescence would enable advancement of the subcellular capabilities of the micro-FACS.