13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Figure 6. Side view showing the ejection of a mineral oil droplet. The spacing between Teflon coated glass substrate (bottom) and textured Si substrate (top) is 200 μm. The applied voltage is 560 Vrms 100 Hz AC. The ejected oil droplet on open surface also sticks to the top plate. A beveled edge on the top plate may facilitate the detachment of the oil droplet. Further effort to fabricate an oleophobic beveled structure on the top plate is needed. 4. Conclusion In this paper, we present theoretical predictions and experimental results on droplet ejection from covered into open microfluidic systems. We use a force balance analysis to predict the condition required to achieve such a motion. This model shows that it is easy to move a water droplet from covered to open section, but oil droplets always tend to stay in a covered section. Ejecting an oil droplet by direct DEP actuation is impossible on Teflon coated surfaces. An oleophobic surface must be used to complete this movement for oil droplet. Experimental testing of the various predictions is done to demonstrate droplet ejection movements. To detach the water droplet from the top plate, we use a bevelled edge to minimize water-Teflon contact area and thus reduce the applied voltage. For oil droplets, we successfully use an oleophobic Si structure to achieve covered to open movement. Further study to bevel the Si structure is needed. Acknowledgements This work was supported by the U.S. Department of Energy, Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302, the University of Rochester, and the New York State Energy Research and Development Authority. References [1] M.G. Pollack, R.B. Fair, A.D. Shenderov, Electrowetting-based actuation of liquid droplets for microfluidic applications. Appl Phys Lett, 77 (2000) 1725–1727. [2] M.G. Pollack, A.D. Shendorov, R.B. Fair, Electro-wetting-based actuation of droplets for integrated microfluidics. Lab Chip, 2 (2002) 96–101. [3] S.K. Cho, H. Moon, C.J. Kim, Creating, transporting, cutting, and merging liquid droplets by electrowetting-based actuation for digital microfluidic circuits, J Microelectromech Syst, 12 (2003) 70-80. [4] H. Ren, R.B. Fair, M.G. Pollack, Automated on-chip droplet dispensing with volume control by electro-wetting actuation and capacitance metering. Sens Actuators B, 98 (2004) 319–327. [5] J. Berthier, Ph. Clementz, O. Raccurt, D. Jary, P. Claustre, C. Peponnet, Y. Fouillet, Computer aided design of an EWOD microdevice. Sens Actuators A, 127 (2006) 283–294.
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