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Modeling microdrop motion between covered and open regions of EWOD microsystems

J. Berthier, Ph. Clementz, J-M Roux, Y. Fouillet and C. Peponnet

digital microfluidics, electrowetting, Laplace law, open EWOD, covered EWOD

Digital microfluidics is a promising way to manipulate biological targets like DNA, proteins or cells in very small liquid volumes. The advantages of such devices are the use of lesser quantities of costly reagents, better biochemical reaction efficiency and shorter operating times. A quickly developing technology is ElectroWetting On Dielectric (EWOD) microsystems [Pollack et al., 2000, Moon et al., 2002]. It has been shown that basic manipulations of drops can be achieved in such Microsystems [Cho et al., 2003].
Two different types of EWOD Microsystems have been developed: covered systems where the droplets are confined between two plates and open systems where the sessile droplet is sitting freely on an horizontal solid substrate (fig 1). Each one of these systems has his own advantages. Drop dispense, motion and splitting are easier in covered EWOD systems whereas mixing and evaporation (for species concentration) are preferably performed in the open configuration [Clementz et al., 2005]. Thus, the concept of a dual open/covered EWOD microsystem has been developed. This concept relies on the fact that motion between a covered and open region is possible under electrowetting actuation. In this work, we analyze the possibilities of such a motion. The approach is performed in three steps. First, we use the Surface Evolver software [Brakke, 1992] to model the displacement of the droplet from one region to the other. This simplified model assumes that the capillary forces – including the electrical forces - are dominant over inertial and viscous forces (i.e. the Weber number and the Ohnesorge numbers are small). Second, from the Evolver results, and using the Laplace law, we deduce a very simple condition for the possibility of droplet motion. Finally experimental results are compared with the results of the modeling steps.

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