13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- side views in Figure 1 (a) and (b). The bottom plate consists of a patterned array of addressable electrodes, and the top plate in the covered section is coated with a transparent indium tin oxide (ITO) layer as a continuous ground electrode. The electrodes on bottom plate are coated with a dielectric layer, and both the top and bottom surfaces are covered with Teflon-AF as a hydrophobic film. To evaluate the prospect of droplet movement from covered to open section by electrical actuation, we conduct force analyses to compare the driving forces with resisting forces. These results provide an estimate for the required voltage for a droplet to achieve such a motion. Figure 1. Force balance analysis of a water droplet ejection from covered into an open section. (a) Top view of a water droplet at the covered/open boundary. (b) Forces exerted on a water droplet when it is being moved toward the open section. (c) Liquid-solid separation with beveled edge. 2.1. Force balance analysis The forces exerted on the droplet at the covered/open interface are illustrated in Figure 1 (b). We can distinguish two distinct contributions in the plane of the structure: the capillary force and the EW force. The capillary force is 1 2 cos cos cF F F θ θ = + (1) and the EW force is 3 1 5 cos ( ) cos EWOD V F F F F θ θ = + − (2) where F1 = F2 = γw, F3 = γa, F5 = γ(w-a). w and a are contact lengths of the droplet in covered and open sections respectively, γ is the surface tension of the liquid droplet. θV and θ are the droplet contact angle with and without applied voltage. When the water droplet is being moved from the covered section to the open substrate, the influence of F4 can be neglected as there is no motion for the upper portion of droplet that contacts the edge of the ITO plate. Thus, the total force acting on the drop is: 3 1 5 2 cos ( ) cos cos total V F F F F F θ θ θ = + − + (3) The three force components in equation (3) tend to move the droplet toward the open section, so we may expect that the droplet gets out the covered section easily. In fact, the water droplet tends to move out even without external applied voltage, in which case the net force becomes F2|cosθ|. (a) (b) (c)
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