13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- When the droplet emerges from the covered section, it still tends to stick to the Teflon-coated ITO plate due to the low interfacial tension between water and Teflon. To facilitate the separation of the liquid droplet and the top plate, we bevel the edge of the top plate as shown in Figure 1 (c). Here the total force acting on the droplet is 1 3 4 ( cos cos ) (cos cos ) total V V F F F F a a θ θ γ θ θ γ = + − = + − (4) Using θ = 115°, Ftotal > 0 gives θV < 55°. Because of the contact angle saturation effect, it is not easy to reduce the contact angle of a water droplet to lower than 55° by EW actuation. However, the electromechanical approach of EW theory shows that the EW force experienced by the droplet is independent of the surface profile. [13,14] This result indicates that contact angle saturation does not necessarily limit the maximum EW force that can be applied to a droplet. [17] Converting the contact angle to applied electric voltage by the Lippmann-Young equation, we get V > 105V as the voltage requirement for liquid-solid separation. 2.2. Experimental results of water droplet ejection We fabricate a number of covered/open integrated devices on glass substrates to test droplet ejection motion. The bottom substrate consists of aluminum electrodes, a 0.5 μm spin-on-glass (SOG) dielectric layer, and a 1μm Teflon-AF layer. The top substrate is coated with an ITO layer and also a 1μm Teflon-AF layer. The spacing between top and bottom substrate is set to be 100 μm. Figure 2. Moving a water drop from covered to open section. (a) The water droplet is initially placed in covered section. (b) The droplet is moved towards covered/open boundary by EW actuation using 100 Vrms 100 Hz AC. (c) The droplet enters open section as soon as it arrives at the boundary. (d) The droplet is successfully separated from top plate after applying 300 Vrms voltage. Figure 2 shows video frames of water droplet motion from the covered to the open section. A water droplet is first injected into the covered section, and moved toward the covered/open interface by EW actuation. When the droplet reaches the interface, it is ejected rapidly but still sticks to the top plate. See Figure 2 (c). We then apply voltages ranging from 100 to 300 Vrms to separate the droplet and the top ITO plate. Separation is only successful at 300 Vrms. To lower the required voltage for droplet/top plate separation, we bevel the edge of the top ITO plate to minimize the contact area between water droplet and Teflon surface. See Figure 3. Now the droplet can be easily separated by applying a voltage of 90 Vrms, which is even lower than the predicted value of 105 V. This is probably because the direction of force F4 in Figure 1 (c) is not exactly horizontal, making the separation voltage somewhat lower.
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