13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 3. Separating a water drop and ITO plate with bevelled edges. The voltage applied is 90 Vrms 100 Hz AC. 3. Oil droplet ejection Oil droplets have low surface tension and usually small contact angles on solid surfaces. It is likely that an oil droplet will find its way into cracks or small channels due to the capillary effect. As a consequence, it is very difficult to eject an oil droplet from covered into an open section. 3.1. Force balance analysis The tendency of a liquid to enter narrow channels is commonly recognized as the capillary effect. A classic textbook example is the liquid rising in a vertical glass tube as shown in Figure 4 (a). For our apparatus, the liquid droplet moves horizontally and the goal is to use electrical actuation to overcome the capillary effect. To evaluate the requirements to accomplish this operation, we conduct a force analysis to compare the driving force and resisting forces as shown in Figure 4 (b). The total force acting on the oil drop is: 2 4 3 5 1 cos sin ( cos cos cos ) total DEP F F F F F F F θ θ θ θ θ = − − + + − 2 4 cos sin DEP F F F θ θ ≈ − − (5) Figure 4. (a) Force analysis for a capillary tube. (b) Force analysis of an oil droplet at the covered/open boundary.
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