13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- According to our device setup, [18,19] the minimum required voltage to achieve Ftotal > 0 is 1292 V and 1555 V for silicone oil and mineral oil, respectively. These high voltages are much greater than the dielectric breakdown voltages of 750 V, so it is impossible to achieve the desired movement by direct actuation. Furthermore, the oil droplet tends to get separated even the DEP force is large enough to overcome the resisting forces. We can understand this issue by considering a quasi-static process where the two plates are slowly separated as shown in Figure 5. The oil droplet sandwiched between parallel plates has a contact angle θ < 90°. The vertical component of the surface tension forces at the upper (or lower) edge and the middle of drop are respectively: 2 sin edge F R γ π θ = (6) 2 mF r γ π = (7) As the plates separate, both R and r decrease. Gradually Fedge will exceed Fm because r can approach 0. Then, the droplet breaks into two droplets, one each on the top and bottom substrates. Figure 5. Breaking up of the oil droplet in parallel-plate structure when the two plates are separated far enough from each other. A solution to this problem is to employ an oleophobic surface for the top plate. When the contact angle is > 90°, R at the upper edge is smaller than r, the oil droplet can be ejected without separation. And the total force acting on the oil droplet become * 2 4 cos sin total DEP F F F F θ θ = + − (8) where θ* is the oil contact angle against top surface, θ* > 90°. As F2 and F4 counterbalance each other mostly, the oil droplet can be ejected into an open section by DEP actuation. 3.2. Experimental results of oil droplet ejection We fabricate an oleophobic surface on textured Si wafer and use it as the top plate for oil droplet ejection tests. The fabrication process of oleophobic Si microstructures is similar to that of Tuteja [22] and of Wu [23]. Figure 6 shows a mineral oil droplet sandwiched between a Teflon coated glass substrate (bottom) and a textured Si substrate (top) actuated by DEP. When the oil droplet is delivered to the covered/open boundary, continuous actuation moves the oil droplet to the open section, see Figure 6 (c).
RkJQdWJsaXNoZXIy MjM0NDE=