School of Mechanical Engineering, Korea University, Seoul 136-713, Korea.
Langmuir. 2013 Jun 25;29(25):7758-67. doi: 10.1021/la401669w. Epub 2013 Jun 7.
Dynamic electrowetting-on-dielectric (DEWOD) of the unstretched and stretched Teflon is reported in the experiments with water drop impact and rebound. We explore experimentally and theoretically the situation with the capacitance different from the standard static electrowetting. Deionized water drops impact onto either an unstretched hydrophobic Teflon surface or Teflon stretched up to 250% strain normally to the impact direction. The surface roughness of the unstretched Teflon increased after stretching from 209.9 to 245.6 nm resulting in the increase in the equilibrium water contact angle from 96 ± 4° to 147 ± 5°, respectively. The electric arrangement used in the drop impact experiments on DEWOD results in a dramatically reduced capacitance and requires a much higher voltage to observe EW in comparison with the standard static case of a drop deposited on a dielectric layer and attached to an electrode. In the dynamic situation we found that as the EW sets in it can greatly reduce the superhydrophobicity of the unstretched and stretched Teflon. At 0 kV, the water drop rebound height (hmax) is higher for the stretched Teflon (hmax ≈ 5.13 mm) and lower for the unstretched Teflon (hmax ≈ 4.16 mm). The EW response of unstretched Teflon is weaker than that of the stretched one. At the voltage of 3 kV, the water drop sticks to the stretched Teflon without rebound, whereas water drops still partially rebound (hmax ≈ 2.8 mm) after a comparable impact onto the unstretched Teflon. We found a sharp dynamic EW response for the stretched Teflon. The contact angle of deionized water ranged from 147 ± 5° (superhydrophobic) to 67 ± 5° (partially hydrophilic) by applying external voltage of 0 and 3 kV, respectively. Dynamic electrowetting introduced in this work for the first time can be used to control spray cooling, painting, and coating and for drop transport in microfluidics.
本文报道了在水滴滴冲击和回弹实验中，未拉伸和拉伸聚四氟乙烯的动态电润湿效应（DEWOD）。我们通过实验和理论探索了电容不同于标准静态电润湿的情况。去离子水滴冲击到未拉伸的疏水性聚四氟乙烯表面或垂直于冲击方向拉伸至 250%应变的聚四氟乙烯。未拉伸聚四氟乙烯的表面粗糙度从 209.9nm 增加到 245.6nm，导致平衡水接触角从 96±4°分别增加到 147±5°。在 DEWOD 水滴冲击实验中使用的电极布置导致电容显著降低，并且与沉积在介电层上并连接到电极的标准静态情况相比，需要更高的电压才能观察到 EW。在动态情况下，我们发现，随着 EW 的建立，它可以大大降低未拉伸和拉伸聚四氟乙烯的超疏水性。在 0kV 时，拉伸聚四氟乙烯的水滴回弹高度（hmax）较高（hmax≈5.13mm），未拉伸聚四氟乙烯的水滴回弹高度（hmax≈4.16mm）较低。未拉伸聚四氟乙烯的 EW 响应比拉伸聚四氟乙烯弱。在 3kV 电压下，拉伸聚四氟乙烯上的水滴不回弹，而在可比冲击到未拉伸聚四氟乙烯后，水滴仍部分回弹（hmax≈2.8mm）。我们发现拉伸聚四氟乙烯的动态 EW 响应非常灵敏。通过施加 0 和 3kV 的外部电压，去离子水的接触角分别从 147±5°（超疏水性）变化到 67±5°（部分亲水性）。本文首次引入的动态电润湿可用于控制喷雾冷却、喷涂和涂层以及微流控中的液滴传输。
