Dynamic Electrowetting-on-Dielectric (DEWOD) on Unstretched and Stretched Teflon

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.本实验通过液滴撞击与反弹工况，研究了未拉伸与拉伸聚四氟乙烯（Teflon）的动态介质上电润湿（Dynamic electrowetting-on-dielectric, DEWOD）特性。本研究从实验与理论层面探究了与标准静态电润湿工况存在差异的电容特性场景。实验采用去离子水液滴，以垂直于表面的撞击方向分别冲击未拉伸疏水聚四氟乙烯表面，以及拉伸至250%应变的聚四氟乙烯表面。未拉伸聚四氟乙烯经拉伸后，表面粗糙度从209.9 nm提升至245.6 nm，对应的平衡水接触角也从96±4°升高至147±5°。相较于将液滴沉积于介质层并附着于电极的标准静态电润湿工况，本DEWOD液滴撞击实验所采用的电学布局会使电容大幅降低，且观测到电润湿现象所需施加的电压也显著更高。在动态工况下，我们发现电润湿效应一旦触发，便可大幅降低未拉伸与拉伸聚四氟乙烯表面的超疏水性能。当施加电压为0 kV时，拉伸聚四氟乙烯表面的液滴最大反弹高度（hmax）约为5.13 mm，高于未拉伸聚四氟乙烯表面的约4.16 mm。未拉伸聚四氟乙烯的电润湿响应弱于拉伸聚四氟乙烯。当施加电压为3 kV时，拉伸聚四氟乙烯表面的液滴会完全粘附于表面，无反弹现象；而以相近撞击参数冲击未拉伸聚四氟乙烯表面时，液滴仍可发生部分反弹，最大反弹高度约为2.8 mm。我们观测到拉伸聚四氟乙烯具有显著的动态电润湿响应。当分别施加0 kV与3 kV的外接电压时，去离子水的接触角可在147±5°（超疏水状态）至67±5°（部分亲水状态）之间变化。本研究首次提出的动态介质上电润湿技术，可应用于喷雾冷却、涂装与镀膜工艺，以及微流控系统中的液滴输运。