School of Mechanical Engineering and Birck Nanotechnology Center , Purdue University , West Lafayette , Indiana 47907 , United States.
Langmuir. 2018 Oct 30;34(43):12787-12793. doi: 10.1021/acs.langmuir.8b02675. Epub 2018 Oct 15.
Electrowetting-based droplet actuation has applications in digital microfluidics. Mobility of droplets on surfaces can be enhanced using structured superhydrophobic surfaces that offer inherently low adhesion to droplets in the Cassie state. However, these surfaces must be designed to prevent transition to the Wenzel state (in which droplets are immobile) at high electrowetting actuation voltages. The electrowetting behavior of cylindrical microposts and mushroom-shaped re-entrant microstructures, both of which afford excellent superhydrophobicity, is investigated and compared. A surface-energy-based model is employed to estimate the energy barrier for the Cassie-to-Wenzel transition and thus the electrowetting voltage required to initiate this transition. The mushroom structures are predicted to be more resilient to transition (i.e., transition occurs at a voltage that is up to 1.5 times higher) than microposts. Both types of microstructured surfaces are fabricated and electrowetting experiments performed to demonstrate that mushroom structures indeed inhibit the Cassie-to-Wenzel transition at voltages that induce such transition on the cylindrical microposts.
基于电润湿的液滴驱动在数字微流控中有应用。在 Cassie 状态下，表面的液滴可以通过结构化的超疏水表面增强其迁移率，因为这种表面对液滴具有固有的低附着力。然而，这些表面必须设计成防止在高电润湿驱动电压下过渡到 Wenzel 状态（此时液滴不能移动）。本文研究并比较了圆柱形微柱和蘑菇形倒凹微结构的电润湿行为，这两种结构都具有极好的超疏水性。采用基于表面能的模型来估计 Cassie 到 Wenzel 转变的能垒，从而估计引发这种转变所需的电润湿电压。预测蘑菇结构比微柱更能抵抗转变（即，在电压升高 1.5 倍时才会发生转变）。这两种微结构表面都被制造出来，并进行了电润湿实验，以证明蘑菇结构确实可以抑制在圆柱形微柱上诱导这种转变的电压下的 Cassie 到 Wenzel 转变。
