Quinn Anthony, Sedev Rossen, Ralston John
Ian Wark Research Institute, University of South Australia, Mawson Lakes, Adelaide, South Australia 5095, Australia.
J Phys Chem B. 2005 Apr 7;109(13):6268-75. doi: 10.1021/jp040478f.
Electrowetting is the phenomenon of contact angle decrease under the influence of an external voltage applied across the solid/liquid interface. Electrowetting offers an interesting possibility to enhance the wettability of hydrophobic materials without altering the chemical composition of the system and thus could be incorporated in various microfluidic devices. Electrowetting is fundamentally an electrocapillary effect occurring on an insulated solid electrode (hence the change of the solid/liquid interfacial tension with voltage follows Lippmann's equation). A limiting contact angle value larger than zero is achieved even at very large external voltages. Saturation precludes full wetting of the substrate and restricts the magnitude of the capillary force variation. Contact angle saturation has been given various interpretations (e.g., charge trapping, air ionization) but appears to reflect a natural thermodynamic limit rather than being simply a defective property. The limiting value of the contact angle is given by the Young equation when the value of the solid/liquid interfacial tension reaches zero. The model is in excellent agreement with our own results and often gives an adequate description of published data. It also suggests that the saturation limit is determined by the material properties of the system and electrowetting at voltages exceeding this threshold is essentially a nonequilibrium process.
电润湿是指在施加于固/液界面的外部电压影响下接触角减小的现象。电润湿提供了一种有趣的可能性，即在不改变系统化学成分的情况下提高疏水材料的润湿性，因此可应用于各种微流控设备中。从根本上讲，电润湿是一种发生在绝缘固体电极上的电毛细效应（因此，固/液界面张力随电压的变化遵循 Lippmann 方程）。即使在非常高的外部电压下，也能实现大于零的极限接触角值。饱和度会阻止基底的完全润湿，并限制毛细力变化的幅度。接触角饱和度有多种解释（例如，电荷俘获、空气电离），但似乎反映了一种自然的热力学极限，而不仅仅是一种缺陷性质。当固/液界面张力的值达到零时，接触角的极限值由杨氏方程给出。该模型与我们自己的结果非常吻合，并且常常能对已发表的数据给出充分的描述。它还表明，饱和度极限由系统的材料特性决定，在超过该阈值的电压下进行电润湿基本上是一个非平衡过程。
