McHale G, Herbertson D L, Elliott S J, Shirtcliffe N J, Newton M I
School of Biomedical and Natural Sciences, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, United Kingdom.
Langmuir. 2007 Jan 16;23(2):918-24. doi: 10.1021/la061920j.
Transport of a water droplet on a solid surface can be achieved by differentially modifying the contact angles at either side of the droplet using capacitive charging of the solid-liquid interface (i.e., electrowetting-on-dielectric) to create a driving force. Improved droplet mobility can be achieved by modifying the surface topography to enhance the effects of a hydrophobic surface chemistry and so achieve an almost complete roll-up into a superhydrophobic droplet where the contact angle is greater than 150 degrees . When electrowetting is attempted on such a surface, an electrocapillary pressure arises which causes water penetration into the surface features and an irreversible conversion to a state in which the droplet loses its mobility. Irreversibility occurs because the surface tension of the liquid does not allow the liquid to retract from these fixed surface features on removal of the actuating voltage. In this work, we show that this irreversibility can be overcome by attaching the solid surface features to the liquid surface to create a liquid marble. The solid topographic surface features then become a conformable "skin" on the water droplet both enabling it to become highly mobile and providing a reversible liquid marble-on-solid system for electrowetting. In our system, hydrophobic silica particles and hydrophobic grains of lycopodium are used as the skin. In the region corresponding to the solid-marble contact area, the liquid marble can be viewed as a liquid droplet resting on the attached solid grains (or particles) in a manner similar to a superhydrophobic droplet resting upon posts fixed on a solid substrate. When a marble is placed on a flat solid surface and electrowetting performed it spreads but with the water remaining effectively suspended on the grains as it would if the system were a droplet of water on a surface consisting of solid posts. When the electrowetting voltage is removed, the surface tension of the water droplet causes it to ball up from the surface but carrying with it the conformable skin. A theoretical basis for this electrowetting of a liquid marble is developed using a surface free energy approach.
通过利用固液界面的电容充电（即介电电润湿）来差异地改变液滴两侧的接触角，从而在固体表面实现水滴的传输，以产生驱动力。通过改变表面形貌以增强疏水表面化学的作用，进而实现几乎完全卷缩成接触角大于150度的超疏水液滴，可提高液滴的迁移率。当在这样的表面上尝试进行电润湿时，会产生一种电毛细管压力，导致水渗透到表面特征中，并不可逆地转变为液滴失去迁移率的状态。不可逆性的出现是因为在去除驱动电压后，液体的表面张力不允许液体从这些固定的表面特征中缩回。在这项工作中，我们表明通过将固体表面特征附着到液体表面以形成液滴弹珠，可以克服这种不可逆性。然后，固体地形表面特征在水滴上形成一层贴合的“外皮”，这既使水滴具有高迁移率，又为电润湿提供了一个可逆的固体上液滴弹珠系统。在我们的系统中，疏水性二氧化硅颗粒和石松子的疏水颗粒被用作外皮。在与固体弹珠接触区域相对应的区域中，液滴弹珠可被视为以类似于超疏水液滴搁置在固定于固体基板上的支柱上的方式，搁置在附着的固体颗粒（或微粒）上的液滴。当一个弹珠放置在平坦的固体表面上并进行电润湿时，它会展开，但水仍有效地悬浮在颗粒上，就好像该系统是由固体支柱组成的表面上的一滴水一样。当去除电润湿电压时，水滴的表面张力使其从表面上形成球状，但同时携带贴合的外皮。利用表面自由能方法为这种液滴弹珠的电润湿建立了理论基础。
