Department of Power Mechanical Engineering, National Tsing Hua University, Taiwan.
Analyst. 2013 Apr 21;138(8):2372-7. doi: 10.1039/c3an36832g.
The change of contact angle is one of the major subjects in the studies of electrowetting on dielectrics. A larger change in contact angle with a less applied electric potential has been pursued by the researchers on digital microfluidics. From previous research it is concluded that the effect of free charges in electrolytes on contact angles can almost be neglected. In this article, obvious influences of free charges on contact angles are presented and discussed. To verify the influence of free charges, both weak electrolyte (boric acid) and strong electrolyte (sodium chloride) are used as sources of free charges in our experiment. It was found that the increase of ion concentration enhances the contact angle variation due to the attraction between the bound surface charges in the dielectric layer and the free counter-ions in the solution. The saturated contact angle occurs with a lower electric potential compared with de-ionized water due to the shielding of the electric field by the free counter-ions. When a strong electrolyte is used, the contact angle varies at a much higher rate than with de-ionized water, and the huge amount of accumulated free ions shields the driving field, causing the contact angle to saturate at a much lower electric potential. The saturated contact angle in strong electrolyte solution is markedly larger than those in weak electrolyte solutions and de-ionized water.
接触角的变化是电润湿研究中的主要课题之一。在数字微流控领域的研究中，研究人员一直在追求用较小的外加电场实现较大的接触角变化。从以往的研究中可以得出结论，电解质中的自由电荷对接触角的影响可以忽略不计。本文提出并讨论了自由电荷对接触角的明显影响。为了验证自由电荷的影响，我们在实验中分别使用弱电解质（硼酸）和强电解质（氯化钠）作为自由电荷的来源。实验发现，随着离子浓度的增加，由于电介质层中的束缚表面电荷与溶液中的自由反离子之间的吸引力，接触角的变化增大。与去离子水相比，由于自由反离子对电场的屏蔽作用，饱和接触角出现在较低的电势下。当使用强电解质时，接触角的变化速率远高于去离子水，大量累积的自由离子屏蔽了驱动电场，导致接触角在较低的电场强度下达到饱和。强电解质溶液中的饱和接触角明显大于弱电解质溶液和去离子水中的饱和接触角。
