Eaker Collin B, Khan M Rashed, Dickey Michael D
Department of Chemical and Biomolecular Engineering, North Carolina State University.
Department of Chemical and Biomolecular Engineering, North Carolina State University;
J Vis Exp. 2016 Jan 26(107):e53567. doi: 10.3791/53567.
Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where interfacial tension is a dominant force. A variety of methods exist for controlling the interfacial tension of aqueous and organic liquids on this scale; however, these techniques have limited utility for liquid metals due to their large interfacial tension. Liquid metals can form soft, stretchable, and shape-reconfigurable components in electronic and electromagnetic devices. Although it is possible to manipulate these fluids via mechanical methods (e.g., pumping), electrical methods are easier to miniaturize, control, and implement. However, most electrical techniques have their own constraints: electrowetting-on-dielectric requires large (kV) potentials for modest actuation, electrocapillarity can affect relatively small changes in the interfacial tension, and continuous electrowetting is limited to plugs of the liquid metal in capillaries. Here, we present a method for actuating gallium and gallium-based liquid metal alloys via an electrochemical surface reaction. Controlling the electrochemical potential on the surface of the liquid metal in electrolyte rapidly and reversibly changes the interfacial tension by over two orders of magnitude ( ̴500 mN/m to near zero). Furthermore, this method requires only a very modest potential (< 1 V) applied relative to a counter electrode. The resulting change in tension is due primarily to the electrochemical deposition of a surface oxide layer, which acts as a surfactant; removal of the oxide increases the interfacial tension, and vice versa. This technique can be applied in a wide variety of electrolytes and is independent of the substrate on which it rests.
控制界面张力是在亚毫米长度尺度上操纵流体形状、位置和流动的有效方法，在该尺度下界面张力是主导力。有多种方法可用于控制此尺度下水溶液和有机液体的界面张力；然而，由于液态金属的界面张力较大，这些技术对液态金属的实用性有限。液态金属可在电子和电磁设备中形成柔软、可拉伸且形状可重构的组件。虽然可以通过机械方法（如泵送）来操纵这些流体，但电方法更易于小型化、控制和实施。然而，大多数电技术都有其自身的限制：介电层上电润湿需要较大（千伏）的电势才能实现适度的驱动，电毛细现象只能影响界面张力相对较小的变化，并且连续电润湿仅限于毛细管中的液态金属塞。在此，我们提出一种通过电化学表面反应驱动镓和镓基液态金属合金的方法。控制电解质中液态金属表面的电化学电势可快速且可逆地使界面张力改变两个数量级以上（从约500 mN/m降至接近零）。此外，相对于对电极，该方法仅需施加非常适度的电势（<1 V）。由此产生的张力变化主要是由于作为表面活性剂的表面氧化层的电化学沉积；去除氧化物会增加界面张力，反之亦然。该技术可应用于多种电解质中，且与它所依托的基底无关。
