Jumping droplets are interesting because of their applications in energy harvesting, heat transfer, anti-icing surfaces, and displays. Typically, droplets “jump” from a surface when two or more drops coalesce. Here, we demonstrate an approach to get a single droplet of liquid metal (eutectic gallium indium) to jump by using electrochemistry in a solution of 1M NaOH. Applying a positive potential to the metal (∼1 V relative to the open circuit potential) drives electrochemical surface oxidation that lowers the interfacial tension from ∼450 mN/m to ∼0 mN/m. In the low interfacial tension state, the droplet flattens due to gravity. Rapid switching to a negative potential (relative to the open circuit potential) reduces the surface oxide, returning the deformed droplet to a state of high interfacial tension. This rapid change in interfacial tension in the flattened state generates excess surface energy, which drives the droplet to return to a spherical shape with enough momentum that the liquid droplet jumps. This work is unique because (1) the jumping is controlled and tuned electrically, (2) the approach works with a single droplet, (3) it does not require a superhydrophobic surface, which is typically used to prevent droplets from adhering to the substrate, (4) the drops jump through a viscous medium rather than air, and (5) the potential energy obtained by the jumping drops is one order of magnitude higher than previous approaches. Yet, a limitation of this approach relative to conventional jumping drops is the need for electrolyte and a source of electricity to enable jumping. Herein, we characterize and optimize the jumping height (∼6 mm for a 3.6 mm diameter drop) by changing the reductive and oxidative potential and time.

中文翻译：

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跳跃式液态金属液滴的电化学控制

跳跃液滴之所以有趣，是因为它们在能量收集，传热，防冰表面和显示器中的应用。通常，当两个或多个液滴聚结时，液滴会从表面“跳出”。在这里，我们演示了一种通过在1M NaOH溶液中使用电化学方法使液态金属（共晶镓铟）液滴跳跃的方法。向金属施加正电势（相对于开路电势约为1 V）会驱动电化学表面氧化，从而将界面张力从约450 mN / m降低至约0 mN / m。在低界面张力状态下，液滴由于重力而变平。快速切换到负电位（相对于开路电位）会减少表面氧化物，使变形的液滴返回到高界面张力的状态。处于展平状态时，界面张力的这种快速变化会产生多余的表面能，从而驱动液滴以足够的动量返回球形，从而使液滴跳动。这项工作是独特的，因为（1）跳跃是受电控制和调谐的，（2）该方法仅处理单个液滴，（3）不需要超疏水表面，该表面通常用于防止液滴粘附到基材上，（4）液滴通过粘性介质而不是空气跳跃，并且（5）跳跃的液滴获得的势能比以前的方法高一个数量级。然而，相对于常规跳跃液滴，该方法的局限性在于需要电解质和能够跳跃的电力源。在此处，