方法通过表面氧化还原操纵液态金属的表面张力

Summary

We present a method to control the interfacial energy of a liquid metal in an electrolyte via electrochemical deposition (or removal) of a surface oxide layer. This simple method can control the capillary behavior of gallium-based liquid metals by tuning the interfacial energy rapidly, significantly, and reversibly using modest voltages.

Abstract

控制界面张力是一种有效的方法，用于操纵在亚毫米尺度，其中界面张力是一个主导力的形状，位置和流量的流体。有多种方法存在用于控制这种规模的含水和有机液体的界面张力;然而，这些技术具有有限的效用的液态金属，由于其大的界面张力。

液态金属可以形成在电子和电磁设备的软的，可拉伸和形状重构组件。虽然是可能的操纵通过机械方法 （例如，泵送）这些流体，电的方法是比较容易的小型化，控制和执行。然而，大多数电气技术有自己的约束：电式电介质需要大量的（千伏）潜力适度的驱动，电毛细管会影响界面张力相对较小的变化，而不断ELEctrowetting仅限于液态金属在毛细管的插头。

这里，我们提出用于通过电化学表面反应致动镓和镓基液体金属合金的方法。控制液体金属在电解质迅速的表面上的电化学势和可逆地变化的界面张力数量级超过两个数量（̴50020mN / m至接近零）。此外，这种方法只需要一个非常温和的电位（<1 V）的施加相对于反电极。在张力所得变化主要是由于表面氧化层，其作为表面活性剂的电化学沉积;除去氧化的增加的界面张力，且反之亦然。这种技术可以在各种电解质被应用，是独立于所依靠的基板。

Introduction

This method provides a simple way to control the surface tension of liquid metals containing gallium. The method uses modest voltages (~1 V) applied directly to the liquid metal (relative to a counter electrode in the presence of electrolyte) to achieve enormous and reversible changes to the surface tension of the metal¹.

Surface tension is a dominant force for liquids at small length scales and is important for a number of capillary phenomena including wetting, spreading, and surface-tension driven flow. Consequently, the ability to control surface tension is a sensible way to manipulate the shape, position, and flow of liquids at sub-mm length scales. The most common way to alter surface tension between two fluids is to use a surfactant, which is a molecule that spans the interface between the fluids. Surfactants lower surface tension, but in a way that is not easy to reverse since it is difficult to remove surfactants from the interface. Surface tension can also be altered using a variety of techniques, including temperature gradients^2,3, light⁴, surface chemistry^5–7,and voltage⁸. But most of these methods result in modest changes to surface tension, particularly for liquid metals, which have notably large surface tensions.

The ability to control the surface tension of liquid metal could enable new opportunities for creating shape reconfigurable structures with metallic properties for electronic, thermal, and optical applications^9–14. The most common liquid metal is Hg, which is noted for its toxicity. The methods described here are relevant for liquid metals based on gallium. These metals have low viscosity, large surface tension, low volatility (low vapor pressure), and low toxicity¹⁵. Importantly, these metals form surface oxides composed of gallium oxide that are a few nm thick in air¹⁶. This oxide layer creates a physical skin that historically has been a nuisance for electrochemical and fluid dynamic applications¹⁷. The method here utilizes the oxide in new ways to control surface tension.

The most common way to manipulate liquid metals in electrolyte is to apply a potential to the metal relative to a counter electrode¹⁸. Oppositely charged ions from the electrolyte match the charges on the metal, causing the interfacial tension to drop. This phenomenon-termed electrocapillarity-has been known since the 1870s as described by Lippman¹⁹and has been utilized for alloys of gallium²⁰. Typically, electrocapillarity achieves modest changes to surface tension, since undesirable electrochemical reactions limit the range of voltages applied to the metal. In contrast, the method described here utilizes the surface oxidation of the metal (or conversely, the reduction of the surface oxide) as a way to achieve enormous changes in surface tension above and beyond changes resulting from electrocapillarity. The leading explanation for this phenomenon is that the oxide is asymmetric; that is, the outer surface of the oxide terminates with hydroxyl groups (making a low interfacial tension interface with the aqueous electrolyte), and the interior surface of the oxide terminates with gallium atoms (making a low interfacial tension interface with the metal). In contrast, the removal of the oxide via electrochemical reduction results in a bare metal-electrolyte interface, which returns the metal back to a state of high surface tension. We characterize the interfacial tension of the metal by analyzing the shape of sessile droplets as a function of voltage while assuming that gravity and surface tension are the dominant forces that define the curvature of its surface.

The advantage of this technique relative to classic electrocapillarity is that it can reversibly tune the tension of low toxicity liquid metals over enormous ranges (from ~500 mN/m to near zero). This delta change in surface tension may be the largest ever reported in literature for any fluid and it can be accomplished in a tunable and reversible manner. These large changes in surface tension are useful for manipulating the capillary behavior of metals; for example, it can induce the metal to spread on a surface, withdraw the metal from microchannels, fill microchannels with metal, and overcome the Rayleigh instabilities to form liquid metal fibers^1,21.

A drawback of this technique is that it requires electrolyte. It works best in acidic or basic conditions, because these electrolytes remove excess surface oxide that would otherwise contaminate the surface of the metal and mechanically restrict the movement of the metal. The simultaneous removal and deposition of the oxide layer complicates the analysis of the interfacial phenomena and it is our hope the methods described in this paper empowers additional analysis. Another disadvantage is that the electrochemical reactions at the surface of the metal must be matched by complimentary half-reactions at the counter electrode^22,23. This can lead to hydrogen bubbles forming at the counter electrode.

Protocol

1.操纵液态金属在电解液的界面张力氧化倾的含水电解质（酸性或碱性）到培养皿。以确保该氧化物完全除去，可使用的酸或碱与浓度大于0.1M的24（如1 M氢氧化钠或1M的HCl）。使用体积，将填补菜到大约1-3毫米的深度。避免与这些解决方案的皮肤接触。 用注射器放置在该电解质的镓基合金的下降（最佳之间10-500微升）。实例包括共晶镓铟（EGAIN）或镓铟锡（Galins…

Representative Results

图 1A示出了用于氧化和还原的简单的两电极技术的一个例子。在这种情况下，液态金属的70微升滴置于1 M氢氧化钠溶液接触的铜线，以建立电连接。的1 M氢氧化钠去除表面氧化物从金属和能使金属水珠，由于其界面张力。施加降和铂网状反电极之间一个2.5伏电势使液滴的表面氧化并在迁移朝向反电极（图1A…

Discussion

此方法控制使用小电压来驱动的表面氧化物的沉积和去除镓基液体金属的表面张力。尽管该方法只适用于在电解质溶液，它简单，和工作在多种不同的条件，但也有微妙之处值得​​留意。在无电势的，酸性和碱性溶液蚀刻掉氧化物^27。的氧化电势的应用驱动表面氧化物在所有水电解液，包括酸性和碱性溶液的形成。然而，该氧化物在酸性或碱性溶液溶解竞争与氧化物的沉积，以防止…

Materials

Eutectic Gallium Indium	Indium Corporation
Sodium Hydroxide	Fisher Scientific	2318-3
Hydrochloric Acid	Fisher Scientific	A481-212
Sodium Fluoride	Sigma-Aldrich	201154
Optical Adhesive	Norland	NOA81
Polydimethylsiloxane (Sylgard-184)	Dow Corning		Silicone Elastomer Kit
Borosilicate Glass Capillaries	Friedrich and Dimmoch	B41972
Ag/AgCl Reference Electrode	Microelectrodes Inc.	MI-401F
Voltage Source	Keithley	3390
Potentiostat	Gamry	Ref 600
Laser Cutter	Universal Laser Systems	VLS 3.50