表面酸化と還元を経て液体金属の表面張力を操作する方法

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

界面張力を制御することは界面張力が支配的な力であるサブミリメートルの長さのスケールでの形状、位置、及び流体の流れを操作するための有効な方法です。この規模での水と有機液体の界面張力を制御するための様々な方法が存在します。しかし、これらの技術は、それらの大きな界面張力による液体金属の有用性が限られています。

液体金属は電気及び電磁機器で、柔らかい伸縮性、および再構成可能な形状の部品を形成することができます。それは機械的な方法（例えば、ポンプ）を介してこれらの流体を操作することが可能であるが、電気的な方法は、コントロールを小型化し、実装が容易です。しかし、ほとんどの電気技術は、独自の制約を持っている：オン誘電体エレクトロウェッティングは、ささやかな作動のための大規模な（キロボルト）電位、電気毛管現象は、界面張力の比較的小さな変化に影響を与えることができ、連続ELEが必要ですctrowettingは毛細血管内の液体金属のプラグに限定されています。

ここでは、電気化学的な表面反応を介して、ガリウムおよびガリウム系液体金属合金を作動させるための方法を提示します。迅速かつ可逆的に電解液中の液体金属の表面に電気化学ポテンシャルを制御すること（ゼロ近くまで̴500MN / m）の大きさの2以上のご注文によって界面張力を変化させます。さらに、この方法は、非常に控えめな電位（<1 V）を必要と対向電極に対して印加されます。張力の結果として生じる変化は、主に界面活性剤として作用する表面酸化物層の電気化学的堆積によるものです。酸化物の除去は、界面張力、およびその逆を増加させます。この技術は、電解質の多種多様に適用され、それが置かれた基板とは無関係であることができます。

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

電解液中の液体金属の界面張力の1.操作酸化ペトリ皿に（酸性または塩基性）水性電解質を注ぎます。酸化物が完全に除去されたことを確認するために、0.1 M 24よりも高い濃度（例えば、1M NaOHまたは1MのHCl）で、酸または塩基を使用しています。約1〜3ミリメートルの深さに料理を記入しますボリュームを使用してください。これらのソリューションで皮膚を触れ?…

Representative Results

図 1Aは、酸化と還元のための単純な二電極法の一例を示す図です。この例では、液体金属の70μlの滴を1M NaOH溶液に接触して電気接続を確立するための銅線を入れました。 1 M NaOHが金属から表面酸化物を除去し、金属は、その界面張力までビーズにできます。ドロップ白金メッシュ対電極との間に2.5Vの電位を印加すると、液滴…

Discussion

この方法は、表面酸化物の堆積及び除去を駆動するために小さな電圧を使用ガリウム系液体金属の表面張力を制御します。メソッドは、電解質溶液中で動作しますが、それは単純で、異なる多種多様な状態で動作しますが、注目すべき微妙な点があります。電位の非存在下で、酸性および塩基性の両方の溶液は、酸化物^{27をエッチング} 。酸化電位の印加は、酸性および塩基性の?…

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