Summary

Une méthode pour manipuler tension de surface du liquide par l'intermédiaire d'un métal oxydation de surface et la réduction

Published: January 26, 2016
doi:

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

Commande de la tension interfaciale est une méthode efficace pour manipuler la forme, la position et l'écoulement des fluides à des échelles de longueur submillimétriques, où la tension interfaciale est une force dominante. Une variété de procédés existent pour commander la tension interfaciale des liquides aqueux et organiques sur cette échelle; Cependant, ces techniques ont une utilité limitée pour les métaux liquides, en raison de leur grande tension interfaciale.

Métaux liquides peuvent former des composants souples, extensibles, et la forme reconfigurable dans des dispositifs électroniques et électromagnétiques. Bien qu'il soit possible de manipuler ces fluides par des méthodes mécaniques (par exemple, pompage), méthodes électriques sont plus faciles à miniaturiser, le contrôle et la mise en œuvre. Cependant, la plupart des techniques électriques ont leurs propres contraintes: électromouillage sur diélectrique nécessite de grandes (kV) potentiels pour modeste actionnement, électrocapillarité peut affecter relativement petits changements dans la tension interfaciale, et ele continuctrowetting est limitée à fiches du métal liquide dans les capillaires.

Ici, nous présentons une méthode pour actionner le gallium et des alliages de métaux liquide à base de gallium par une réaction électrochimique de surface. Le contrôle du potentiel électrochimique sur la surface du métal liquide dans l'électrolyte modifie rapidement et de manière réversible la tension interfaciale de plus de deux ordres de grandeur (̴500 mN / m à près de zéro). En outre, ce procédé nécessite seulement un potentiel très modeste (<1 V) appliquée par rapport à une contre-électrode. La variation résultante de la tension est due principalement au dépôt électrochimique d'une couche d'oxyde de surface, qui agit comme un agent tensio-actif; enlèvement de l'oxyde augmente la tension interfaciale, et vice versa. Cette technique peut être appliquée dans une grande variété d'électrolytes et est indépendante du substrat sur lequel elle repose.

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 gradients2,3, light4, surface chemistry57,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 applications914. 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 fibers1,21.

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 electrode22,23. This can lead to hydrogen bubbles forming at the counter electrode.

Protocol

1. La manipulation de la tension interfaciale de Liquid Metal dans l'électrolyte Oxydation Verser un électrolyte aqueux (acide ou basique) dans une boîte de Petri. Pour faire en sorte que l'oxyde est complètement enlevée, utiliser un acide ou une base à une concentration supérieure à 0,1 M 24 (par exemple NaOH 1 M ou HCl 1 M). Utiliser un volume qui permettra de combler le plat à une profondeur d'environ 1-3 mm. Éviter tout contact avec la peau avec ces solut…

Representative Results

A la figure 1 montre un exemple de la technique simple à deux électrodes pour oxydation et de réduction. Dans ce cas, une goutte de 70 ul du métal liquide placé dans une NaOH 1 M de solution de contacts un fil de cuivre pour établir une connexion électrique. Le NaOH 1 M enlève l'oxyde de métal de la surface et permet au métal à perler en raison de sa tension interfaciale. L'application d'un potentiel de 2,5 V …

Discussion

Cette méthode contrôle la tension de surface des métaux liquides à base de gallium en utilisant de petites tensions pour entraîner le dépôt et le retrait d'un oxyde de surface. Bien que la méthode ne fonctionne que dans les solutions électrolytiques, il est simple, et travaille dans une grande variété de conditions différentes, mais il ya des subtilités à noter. En l'absence de tension électrique, les deux solutions acides et basiques etch à une distance de l'oxyde 27. L'…

Disclosures

The authors have nothing to disclose.

Acknowledgements

The authors acknowledge support from Samsung, the NC State Chancellors Innovation Funds, NSF (CAREER CMMI-0954321 and Triangle MRSEC DMR-1121107), and Air Force Research Labs.

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

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Cite This Article
Eaker, C. B., Khan, M. R., Dickey, M. D. A Method to Manipulate Surface Tension of a Liquid Metal via Surface Oxidation and Reduction. J. Vis. Exp. (107), e53567, doi:10.3791/53567 (2016).

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