Summary

Un método para manipular tensión superficial de un metal líquido a través de la oxidación de la superficie y Reducción

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

El control de la tensión interfacial es un método eficaz para la manipulación de la forma, posición, y el flujo de líquidos a escalas de longitud submilimétricas, donde la tensión interfacial es una fuerza dominante. Una variedad de métodos para controlar la tensión interfacial de líquidos acuosos y orgánicos en esta escala; Sin embargo, estas técnicas han limitado la utilidad para metales líquidos debido a su gran tensión interfacial.

Metales líquidos pueden formar componentes blandos, elásticos y con forma reconfigurable en dispositivos electrónicos y electromagnéticos. A pesar de que es posible manipular estos fluidos a través de métodos mecánicos (por ejemplo, bombeo), métodos eléctricos son más fáciles de miniaturizar, control, e implementar. Sin embargo, la mayoría de las técnicas eléctricos tienen sus propias limitaciones: electrohumectación-on-dieléctrico requiere grandes (kV) potenciales para el accionamiento modesta, electrocapillarity puede afectar a cambios relativamente pequeños en la tensión interfacial, y ele continuoctrowetting se limita a los enchufes del metal líquido en los capilares.

A continuación, presentamos un método para el accionamiento de galio y aleaciones de metal líquido a base de galio mediante una reacción electroquímica superficie. Controlar el potencial electroquímico en la superficie del metal líquido en un electrólito rápidamente y de forma reversible cambia la tensión interfacial por más de dos órdenes de magnitud (̴500 mN / m a cerca de cero). Además, este método requiere sólo un potencial muy modesta (<1 V) aplicada respecto a un electrodo contador. El cambio resultante en la tensión se debe principalmente a la deposición electroquímica de una capa de óxido de la superficie, que actúa como un agente tensioactivo; la eliminación del óxido aumenta la tensión interfacial, y viceversa. Esta técnica se puede aplicar en una amplia variedad de electrolitos y es independiente del sustrato sobre la que descansa.

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 manipulación de la tensión interfacial de Liquid Metal en electrolitos Oxidación Verter un electrolito acuoso (ácido o básico) en una placa de Petri. Para asegurar que el óxido se elimina por completo, utilice un ácido o base con una concentración superior a 0,1 M 24 (por ejemplo, 1 M de NaOH o HCl 1 M). Utilice un volumen que va a llenar el plato hasta una profundidad de aproximadamente 1-3 mm. Evite el contacto con la piel con estas soluciones. Utilice un…

Representative Results

Figura 1 A muestra un ejemplo de la técnica simple de dos electrodos para la oxidación y reducción. En este caso, una caída de 70 l del metal líquido colocado en una relación 1 M de NaOH solución entra en contacto un alambre de cobre para establecer una conexión eléctrica. El 1 M NaOH elimina el óxido de la superficie del metal y permite que el metal a talón hasta debido a su tensión interfacial. La aplicación de un pot…

Discussion

Este método controla la tensión superficial del líquido a base de metales de galio utilizando pequeñas tensiones para conducir la deposición y la eliminación de un óxido superficial. Aunque el método sólo funciona en soluciones de electrolitos, es simple, y trabaja en una amplia variedad de diferentes condiciones, pero hay matices dignos de mención. En ausencia de potencial eléctrico, ambas soluciones ácidas y básicas etch distancia El óxido de 27. La aplicación de un potencial oxidativo …

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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