En metode for å manipulere overflatespenningen i en Liquid Metal via Surface Oksidasjon og reduksjon

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

Kontrollere grenseflatespenning er en effektiv metode for å manipulere form, posisjon, og strømmen av væske på sub-millimeter lengdeskala, der overflatespenningen er en dominerende kraft. En rekke metoder finnes for styring av grenseflatespenning i vandige og organiske væsker på denne skalaen; imidlertid har disse teknikkene begrenset nytte for flytende metaller på grunn av deres store grenseflatespenningen.

Flytende metaller kan danne myke, elastiske, og formrekonfigurer komponenter i elektroniske og elektromagnetiske enheter. Selv om det er mulig å manipulere disse fluider via mekaniske metoder (for eksempel pumping), elektriske metoder er enklere å miniatyrisere, kontroll, og implementere. Men de fleste elektriske teknikker har sine egne begrensninger: electrowetting-on-dielektrisk krever store (kV) potensial for moderat aktuering, electrocapillarity kan påvirke forholdsvis små endringer i grenseflatespenning, og kontinuerlig electrowetting er begrenset til plugger av det flytende metall i kapillærer.

Her presenterer vi en fremgangsmåte for aktivering av gallium og gallium-baserte flytende metall-legeringer ved hjelp av en elektrokjemisk overflatereaksjon. Regulering av elektrokjemisk potensial på overflaten av det flytende metall på elektrolytt hurtig og reversibelt forandrer grenseflatespenningen med mer enn to størrelsesordener (̴500 mN / m til nær null). Videre krever denne fremgangsmåte bare en meget beskjeden potensial (<1 V) anvendes i forhold til en motelektrode. Den resulterende endring i spenning skyldes i første rekke den elektrokjemiske avsetning av et overflate oksydlag, som virker som et overflateaktivt middel; fjerning av oksyd øker grenseflatespenning, og vice versa. Denne teknikken kan brukes i en rekke forskjellige elektrolytter og er uavhengig av underlaget på hvilket det hviler.

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. Manipulering av overflatespenningen av Liquid Metal i elektrolytt Oksidasjon Helle en vandig elektrolytt (sur eller basisk) i en petriskål. For å sikre at oksydet er helt fjernet ved å bruke en syre eller base med en konsentrasjon høyere enn 0,1 M 24 (for eksempel 1 M NaOH og 1 M HCl). Bruker et volum som vil fylle formen til en dybde på omtrent 1-3 mm. Unngå å kontakte huden med disse løsningene. Benytte en sprøyte for å plassere en dråpe (optimalt mellom …

Representative Results

Figur 1 A viser et eksempel på det enkle to-elektrode teknikk for oksydasjon og reduksjon. I dette tilfellet, til en 70 pl dråpe av det flytende metall anbragt i en 1 M NaOH-løsning i kontakt med en kobbertråd etablere en elektrisk forbindelse. Den 1 M NaOH fjerner overflateoksydet fra metallet og tillater metallet å perle opp på grunn av dens grenseflatespenning. Bruk av en 2,5 V potensial mellom dråpen og en platina motelek…

Discussion

Denne fremgangsmåten kontrollerer overflatespenningen av gallium-baserte flytende metaller ved hjelp av små spenninger for å drive avsetning og fjernelse av en overflate oksyd. Selv om fremgangsmåten virker bare i elektrolyttoppløsninger, er det enkelt, og arbeider i et bredt utvalg av forskjellige betingelser, men det er verdt å merke seg nyanser. I fravær av elektrisk potensial, både sure og basiske løsninger etse bort oksyd ^27. Anvendelsen av en oksyderende potensial driver dannelse av overf…

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