Summary

Un metodo per manipolare Tensione superficiale di un metallo liquido tramite ossidazione superficiale e riduzione

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

Controllo tensione interfacciale è un metodo efficace per manipolare la forma, la posizione e il flusso di fluidi su scale sub-millimetriche, dove tensione interfacciale è una forza dominante. Una varietà di metodi esistenti per il controllo della tensione interfacciale di liquidi acquosi e biologici su questa scala; Tuttavia, queste tecniche hanno un'utilità limitata per metalli liquidi a causa della loro grande tensione interfacciale.

Metalli liquidi possono formare componenti morbide, estensibili, e di forma-riconfigurabile in dispositivi elettronici ed elettromagnetici. Sebbene sia possibile manipolare questi fluidi attraverso metodi meccanici (ad esempio, di pompaggio), metodi elettrici sono più facili da miniaturizzare, controllo e attuazione. Tuttavia, la maggior parte delle tecniche elettrici hanno i loro limiti: elettrowetting-on-dielettrico richiede grandi (kV) potenzialità di modesta attuazione, electrocapillarity può incidere relativamente piccoli cambiamenti nella tensione superficiale, e ele continuactrowetting è limitata alle spine del metallo liquido in capillari.

Qui, presentiamo un metodo per l'azionamento di gallio e leghe metalliche liquido a base di gallio-via una reazione superficiale elettrochimica. Controllo del potenziale elettrochimico sulla superficie del metallo liquido in elettrolita rapidamente e reversibilmente cambia la tensione interfacciale di oltre due ordini di grandezza (̴500 mN / m quasi a zero). Inoltre, questo metodo richiede solo un potenziale molto modesta (<1 V) applicato relativamente ad un controelettrodo. La variazione conseguente tensionamento è dovuto principalmente alla deposizione elettrochimica di uno strato di ossido superficiale, che agisce come un tensioattivo; rimozione dell'ossido aumenta la tensione interfacciale, e viceversa. Questa tecnica può essere applicata in una vasta gamma di elettroliti ed è indipendente dal substrato su cui poggia.

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 manipolazione della tensione superficiale del metallo liquido in elettrolita Ossidazione Versare un elettrolita acquoso (acida o basica) in una capsula di Petri. Per assicurare che l'ossido viene completamente rimosso, usare un acido o base con una concentrazione maggiore di 0.1 M 24 (ad esempio 1 M NaOH o HCl 1 M). Utilizzare un volume che riempirà il piatto ad una profondità di circa 1-3 mm. Evitare il contatto con la pelle con queste soluzioni. Utilizzare …

Representative Results

Figura 1 A mostra un esempio della semplice tecnica a due elettrodi per l'ossidazione e riduzione. In questo caso, una goccia 70 ml di metallo liquido in una camera 1 M NaOH soluzione entra in contatto un filo di rame per formare una connessione elettrica. Il 1 M NaOH rimuove l'ossido superficiale del metallo e permette il metallo da bordare fino a causa della sua tensione interfacciale. L'applicazione di un potenziale 2…

Discussion

Questo metodo controlla la tensione superficiale di metalli liquidi a base di gallio-utilizzando piccole tensioni per guidare la deposizione e la rimozione di un ossido superficiale. Anche se il metodo funziona solo in soluzioni elettrolitiche, è semplice, e lavora in un'ampia varietà di condizioni diverse, ma ci sono sottigliezze degne di nota. In assenza di potenziale elettrico, sia soluzioni acide e basiche etch via l'ossido 27. L'applicazione di un potenziale ossidativo guida la formaz…

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

References

  1. Khan, M. R., Eaker, C. B., Bowden, E. F., Dickey, M. D. Giant and switchable surface activity of liquid metal via surface oxidation. Proc. Natl. Acad. Sci. 111 (39), 14047-14051 (2014).
  2. Kataoka, D. E., Troian, S. M. Patterning liquid flow on the microscopic scale. Nature. 402 (6763), 794-797 (1999).
  3. Daniel, S., Chaudhury, M. K., Chen, J. C. Fast Drop Movements Resulting from the Phase Change on a Gradient Surface. Science. 291 (5504), 633-636 (2001).
  4. Ichimura, K., Oh, S. K., Nakagawa, M. Light-driven motion of liquids on a photoresponsive surface. Science. 288 (5471), 1624-1626 (2000).
  5. Gallardo, B. S., et al. Electrochemical principles for active control of liquids on submillimeter scales. Science. 283 (5398), 57-60 (1999).
  6. Zhao, B., Moore, J. S., Beebe, D. J. Surface-Directed Liquid Flow Inside Microchannels. Science. 291 (5506), 1023-1026 (2001).
  7. Chaudhury, M. K., Whitesides, G. M. How to Make Water Run Uphill. Science. 256 (5063), 1539-1541 (1992).
  8. Lahann, J., et al. A reversibly switching surface. Science. 299 (5605), 371-374 (2003).
  9. Rogers, J. A., Someya, T., Huang, Y. Materials and Mechanics for Stretchable Electronics. Science. 327 (5973), 1603-1607 (2010).
  10. Bauer, S., et al. 25th Anniversary Article: A Soft Future: From Robots and Sensor Skin to Energy Harvesters. Adv. Mater. 26 (1), 149-162 (2013).
  11. Ozbay, E. Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions. Science. 311 (5758), 189-193 (2006).
  12. Monat, C., Domachuk, P., Eggleton, B. J. Integrated optofluidics: A new river of light. Nat. Photonics. 1 (2), 106-114 (2007).
  13. Schurig, D., et al. Metamaterial Electromagnetic Cloak at Microwave Frequencies. Science. 314 (5801), 977-980 (2006).
  14. Dickey, M. D. Emerging Applications of Liquid Metals Featuring Surface Oxides. ACS Appl. Mater. Interfaces. 6 (21), 18369-18379 (2014).
  15. Dickey, M. D., et al. Eutectic gallium-indium (EGaIn): A liquid metal alloy for the formation of stable structures in microchannels at room temperature. Adv. Funct. Mater. 18 (7), 1097-1104 (2008).
  16. Regan, M. J., et al. X-ray study of the oxidation of liquid-gallium surfaces. Phys. Rev. B. 55 (16), 10786-10790 (1997).
  17. Giguère, P. A., Lamontagne, D. Polarography with a Dropping Gallium Electrode. Science. 120 (3114), 390-391 (1954).
  18. Frumkin, A., Polianovskaya, N., Grigoryev, N., Bagotskaya, I. Electrocapillary phenomena on gallium. Electrochim. Acta. 10 (8), 793-802 (1965).
  19. Lippmann, G. . Relations entre les phénomènes électriques et capillaires. , (1875).
  20. Tsai, J. T. H., Ho, C. M., Wang, F. C., Liang, C. T. Ultrahigh contrast light valve driven by electrocapillarity of liquid gallium. Appl. Phys. Lett. 95 (25), 251110 (2009).
  21. Khan, M. R., Trlica, C., Dickey, M. D. Recapillarity: Electrochemically Controlled Capillary Withdrawal of a Liquid Metal Alloy from Microchannels. Adv. Funct. Mater. 25 (5), 671-678 (2015).
  22. Saltman, W., Nachtrieb, N. The Electrochemistry of Gallium. J. Electrochem. Soc. 100, 126-130 (1953).
  23. Perkins, R. Anodic-Oxidation of Gallium in Alkaline-Solution. J. Electroanal. Chem. 101, 47-57 (1979).
  24. Xu, Q., Oudalov, N., Guo, Q., Jaeger, H. M., Brown, E. Effect of oxidation on the mechanical properties of liquid gallium and eutectic gallium-indium. Phys. Fluids. 24, 063101 (2012).
  25. Rotenberg, Y., Boruvka, L., Neumann, A. W. Determination of surface tension and contact angle from the shapes of axisymmetric fluid interfaces. J. Colloid Interface Sci. 93, 169-183 (1983).
  26. Xia, Y., Whitesides, G. M. Soft Lithography. Annu. Rev. Mater. Sci. 28 (1), 153-184 (1998).
  27. Pourbaix, M. . Atlas of Electrochemical Equilibria in Aqueous Solutions. , (1974).
  28. Gough, R. C., et al. Rapid electrocapillary deformation of liquid metal with reversible shape retention. Micro Nano Syst. Lett. 3 (1), 1-9 (2015).
  29. Wang, M., Trlica, C., Khan, M. R., Dickey, M. D., Adams, J. J. A reconfigurable liquid metal antenna driven by electrochemically controlled capillarity. J. Appl. Phys. 117 (19), 194901 (2015).

Play Video

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).

View Video