A طريقة لمعالجة التوتر السطحي من المعدن السائل عن طريق الأكسدة السطحية والحد من
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
السيطرة على التوتر السطحي هو وسيلة فعالة لمعالجة الشكل، والموقف، وتدفق السوائل في جداول طول ملليمتر من الباطن، حيث التوتر السطحي هو القوة المهيمنة. مجموعة متنوعة من أساليب وجود للسيطرة على التوتر السطحي للسوائل المائية والعضوية في هذا النطاق. ومع ذلك، فإن هذه التقنيات فائدة للمعادن سائلة محدودة بسبب التوتر السطحي كبير بهم.
يمكن أن المعادن السائلة تشكل مكونات لينة، للمط، وشكل إعادة التشكيل في الأجهزة الإلكترونية والكهرومغناطيسية. على الرغم من أنه من الممكن التعامل مع هذه السوائل عبر الطرق الميكانيكية (على سبيل المثال، وضخ)، وأساليب الكهربائية هي أسهل للتصغير، والسيطرة، وتنفيذها. ومع ذلك، فإن معظم تقنيات الكهربائية لديها قيود الخاصة: يتطلب electrowetting على اساس عازلة كبيرة (كيلو فولت) إمكانات يشتغل متواضعة، electrocapillarity يمكن أن تؤثر التغيرات الصغيرة نسبيا في التوتر السطحي، وايلى المستمرctrowetting يقتصر على المقابس من المعدن السائل في الشعيرات الدموية.
هنا، فإننا نقدم وسيلة لالمشغلات الغاليوم وسبائك المعدن السائل القائم على الغاليوم عبر رد فعل السطح الكهروكيميائية. السيطرة على إمكانات الكهروكيميائية على سطح المعدن السائل في بالكهرباء بسرعة وعكسية يغير التوتر السطحي بما يزيد عن اثنين من حيث الحجم (̴500 مليون / م إلى ما يقارب الصفر). وعلاوة على ذلك، يتطلب هذا الأسلوب فقط إمكانات متواضعة جدا (<1 V) تطبق نسبة إلى القطب المضاد. ومن المقرر أساسا إلى ترسب الكهروكيميائية من طبقة أكسيد السطح، الذي يعمل بمثابة السطحي التغيير مما أدى إلى التوتر. إزالة أكسيد يزيد من التوتر السطحي، والعكس بالعكس. هذه التقنية يمكن تطبيقها في مجموعة واسعة من الشوارد ومستقلة عن الركيزة التي تستند إليها.
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 chemistry5–7,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 applications9–14. 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 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
Representative Results
Discussion
يتحكم هذا الأسلوب التوتر السطحي للمعادن على أساس الغاليوم السائلة باستخدام الفولتية صغيرة لدفع ترسب وإزالة أكسيد السطح. على الرغم من أن الأسلوب يعمل فقط في حلول بالكهرباء، أنها بسيطة، ويعمل في مجموعة واسعة من ظروف مختلفة، ولكن هناك الدقيقة الجدير بالذكر. في غياب…
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
- 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).
- Kataoka, D. E., Troian, S. M. Patterning liquid flow on the microscopic scale. Nature. 402 (6763), 794-797 (1999).
- 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).
- Ichimura, K., Oh, S. K., Nakagawa, M. Light-driven motion of liquids on a photoresponsive surface. Science. 288 (5471), 1624-1626 (2000).
- Gallardo, B. S., et al. Electrochemical principles for active control of liquids on submillimeter scales. Science. 283 (5398), 57-60 (1999).
- Zhao, B., Moore, J. S., Beebe, D. J. Surface-Directed Liquid Flow Inside Microchannels. Science. 291 (5506), 1023-1026 (2001).
- Chaudhury, M. K., Whitesides, G. M. How to Make Water Run Uphill. Science. 256 (5063), 1539-1541 (1992).
- Lahann, J., et al. A reversibly switching surface. Science. 299 (5605), 371-374 (2003).
- Rogers, J. A., Someya, T., Huang, Y. Materials and Mechanics for Stretchable Electronics. Science. 327 (5973), 1603-1607 (2010).
- 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).
- Ozbay, E. Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions. Science. 311 (5758), 189-193 (2006).
- Monat, C., Domachuk, P., Eggleton, B. J. Integrated optofluidics: A new river of light. Nat. Photonics. 1 (2), 106-114 (2007).
- Schurig, D., et al. Metamaterial Electromagnetic Cloak at Microwave Frequencies. Science. 314 (5801), 977-980 (2006).
- Dickey, M. D. Emerging Applications of Liquid Metals Featuring Surface Oxides. ACS Appl. Mater. Interfaces. 6 (21), 18369-18379 (2014).
- 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).
- Regan, M. J., et al. X-ray study of the oxidation of liquid-gallium surfaces. Phys. Rev. B. 55 (16), 10786-10790 (1997).
- Giguère, P. A., Lamontagne, D. Polarography with a Dropping Gallium Electrode. Science. 120 (3114), 390-391 (1954).
- Frumkin, A., Polianovskaya, N., Grigoryev, N., Bagotskaya, I. Electrocapillary phenomena on gallium. Electrochim. Acta. 10 (8), 793-802 (1965).
- Lippmann, G. . Relations entre les phénomènes électriques et capillaires. , (1875).
- 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).
- 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).
- Saltman, W., Nachtrieb, N. The Electrochemistry of Gallium. J. Electrochem. Soc. 100, 126-130 (1953).
- Perkins, R. Anodic-Oxidation of Gallium in Alkaline-Solution. J. Electroanal. Chem. 101, 47-57 (1979).
- 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).
- 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).
- Xia, Y., Whitesides, G. M. Soft Lithography. Annu. Rev. Mater. Sci. 28 (1), 153-184 (1998).
- Pourbaix, M. . Atlas of Electrochemical Equilibria in Aqueous Solutions. , (1974).
- Gough, R. C., et al. Rapid electrocapillary deformation of liquid metal with reversible shape retention. Micro Nano Syst. Lett. 3 (1), 1-9 (2015).
- 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).
