Article pubs.acs.org/Langmuir
Bubble Formation at a Gas-Evolving Microelectrode Damaris Fernández, Paco Maurer, Milena Martine, J. M. D. Coey, and Matthias E. Möbius* School of Physics, Trinity College, Dublin 2, Ireland S Supporting Information *
ABSTRACT: The electrolytic production of gas bubbles involves three stepsnucleation, growth, and detachment. Here the growth of hydrogen bubbles and their detachment from a platinum microelectrode of diameter 125 μm are studied using high-speed photography and overpotential frequency spectrum (noise) analysis. The periodic release of large 1 (mN m−1
Ȧ = 6V̇ /d = 3jπde 2kBT /(4eP0d)
(11)
This is the quantity of interest for solid−liquid separation techniques such as electroflotation. The amount of gas−liquid interface produced is greatly enhanced in the aperiodic regime, where the bubble size can be less than 50 μm.
5. CONCLUSIONS By combining electrochemical measurements, frequency analysis, and high-speed photography, it has been possible to build up a picture of the growth and release of bubbles of hydrogen gas at a platinum microelectrode. The electrode is always covered by a gas bubble in the periodic gas oscillator regime, but the contact angle changes with time. Lift-off occurs for bubble diameters that are invariably smaller than expected from a balance of buoyancy and surface tension forces, and a possible explanation has been suggested, namely, a broken triple line of contact between the large bubble and the solid surface because of the presence of tiny bubbles adhering to the surface. These bubbles have not been observed directly, but their presence is inferred from the fact that the footprint of the bubble can exceed the area of the microelectrode while hydrogen is still being generated and the bubble is growing.28 The supersaturated electrolyte is continuously nucleating small bubbles, some of which coalesce directly with the growing bubble, leaving a trace of sessile droplets. Further investigation using techniques with improved spatial resolution53,54 will be needed to reveal, for example, whether nanobubbles23,43,44 or a dynamic submicrometer-scale foam of normal bubbles is present. Optical methods are able to visualize flat nanobubbles.54 The critical property, which controls the transition from a periodic regime with the evolution of large bubbles to an aperiodic regime with many smaller ones, appears to be the small change in surface tension produced by various additives. When it falls below 70 mN/m, the gas oscillator behavior cannot be sustained with the 125 μm microelectrode. Bubbles of various sizes jostle over the surface, but their coalescence is inhibited by additives, which create a gradient of surface tension in the interstitial electrolyte. A study of the effect of the electrode diameter should be undertaken next, and the effects of different categories of ionic and nonionic additives need to be investigated systematically, as well as direct evidence for the scavenging growth mechanism. The novel observation of sessile liquid droplets within the footprint of a large bubble, due to the ejection of the interface 13072
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electrolyte when the interface ruptures on coalescence, provides a new method for measuring the thickness of the interface layer as a function of bubble size. The results reported here can help in the design of lithographically patterned microelectrode arrays to produce streams of hydrogen gas bubbles with a desired size or size distribution, which may have applications in areas such as particle separation5 and ultrasonic imaging.55
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ASSOCIATED CONTENT
S Supporting Information *
A video from bubble formation at the transparent electrode. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
This article is based on contributions by all of the authors. It was written by D.F., J.M.D.C., and M.E.M. All authors have given approval to the final version of the article. Funding
The work was supported by Science Foundation under contract numbers 10/IN1/I3002 and 13/ERC/12561 and by Science foundation Ireland RFP grant 11/RFP-1/MTR/3135. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.M.D.C. is grateful to Siddhartha Sen for helpful discussions. REFERENCES
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