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Water vapor from the ambient air condenses spontaneously inside porous materials or between contacting surfaces. But since the liquid layer is only a few molecules thick, this ubiquitous and important phenomenon has so far not been understood.
Researchers at the University of Manchester led by Nobel Laureate Andre Geim – who, along with Kostya Novoselov, received the Nobel Prize in Physics 10 years ago this month – have made artificial capillaries small enough that water vapor condenses there under normal ambient conditions. .
The Manchester study is titled “Capillary Condensation Under Confinement at the Atomic Scale” and will be published in Nature. The research provides a solution to the century-and-a-half puzzle of why capillary condensation, a fundamentally microscopic phenomenon involving a few molecular layers of water, can be fairly well described using macroscopic equations and macroscopic characteristics of the bulk water. Is it a coincidence or a hidden law of nature?
Capillary condensation, a classic phenomenon, is ubiquitous in the world around us, and properties as important as friction, adhesion, stiction, lubrication and corrosion are strongly affected by capillary condensation. This phenomenon is important in many technological processes used by the microelectronics, pharmaceutical, food and other industries – and even sandcastles could not be built by children except for capillary condensation.
Scientifically, the phenomenon is often described by the 150-year-old Kelvin equation which has been shown to be remarkably accurate even for capillaries as small as 10 nanometers, a thousandth the width of human hair. However, for condensation to occur under normal humidity of 30% to 50%, for example, the capillaries must be much smaller, about 1 nm in size. This is comparable to the diameter of water molecules (around 0.3nm), so that only a few molecular layers of water can fit inside the pores responsible for common condensation effects.
Kelvin’s macroscopic equation could not be justified to describe properties involving the molecular scale and, in fact, the equation makes little sense at this scale. For example, it is impossible to define the curvature of a water meniscus, which goes into the equation, if the meniscus is only a few molecules wide. As a result, Kelvin’s equation has been used as a poor man’s approach for lack of a proper description. Scientific progress has been hampered by numerous experimental problems and, in particular, by surface roughness which makes it difficult to make and study capillaries of sizes at the required molecular scale.
To create such capillaries, researchers at Manchester painstakingly assembled atomically flat crystals of mica and graphite. They put two of these crystals on top of each other with narrow bands of graphene, another atomically thin and flat crystal, placed in between. The bands acted as spacers and could be of different thickness. This three-layer assembly allowed capillaries of different heights. Some of them were just one atom top, the smallest possible capillaries, and could accommodate a single layer of water molecules.
The Manchester experiments showed that the Kelvin equation can describe capillary condensation even in the smallest capillaries, at least qualitatively. This is not only surprising, but contradicts general expectations, as water changes its properties on this scale and its structure becomes distinctly discrete and layered.
“It was a big surprise. I expected a complete break in conventional physics, ”said Dr. Qian Yang, senior author of the Nature report. “The old equation turned out to work well. A little disappointing but also fascinating to finally solve the century-old mystery.
“So that we can relax, all of these many condensation effects and related properties are now supported by hard evidence rather than a hunch that ‘it seems to work, so it should be okay to use the equation.’
Manchester researchers argue that the agreement reached, while qualitative, is also fortuitous. The pressures involved in capillary condensation under ambient humidity exceed 1000 bars, more than that at the bottom of the deepest ocean. Such pressures cause the capillaries to adjust their sizes by a fraction of an angstrom, which is enough to perfectly accommodate only a whole number of molecular layers within. These microscopic adjustments remove the effects of commensurability, allowing the Kelvin equation to hold up well.
“A good theory often works beyond its limits of applicability,” Geim said.
“Lord Kelvin was a remarkable scientist, making many discoveries, but even he would surely be surprised to find that his theory – originally considering millimeter-sized tubes – even holds the scale of an atom. In fact, in his seminal article, Kelvin commented on exactly this impossibility.
“So our work proved him both right and wrong.”
Lord Kelvin
Sir William Thomson, later Lord Kelvin (1824-1907), first referred to his famous equation in an article entitled “On the equilibrium of vapor at a curved surface of liquid” published in 1871 in the Philosophical Magazine . Kelvin’s significant contributions to science included a major role in the development of the second law of thermodynamics; the absolute temperature scale (measured in Kelvin); the dynamic theory of heat; the mathematical analysis of electricity and magnetism, including the basic ideas of the electromagnetic theory of light; as well as fundamental work in hydrodynamics.
Reference: “Capillary condensation under confinement at the atomic scale” December 9, 2020, Nature.
DOI: 10.1038 / s41586-020-2978-1
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