A recent article (ACS Omega 2021, 6, 8021−8027) confirms the stability of nanobubbles with theoretical principles. Nanobubbles, having a size in the range of 50-500 nm, are reported to be metastable, i.e., stable in various liquids for times ranging up to weeks. Is the inside of the bubble filled with a gaseous form of the liquid surrounding the bubble, or with some other gas? Are there repeatable observations of nanobubbles? Is the theoretical derivation even correct?
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asked Apr 1, 2021 at 14:26
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$\begingroup$ Well, the normal big bubbles can't be born big, can they? $\endgroup$– Ivan NeretinCommented Apr 1, 2021 at 14:30
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$\begingroup$ @Ivan: my ignorance about nanobubbles concerns the distinction between ordinary bubbles, whose behavior is reasonably well understood, and this new (?) class of bubbles, which is claimed to be so different. This resembles "nascent" hydrogen, etc., which isn't really thought of anymore. Maybe there is some connection. But I'm wondering about freely suspended bubbles. $\endgroup$– James GaidisCommented Apr 2, 2021 at 13:29
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$\begingroup$ Is it true that the reference you give does not mention what the bubble are made of, or supposed to be made of? Obviously there should not be vacuum, so either liquid vapors, or air /gases bubbled inside (unless one think of a robust cage extra resistant to inwards collapse, but how could they form at first?) $\endgroup$– AlchimistaCommented Apr 3, 2021 at 8:18
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$\begingroup$ @Alchemista: one other reference suggested that nanobubbles have a negative charge that stabilizes a skin structure against intrusion and expands it to be less dense inside. It is not clear to me whether all nanobubbles are assumed to be in water; sometimes other liquids are mentioned, but whether as the body or the contents of a bubble is not defined. I wonder if there is any similarity to defects or vacancies in solid metals. Water does have an extended hydrogen-bonded structure. $\endgroup$– James GaidisCommented Apr 3, 2021 at 12:50
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Inside nanobubbles would probably be "a high-pressure gas". Whether it be an inert gas or vaporized surrounding liquid depends on the temperature, pressure, components, and many other experimental variables.
Regarding theory on nanobubbles, and starting from a classical, macro-scale standpoint, the Laplace pressure describes the pressure inside a bubble due to surface tension on the curved vapor-liquid surface. The pressure will be necessarily higher inside the bubble in order that it doesn't collapse. The back-of-the-envelope calculation on that Wikipedia page indicates around 10 bar for a 300 nm bubble in water.
For smaller nanobubbles, things get more interesting. The surface tension is not constant with curvature and at the extreme curvatures necessary for nanobubbles this will change things. A first-order approximation to surface tension corrections is the Tolman length. Also, at the higher range of pressures involved gases would not be ideal, so some equation of state is likely necessary to describe the interior phase. As things get even smaller, then you get into the details where the continuum approximations like surface tension break down entirely.
Song et al. comes to my mind of a paper where they look at nanobubbles in an actual experiment. I'm sure there are many other experiments in this area.
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$\begingroup$ This seems like conventional thinking. Micro bubbles, ~10 microns, are observable in carbonated beverages. That's as small as they get; surfactants don't make them smaller. Song's bubbles are pinned to a surface - I'm more interested in free-standing bubbles that don't even float in a liquid. The Tolman length is new to me, but seems to say a chain can have different tensions along its length - interesting theory, but hard to use here. High pressure vapor or other gas should dissolve in the liquid; nanobubbles are claimed to be stable for weeks. What structural elements could exist? $\endgroup$ Commented Apr 2, 2021 at 13:20
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$\begingroup$ @JamesGaidis then it is some kind of cage.... But still the question remains about what the interior is... It is a nice point to think of. It made me think of fullerenes as nanobubbles.... They certainly are, though covalent. I know that my last sentences aren't related to the bubbles at discussion, but the point of view / thinking are. $\endgroup$ Commented Apr 4, 2021 at 11:13