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A comment on Engineers discover new class of materials that passively harvest water from air. Parent comment

Re:Engineers discover new class of materials that passively ... by mppm on Monday May 26, 2025 @01:10PM (#44099346)
Keeping the temperature constant with a thermostat is not an issue here. That would only explain things if the surface were kept cooler than the surrounding air (below the dew point), but from the description in the paper that does not seem to be the case. They basically claim that macroscopic droplets form spontaneously from an unsaturated vapor. And no, this is not something permitted by the second law of thermodynamics.
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Re:Engineers discover new class of materials that passively ... by dotancohen on Monday May 26, 2025 @02:26PM (#44100137) 

  > And no, this is not something permitted by the second law of thermodynamics.
If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations—then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation—well these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.
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Re:Engineers discover new class of materials that passively ... by lolinder on Monday May 26, 2025 @03:22PM (#44100699)
— Sir Arthur Stanley Eddington, The Nature of the Physical World (1927)
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Re:Engineers discover new class of materials that passively ... by robertclaus on Monday May 26, 2025 @04:51PM (#44101563)
While I generally agree that it sounds dubious, this argument depends on whether the entropy of the liquid in the pore is lower than the entropy of the vapor in the air in the pore. I could see a highly hydrophilic capillary restricting a vapor enough to where it has better entropy in a liquid state.

If that's true we just need to balance energy, which the cooler does.

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Re:Engineers discover new class of materials that passively ... by mppm on Monday May 26, 2025 @05:57PM (#44102062)
> I could see a highly hydrophilic capillary restricting a vapor enough to where it has better entropy in a liquid state.

My other comment here (and and a reply to a similar question) has more detail [1], but in short: this is true for capillaries and pores, it is not true for "collectable" droplets on a flat surface.

1. https://news.ycombinator.com/item?id=44099078

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Re:Engineers discover new class of materials that passively ... by ummonk on Monday May 26, 2025 @09:33PM (#44103184)
Replied to that comment as well but per the article they're not droplets on a flat surface, but rather droplets connected to pores by surface tension.
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Re:Engineers discover new class of materials that passively ... by bm62 on Tuesday May 27, 2025 @08:25PM (#44111789)
Practically it just means that the energy to form the droplets is coming from somewhere else, just not via cooling the surface below the dew point. For instance, you could imagine something like squeezing a material that undergoes capillary condensation to get the water out, since you'd pay the requisite energy cost via mechanical work.
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