Photomolecular effect leading to water evaporation exceeding the thermal limit | Hacker News Reader

Photomolecular effect leading to water evaporation exceeding the thermal limit

pnas.org

59 pointscroes21 comments

I’m not even quite an arm chair scientist, but this seems like a big deal to me. It seems like it would affect a lot of assumptions that have been made over time involving evaporation like weather and climate predictions. Can anyone that knows more about this chime in?

Yes, this is a big deal. You can think an analogy to the photoelectric effect where a photon excites an electron and for the photomolecular effect, a photon with the proper 'excites' a molecule, breaking the molecule free from its bulk.

Nothing in the abstract seems to tell the magnitude of the effect or how it differs in hydrogels vs other natural surfaces. Anyone have details on those from behind the paywall?

After checking a widely-used hub of scientific articles, in this case unsuccessfully, a possible next step is to send an email to one of the authors of the article to see if they have a draft or preprint version of the article that they can email back to you. Often the differences between the preprint version and the published version are insignificant.

I found https://arxiv.org/pdf/2201.10385.pdf by the same authors.

If I read it correctly, it says the theoretical limit for thermal evaporation is 1.45 kg/(m²h), while experimentally, values as high as 4-5 kg/(m²h) and over 10 kg/(m²h) have been reported in two-, respectively three-dimensional structures.

That’s a huge difference, but in my uneducated opinion not likely to be completely caused by this effect.

Edit: reading https://news.mit.edu/2023/surprising-finding-light-makes-wat..., which says “and the excess has been significant — a doubling, or even a tripling or more, of the theoretical maximum rate”, I read that correctly. I’m beginning to doubt my “not likely to be completely caused by this effect” hunch, though,

First pdf source says it happens in darkness with hydrogels too, but doesn't directly say the magnitude (has another reference), just that it is smaller.

This is a bit of inside baseball but many climate models use something called a "bulk formula" for evaporation [], which is empirically derived from real observations of the sea surface. Hopefully which (already) captures this novel phenomenon.

[] https://oceanwiki.ethz.ch/doku.php?id=lecture7:bulkformula

>real observations of the sea surface

I suspect this effect is more relevant to plant transpiration, which has a massive effect on terrestrial rainfall patterns.[0]

Do they use similar methodologies to capture land-based evapotranspiration? Does the resulting simplified model have inputs that allow it to capture real-world variations in this effect?

[0] https://www.nature.com/articles/nature11983

Don’t worry, it’ll be no time at all until the climate xrisk people explain why this means even more draconian measures are necessary.

An earlier discovery draft paper is freely available, but only guesses at mechanisms: https://arxiv.org/pdf/2201.10385.pdf

This is one of those things that seems fairly obvious once pointed out: that the energy in light might be sufficient to directly break hydrogen bonds and separate water molecules from surfaces, especially in regions where water's absorption is low. This is a separate process from thermal gain (from any source, including light) heating and phase-changing the water.

I don't know if this is right but I'll put something forth to sanity check it:

0) say the energy required to break water's hydrogen bonds is 6.3 kJ/mol (random quora source)

1) 6.4 kJ/mol / 6.022 x 10^23 molecules/mol = 1.0461641 x 10^-20 J per bond

2) 1.0461641 x 10^-20 J * 6.242 x10^18 eV/J = 0.0653 eV per bond (convenient units for light)

3) widest band of visible light energies I found (per photon): red to violet is 1.63 to 3.26 eV

4) if this holds up then each violet photon could potentially break ~25 H bonds and still remain visible

5) this process might be detectable in visible light itself as a red shift, or maybe not because the red also shifts down (or because various frequencies participate more or less for reasons I don’t know about)

This is interesting to me from an entirely different direction after you sketched out that math and the wavelength relationship. I wonder if artificial red-shifting through this bond-loss would account for some of the weirder distance measurements seen is astronomy.

I wonder if this can be used for desalination.

That's what they're hoping, but it might not be that simple I'd guess. The chemistry of salt water or other saturated minerals might negate the effect or cancel it all together. But we'll see.

Related ongoing thread:

In a surprising finding, light can make water evaporate without heat - https://news.ycombinator.com/item?id=38117556

Rewatching The Expanse, I read it as Protomolecular effect ..

As a sidenote, this is from Gang Chen, former head of MIT's Mechanical Engineering department who was (rather shamefully in my opinion) wrongly targeted by Trump's China initiative. The government finally dropped the case acknowledging there was nothing there.

https://www.nytimes.com/2022/01/20/science/gang-chen-mit-chi...

How does this jive with conservation of energy?

Photons carry energy, so I don't see any reason why there is any reason to suspect this would be problematic for conservation of energy.

From he research paper:

> 4) Temperature of the vapor phase becomes cooler under light illumination and shows a flat region due to breaking-up of the clusters that saturates air.

Assuming no magic... I guess this means that small cluster get airborne and then get split absorbing heat from the air, they absorb energy from the photon and then energy from the air. (???)

Calling it the photomolecular effect is a bit much. Idk what it is about MIT but this is such an egoish thing to do something like that

Why is it a bit much? It seems just plain descriptive to me, and calls back to the obvious parallel of the photoelectric effect.

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