[HN Gopher] Photomolecular effect leading to water evaporation e...
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Photomolecular effect leading to water evaporation exceeding the
thermal limit
Author : croes
Score : 50 points
Date : 2023-11-02 12:30 UTC (10 hours ago)
(HTM) web link (www.pnas.org)
(TXT) w3m dump (www.pnas.org)
| DougN7 wrote:
| 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?
| quickcheque wrote:
| 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.
| cma wrote:
| 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?
| alasarmas wrote:
| 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.
| Someone wrote:
| 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,
| cma wrote:
| 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.
| lainga wrote:
| 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
| schiffern wrote:
| >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
| bilsbie wrote:
| How does this jive with conservation of energy?
| aidenn0 wrote:
| 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.
| gus_massa wrote:
| 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.
| (???)
| svnt wrote:
| 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)
| TrueDuality wrote:
| 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.
| thelastgallon wrote:
| Rewatching The Expanse, I read it as Protomolecular effect ..
| spiritplumber wrote:
| I wonder if this can be used for desalination.
| contemporary343 wrote:
| 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...
| noobermin wrote:
| 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
| kadoban wrote:
| Why is it a bit much? It seems just plain descriptive to me,
| and calls back to the obvious parallel of the photoelectric
| effect.
| dang wrote:
| Related ongoing thread:
|
| _In a surprising finding, light can make water evaporate without
| heat_ - https://news.ycombinator.com/item?id=38117556
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(page generated 2023-11-02 23:01 UTC)