Light Driven Water Evaporation

I always thought that evaporation was governed by relative humidity, the lower the RH the faster the evaporation, with heat just expanding the air molecules and lowering the RH to evaporate the water quicker....A laser light, green or other, is a source of heat, the higher the wattage, the hotter the laser point...I don't see how a cool green light, or any light for that matter, can be produced without heat...so this doesn't make any sense to me...it's a dubious claim without any scientific basis...

..."Hydrogen has several wavelengths, including those in the visible spectrum and in the infrared and ultraviolet spectrums:

• Visible spectrum Hydrogen absorbs light with wavelengths of 410 nm (violet), 434 nm (blue), 486 nm (blue-green), and 656 nm (red). These wavelengths correspond to emissions of photons by electrons in excited states transitioning to the quantum level described by the principal quantum number n equals 2."...

Maybe the light breaking the hydrogen bonds between molecules on the surface is more efficient than heating the whole volume of water until a percentage of molecules on the surface have enough energy to escape. It's putting the energy where it's needed.

Just a guess...

Yeah I get the idea, I just don't think it is being described accurately or completely...but maybe that's by design...anyway can't wait to see if it will scale up to a viable alternative...

I'd say that's a very educated guess!

One clue is that the effect depends on the polarization and angle of incidence.

"The effect is strongest when light hits the water surface at an angle of 45 degrees. It is also strongest with a certain type of polarization, called transverse magnetic polarization. And it peaks in green light — which, oddly, is the color for which water is most transparent and thus interacts the least.

Chen and his co-researchers have proposed a physical mechanism that can explain the angle and polarization dependence of the effect, showing that the photons of light can impart a net force on water molecules at the water surface that is sufficient to knock them loose from the body of water. But they cannot yet account for the color dependence, which they say will require further study."

https://news.mit.edu/2024/how-light-can-vaporize-water-without-heat-0423

TM

TM (Transverse Magnetic) polarized light is characterized by its magnetic field being perpendicular to the plane of incidence. For TM light, the electric field – always perpendicular to the magnetic field in isotropic materials – thus lies in the plane of incidence.

https://ibsen.com/resources/grating-resources/polarization-definitions/#:~:text=TM%20(Transverse%20Magnetic)%20polarized%20light,in%20the%20plane%20of%20incidence.

Yes, a magnetic particle knockout punch delivered to the hydrogen bond...still need a dry desert wind to carry away the heads..

which, oddly, is the color for which water is most transparent and thus interacts the least.

Thus sort of an "inverse resonance."

Here is a paper with more details:

https://www.pnas.org/doi/10.1073/pnas.2312751120

I won't get excited until there is peer review and a better theoretical explanation of what is happening.

Perhaps there is extremely local heating to allow each molecule to evaporate. Maybe the difference in energy input is the water that didn't evaporate, but is now warmer than before.

Your second statement is essentially reinforcing Rixter's very good guess of post #2. If you can make surface molecules evaporate with very little energy going into the un-evaporated water, then the un-evaporated water will be only slightly warmed, saving significant energy.

Who knows? This could be like when French scientist Edmond Becquerel determined light could increase electricity generation when two metal electrodes were placed into a conducting solution. Can you imagine the nay sayers back in 1839, light directly creates electrical current?

It is interesting that green light also travels the furthest through the water. It is why underwater raster imaging laser scanners use a green laser.

https://oceanexplorer.noaa.gov/projects/laser01/welcome.html

https://www.osti.gov/servlets/purl/79751&ved=2ahUKEwiW3f-i_NGGAxU4D1kFHQs2BKM4MhAWegQIEhAB&usg=AOvVaw15weRg80zRLz9tP72dynEP

There is still the matter of the latent heat of evaporation to account for.Water must absorb energy to evaporate,that energy must come from the laser.And while true it may only interact at the surface in a very thin layer,it may be the effect of bouncing off that makes the water evaporate.Like a bullet glancing off of water when striking at a 45 degree angle.Some momentum is lost as heat.The reflected laser will not be as powerful as the original before the bounce.

I think I see where you're coming from. Water has a high capacity because when you heat it, energy is needed to break those pesky hydrogen bonds.

"Water’s high heat capacity is a property caused by hydrogen bonding among water molecules. When heat is absorbed, hydrogen bonds are broken and water molecules can move freely. When the temperature of water decreases, the hydrogen bonds are formed and release a considerable amount of energy."

https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/General_Biology_(Boundless)/02%3A_The_Chemical_Foundation_of_Life/2.14%3A_Water_-_High_Heat_Capacity#:~:text=Water's%20high%20heat%20capacity%20is,a%20considerable%20amount%20of%20energy.

I think the process doesn't remove the water molecules one at a time but removes a microscopic droplet consisting of many molecules still bound together. Heat from the surrounding air finishes the job. The microscopic droplets have a high surface to volume ratio, so it's just as a puddle dries up, only much faster. The laser only has to break a few of the bonds, near the surface of the water where it can escape.

The video below describes how it works, starting at about 5:40.