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Light can make water evaporate without heat

news.mit.edu

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Re: Light can make water evaporate without heat

#131

Post by thsksbd »

elil17 wrote: post #44
frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

Thermodynamics and evaporation are my day job and I think most other explanations here are missing the point. Evaporation normally occurs when individual water molecules have enough thermal energy to break their intermolecular bonds, leaving the bulk liquid and entering the air. In this case, they found strong evidence that water molecules were being removed in groups of several water molecules. Because intermolecula…

Once airborne, the little droplet would be below the critical radius of water and have to absorb heat from air and fully evaporate.

Re: Light can make water evaporate without heat

#132

Post by abdullahkhalids »

klysm wrote: post #62

This is really cool, but I'm confused how we haven't stumbled into this before? Wouldn't this be quite obvious under some napkin math or does it have more to do with the specific hydrogels they were using?

It seems to be an extreme small effect, which only becomes apparent to someone looking for it under specifically controlled experimental conditions.

This sort of thing happens all the time in chemistry, material science and condensed matter physics. There are infinite possibilities of putting things together, and only some of them are interesting. It takes a lot of manual work to isolate some phenomena like this.

Re: Light can make water evaporate without heat

#133

Post by bloopernova »

elil17 wrote: post #44
frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

Thermodynamics and evaporation are my day job and I think most other explanations here are missing the point. Evaporation normally occurs when individual water molecules have enough thermal energy to break their intermolecular bonds, leaving the bulk liquid and entering the air. In this case, they found strong evidence that water molecules were being removed in groups of several water molecules. Because intermolecula…

Do photons hitting the surface of water transfer any kinetic energy?

Does the evaporation occur as soon as the light is switched on, or does it take a while to get started?

Re: Light can make water evaporate without heat

#134

Post by genewitch »

canadianfella wrote: post #94
montjoy wrote: post #71

> The researchers found that the effect varied with color and peaked at a particular wavelength of green light. I wonder if this affects photosynthesis in any way? IIRC plants are green because chlorophyll does not absorb that color. Maybe that’s partially because green light induces too much evaporation?

I’ve always wondered why inefficient green is the way plants evolved. Interesting question.

[green] plants are damaged by green light, is the way i've heard it explained. So they "block it".

Re: Light can make water evaporate without heat

#136

Post by pomtato »

elil17 wrote: post #44
frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

Thermodynamics and evaporation are my day job and I think most other explanations here are missing the point. Evaporation normally occurs when individual water molecules have enough thermal energy to break their intermolecular bonds, leaving the bulk liquid and entering the air. In this case, they found strong evidence that water molecules were being removed in groups of several water molecules. Because intermolecula…

Interesting, reminds me of Microwave Emitter from Batman :D

Re: Light can make water evaporate without heat

#137

Post by thayne »

frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

Because visible light doesn't interact with water very much. It usually just passes through. For many processes the frequency of the light makes a big difference. Longer wavelength microwaves however do interact with water molecules, and that is how microwave ovens work.

Re: Light can make water evaporate without heat

#138

Post by ponyboy123 »

grishka wrote: post #128
m463 wrote: post #80

Earlier quoted context omitted.

> I'm not really sure whether to even call it evaporation I can't help but think of ultrasonic humidifiers/misters, which use vibration to do evaporation-adjacent kinds of things. I also wonder if specific wavelengths of light are involved (sort of how 2.4ghz microwaves work on water)

Water particles produced by an ultrasonic humidifier are larger than those of real steam. I know it because I have one and if I run it for long enough, everything gets covered in a nasty white residue, probably salts from the water. Real evaporation doesn't do that.

I think the difference is that evaporation creates water vapor, whereas an ultrasonic dehumidifier is creating water droplets, some of which are very small, but are still droplets that can carry minerals from the water

Re: Light can make water evaporate without heat

#139

Post by montjoy »

malfist wrote: post #95
montjoy wrote: post #71

> The researchers found that the effect varied with color and peaked at a particular wavelength of green light. I wonder if this affects photosynthesis in any way? IIRC plants are green because chlorophyll does not absorb that color. Maybe that’s partially because green light induces too much evaporation?

Maybe, but evaporation is critical for plants. They don't have a heart to pump blood and supply nutrients, they have to rely on evaporation to move nutrients from the roots up

No doubt. I’m just toying with the idea of how this might impact our understanding of how plants work.
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