Two things to wonder about, that we didn't see mentioned in article, which I'm sure were considered somewhere.
What about the differences in bonding strength between water molecules surrounded by water, as opposed to water in the hydro gel. The other thing, which is sort of related, is partial pressure differences between a pure water surface exposed to air, and a water hydrogel surface exposed to air.
Looks like very interesting further studies required.
Light can make water evaporate without heat
91–100 of 198 posts
Re: Light can make water evaporate without heat
#92Earlier quoted context omitted.
The paper is accounting for the energy that the light is delivering. The very neat thing about this paper is that it does change the bonding energy between water molecules that has to be overcome for evaporation to occur. They observed evaporation of clusters of molecules, not individual molecules. Since whole groups of molecules are flung into the air, not all of the intermolecular bonds need to be broken for them t…
> They observed evaporation of clusters of molecules, not individual molecules. Since whole groups of molecules are flung into the air, not all of the intermolecular bonds need to be broken for them to evaporate. Heat from the air is later used to break those clusters apart into individual molecules. This sounds more like the light is making thin fog, not water vapor.
Well, sort of. I’m conceptualizing it as an intermediate state between a vapor and an aerosol. The aerosol would have way more intermolecular bonds per molecule on average since most of its molecules are in the insides of droplets. But these molecule clusters have all or almost all their molecules exposed on the clusters surface, so they have many fewer intermolecular bonds.
Re: Light can make water evaporate without heat
#93I 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…
What happens if the air is saturated? Does the molecule clump just settle back down where it came from, or would it stay suspended somehow in the air (mist?)?
Alternatively, I wonder if this could be used as a super swamp cooler, I'm picture water dripping or flowing from a tube, a laser causing it to "burst apart", and then the droplets formed rapidly cooling the surroundings due to their surface area.
Re: Light can make water evaporate without heat
#94> 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.
Re: Light can make water evaporate without heat
#95> 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
Re: Light can make water evaporate without heat
#96Earlier quoted context omitted.
> it evaporates much, much more quickly than you'd expect from purely energy per mass From purely "thermal" energy per unit mass. But the light is delivering energy too; the total energy per unit mass being delivered is still the same, it's just being put in in a different form. Nothing about this changes the bonding energy between water molecules that has to be overcome for evaporation to occur. It's just a differen…
The paper is accounting for the energy that the light is delivering. The very neat thing about this paper is that it does change the bonding energy between water molecules that has to be overcome for evaporation to occur. They observed evaporation of clusters of molecules, not individual molecules. Since whole groups of molecules are flung into the air, not all of the intermolecular bonds need to be broken for them t…
Evaporation should be a function of surface area. In a large pool of water the evaporation should occur primarily in the upwards direction. Any molecular activity that is sideways or downwards will not lead to the molecule escape.
Once a water particle escapes, depending on the clump size it should have more degrees of freedom in evaporation and I would think surface tension would be reduced.
Re: Light can make water evaporate without heat
#97I 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…
> I'm not really sure whether to even call it evaporation since I don't think the clusters would fully qualify as vapor until they are broken apart into individual molecules.
It's simple. You have a small puddle of water on the ground that slowly rains up into the air. ;D
Re: Light can make water evaporate without heat
#98I 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?
I think the key paragraph is buried: "Though water itself does not absorb much light, and neither does the hydrogel material itself, when the two combine they become strong absorbers, Chen says. That allows the material to harness the energy of the solar photons efficiently and exceed the thermal limit, without the need for any dark dyes for absorption." So when water is combined with hydrogel, they absorb more light…
yes, but that's the exact opposite of everything else that's being said which is that there is no absorption taking place. It's not a good article in terms of explanatory power
Re: Light can make water evaporate without heat
#99Earlier quoted context omitted.
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…
So a watched kettle might boil faster?
You emmit green light from your eyes?