10 ms·

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 liqui

by frud 3y ago

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?

its-summertime 3y ago
if you leave a container of liquid in a cold dry room, it will eventually become empty and the room will become more humid Heat speeds up this process via excitation, photon bombardment speeds up this process also. I'm guessing its more a matter of, if you heat up stuff, you need to heat up stuff and everything around it. Light can be a lot more controllable and directed. In addition, internal reflection can happen within water against air, meaning a free second (third, fourth, fifth, etc) attempt at depositing energy somewhere.
- pdonis 3y ago
  > I'm guessing its more a matter of, if you heat up stuff, you need to heat up stuff and everything around it. Light can be a lot more controllable and directed Exactly.
fspeech 3y ago
Entropy favors the vapor form. It doesn't necessarily take energy to evaporate.
- frud 3y ago
  It takes [40.66 kJ/mol](https://en.wikipedia.org/wiki/Enthalpy_of_vaporization https://en.wikipedia.org/wiki/Enthalpy_of_vaporization) to vaporize water. There are no shortcuts.
  - stubish 3y ago
    The shortcut is apparently breaking the bonds of groups of molecules, rather than supplying enough energy to break all the bonds of each individual molecule. But still technically correct, with the airborne groups breaking apart into individual modules cooling the air. But you don't have to supply all that energy and get to break some theoretical limits.
- pdonis 3y ago
  > Entropy favors the vapor form. Under the conditions of these experiments (and under most ordinary conditions on Earth), yes. However: > It doesn't necessarily take energy to evaporate. Yes, it does. The water molecules in liquid water are bound to each other; that binding energy has to be supplied to enable evaporation. It just doesn't have to be "thermal" energy. It would be correct to say that it doesn't necessarily take externally applied energy for water to evaporate. Water can evaporate using just its own internal thermal energy. In this case the evaporation process will cause the water to cool.
ethanbond 3y ago
> In recent years, some researchers have been puzzled upon finding that water in their experiments, which was held in a sponge-like material known as a hydrogel, was evaporating at a higher rate than could be explained by the amount of heat, or thermal energy, that the water was receiving. And the excess has been significant — a doubling, or even a tripling or more, of the theoretical maximum rate. Apparently it evaporates much, much more quickly than you'd expect from purely energy per mass.
- deleted 3y ago
  [deleted]
- pdonis 3y ago
  > 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 different method of delivering that energy.
  - elil17 3y ago
    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 to evaporate. Heat from the air is later used to break those clusters apart into individual molecules.
    - pdonis 3y ago
      > 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.
pard68 3y ago
My 8th grade understanding is evaporation doesn't always mean steam.
- frud 3y ago
  What exactly is the distinction between water vapor and steam?
  - wnoise 3y ago
    Temperature
  - px43 3y ago
    Where I'm sitting now, the humidity in the air is about 40%. Would you consider that to be "steam"? I feel like steam generally needs to be hot, or at least somewhat warm. Cold steam exists, but is specifically called out as an exception to the rule. Maybe steam needs to be somewhat translucent?
    - mikewarot 3y ago
      Humidity is a percentage of the amount of water that could be in the air, which is when the vapor pressure of water is equal to the partial pressure.[1] So if you're in a room 20 °C, the partial pressure of water is 0.0231 Atmospheres * 40% --> 0.00924 water (0.924%) Steam is 100% water, and generally can only happen at 100 °C or higher. The highest humidity/temperature I've experienced is about 100% at 99F, which works out to around 7% water in the air. It was a miserable day, and I was a young/healthy kid at the time. [1] https://en.wikipedia.org/wiki/Vapour_pressure_of_water https://en.wikipedia.org/wiki/Vapour_pressure_of_water
  - Sai_ 3y ago
    Isn’t steam just water vapour off boiling water? I.e., water vapour which, on condensing, yields heat to the surface on which it condenses. Steam is hot water vapour, IOW.
  - pard68 3y ago
zwieback 3y ago
Yeah, I have the same question. The blurb is too unclear but suggests it's almost more of a mechanical thing: the photon bumps into some molecules that are almost already on the gas side of things and that's enough to turn them into fog. So it's not like the water is heating up and then jumping out, it's more like it's getting knocked to the gas side, like in a humidifier.
ummonk 3y ago
Presumably the surrounding air is below 100% humidity. So the light isn’t heating up and vaporizing the water, but rather helping the air pick it up faster.
pdonis 3y ago
> Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Yes. > Why does it matter that the energy comes from light? The paper is drawing a distinction between light and "heat", which in the context of these experiments basically involves how you deliver the energy: do you do it by heating up the whole mass of water, or do you do it by shining light at it and having the light interact with individual water molecules? In a practical sense, this would be expected to potentially increase the efficiency of evaporation, since bulk heating of water involves significant losses--much of the energy you expend doesn't go into the water. If you can find particular wavelengths of light that interact strongly with the water and cause evaporation, you can greatly decrease the amount of input energy that gets lost in the process.
elil17 3y ago
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 intermolecular bonds aren't being broken in these groups, the amount of thermal energy needed to cause them to enter the air is less than if they had evaporated as individual molecules. These groups later break apart in the air, absorbing thermal energy from the air and leading the air temperature to decrease slightly a few millimeters away from the sample surface. Evaporation happening as clusters of molecules is weird - it's very different from how evaporation usually works. 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.
- syntaxing 3y ago
  But in a closed system, the energy to boil or evaporate the same amount of water is the same right? As in, you still have to pay the energy price but evaporating all the water is probably easier engineering wise?
  - klysm 3y ago
    (armchair science) it seems like if a bigger bunch breaks off, you get better heat transfer from the increased surface area and it would evaporate much faster. Probably the same energy price but much more rapidly applied
  - elil17 3y ago
    Yes, that’s exactly right! Although I would caveat that and say that we don’t know whether it is actually useful engineering-wise.
    - depereo 3y ago
      'clumped vapor' might be way more effective for cloudseeding
ankitml 3y ago
Quantum effects can change thermodynamic parameters. If something seems bizzare in thermodynamic models, next step is to understand quantum physics. This includes modification of energy needed for reaction to phase change energy needs.
- frud 3y ago
  Quantum or no, there is no shortcut around the 40.66 kJ/mol it takes to evaporate water.
ndonnellan 3y ago
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 -> more light = more energy -> more energy = more evaporation.
- fsckboy 3y ago
  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
labcomputer 3y ago
> 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? Yes. The rest of the energy comes from the bulk water/hydrogel in other words, the bulk water is cooled by this process. What’s happening is that energy is sloshing around between various degrees of freedom of the system (the temperature of the system is not zero). When it sloshes is such a way that a water molecule near the surface has more kinetic energy than the bond strength between it and the bulk, that molecule evaporates. Since the “sloshed” molecule has greater-than-average energy just before evaporation, the average energy of the remaining bulk water is reduced (the bulk cools). But the interesting thing here is that it seems that they have found a resonance where the photon will not just cause the water molecule to evaporate “early” and also carry with it more excess energy than the phone came in with (hence having an evaporation rate 2x expected). I wonder if this has something to do with the hydrogel causing the water to behave more like a solid, and enabling some kind of phonon-photon coupling process that isn’t supported in pure bulk water > Why does it matter that the energy comes from light? Practically, because they want to make a solar desalination system (though this just raises the question of how do you get monochromatic green light from the solar spectrum). Scientifically, because it is interesting that the photon will trigger a water molecule to take off with more energy than the photon. Also, it feels entropically weird.
- kortex 3y ago
  You don't need monochromatic green light - that was just the test condition to find the best wavelength. Broad spectrum sunlight should do the trick.
thayne 3y ago
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.
movpasd 3y ago
Thermodynamics (well, equilibrium thermodynamics) doesn't say too much, because evaporation is a non-equilibrium effect. They're not saying more water vapor is produced for free given the energy, rather that the _rate_ of evaporation is increased — and that is up for grabs.

by frud 3y ago

its-summertime 3y ago

if you leave a container of liquid in a cold dry room, it will eventually become empty and the room will become more humid Heat speeds up this process via excitation, photon bombardment speeds up this process also. I'm guessing its more a matter of, if you heat up stuff, you need to heat up stuff and everything around it. Light can be a lot more controllable and directed. In addition, internal reflection can happen within water against air, meaning a free second (third, fourth, fifth, etc) attempt at depositing energy somewhere.

pdonis 3y ago
> I'm guessing its more a matter of, if you heat up stuff, you need to heat up stuff and everything around it. Light can be a lot more controllable and directed Exactly.

fspeech 3y ago

Entropy favors the vapor form. It doesn't necessarily take energy to evaporate.

frud 3y ago
It takes [40.66 kJ/mol](https://en.wikipedia.org/wiki/Enthalpy_of_vaporization https://en.wikipedia.org/wiki/Enthalpy_of_vaporization) to vaporize water. There are no shortcuts.
- stubish 3y ago
  The shortcut is apparently breaking the bonds of groups of molecules, rather than supplying enough energy to break all the bonds of each individual molecule. But still technically correct, with the airborne groups breaking apart into individual modules cooling the air. But you don't have to supply all that energy and get to break some theoretical limits.
pdonis 3y ago
> Entropy favors the vapor form. Under the conditions of these experiments (and under most ordinary conditions on Earth), yes. However: > It doesn't necessarily take energy to evaporate. Yes, it does. The water molecules in liquid water are bound to each other; that binding energy has to be supplied to enable evaporation. It just doesn't have to be "thermal" energy. It would be correct to say that it doesn't necessarily take externally applied energy for water to evaporate. Water can evaporate using just its own internal thermal energy. In this case the evaporation process will cause the water to cool.

ethanbond 3y ago

> In recent years, some researchers have been puzzled upon finding that water in their experiments, which was held in a sponge-like material known as a hydrogel, was evaporating at a higher rate than could be explained by the amount of heat, or thermal energy, that the water was receiving. And the excess has been significant — a doubling, or even a tripling or more, of the theoretical maximum rate. Apparently it evaporates much, much more quickly than you'd expect from purely energy per mass.

deleted 3y ago
[deleted]
pdonis 3y ago
> 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 different method of delivering that energy.
- elil17 3y ago
  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 to evaporate. Heat from the air is later used to break those clusters apart into individual molecules.
  - pdonis 3y ago
    > 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.

pard68 3y ago

My 8th grade understanding is evaporation doesn't always mean steam.

frud 3y ago
What exactly is the distinction between water vapor and steam?
- wnoise 3y ago
  Temperature
- px43 3y ago
  Where I'm sitting now, the humidity in the air is about 40%. Would you consider that to be "steam"? I feel like steam generally needs to be hot, or at least somewhat warm. Cold steam exists, but is specifically called out as an exception to the rule. Maybe steam needs to be somewhat translucent?
  - mikewarot 3y ago
    Humidity is a percentage of the amount of water that could be in the air, which is when the vapor pressure of water is equal to the partial pressure.[1] So if you're in a room 20 °C, the partial pressure of water is 0.0231 Atmospheres * 40% --> 0.00924 water (0.924%) Steam is 100% water, and generally can only happen at 100 °C or higher. The highest humidity/temperature I've experienced is about 100% at 99F, which works out to around 7% water in the air. It was a miserable day, and I was a young/healthy kid at the time. [1] https://en.wikipedia.org/wiki/Vapour_pressure_of_water https://en.wikipedia.org/wiki/Vapour_pressure_of_water
- Sai_ 3y ago
  Isn’t steam just water vapour off boiling water? I.e., water vapour which, on condensing, yields heat to the surface on which it condenses. Steam is hot water vapour, IOW.
- pard68 3y ago

zwieback 3y ago

Yeah, I have the same question. The blurb is too unclear but suggests it's almost more of a mechanical thing: the photon bumps into some molecules that are almost already on the gas side of things and that's enough to turn them into fog. So it's not like the water is heating up and then jumping out, it's more like it's getting knocked to the gas side, like in a humidifier.

ummonk 3y ago

Presumably the surrounding air is below 100% humidity. So the light isn’t heating up and vaporizing the water, but rather helping the air pick it up faster.

pdonis 3y ago

> Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Yes. > Why does it matter that the energy comes from light? The paper is drawing a distinction between light and "heat", which in the context of these experiments basically involves how you deliver the energy: do you do it by heating up the whole mass of water, or do you do it by shining light at it and having the light interact with individual water molecules? In a practical sense, this would be expected to potentially increase the efficiency of evaporation, since bulk heating of water involves significant losses--much of the energy you expend doesn't go into the water. If you can find particular wavelengths of light that interact strongly with the water and cause evaporation, you can greatly decrease the amount of input energy that gets lost in the process.

elil17 3y ago

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 intermolecular bonds aren't being broken in these groups, the amount of thermal energy needed to cause them to enter the air is less than if they had evaporated as individual molecules. These groups later break apart in the air, absorbing thermal energy from the air and leading the air temperature to decrease slightly a few millimeters away from the sample surface. Evaporation happening as clusters of molecules is weird - it's very different from how evaporation usually works. 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.

syntaxing 3y ago
But in a closed system, the energy to boil or evaporate the same amount of water is the same right? As in, you still have to pay the energy price but evaporating all the water is probably easier engineering wise?
- klysm 3y ago
  (armchair science) it seems like if a bigger bunch breaks off, you get better heat transfer from the increased surface area and it would evaporate much faster. Probably the same energy price but much more rapidly applied
- elil17 3y ago
  Yes, that’s exactly right! Although I would caveat that and say that we don’t know whether it is actually useful engineering-wise.
  - depereo 3y ago
    'clumped vapor' might be way more effective for cloudseeding

ankitml 3y ago

Quantum effects can change thermodynamic parameters. If something seems bizzare in thermodynamic models, next step is to understand quantum physics. This includes modification of energy needed for reaction to phase change energy needs.

frud 3y ago
Quantum or no, there is no shortcut around the 40.66 kJ/mol it takes to evaporate water.

ndonnellan 3y ago

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 -> more light = more energy -> more energy = more evaporation.

fsckboy 3y ago
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

labcomputer 3y ago

> 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? Yes. The rest of the energy comes from the bulk water/hydrogel in other words, the bulk water is cooled by this process. What’s happening is that energy is sloshing around between various degrees of freedom of the system (the temperature of the system is not zero). When it sloshes is such a way that a water molecule near the surface has more kinetic energy than the bond strength between it and the bulk, that molecule evaporates. Since the “sloshed” molecule has greater-than-average energy just before evaporation, the average energy of the remaining bulk water is reduced (the bulk cools). But the interesting thing here is that it seems that they have found a resonance where the photon will not just cause the water molecule to evaporate “early” and also carry with it more excess energy than the phone came in with (hence having an evaporation rate 2x expected). I wonder if this has something to do with the hydrogel causing the water to behave more like a solid, and enabling some kind of phonon-photon coupling process that isn’t supported in pure bulk water > Why does it matter that the energy comes from light? Practically, because they want to make a solar desalination system (though this just raises the question of how do you get monochromatic green light from the solar spectrum). Scientifically, because it is interesting that the photon will trigger a water molecule to take off with more energy than the photon. Also, it feels entropically weird.

kortex 3y ago
You don't need monochromatic green light - that was just the test condition to find the best wavelength. Broad spectrum sunlight should do the trick.

thayne 3y ago

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.

movpasd 3y ago

Thermodynamics (well, equilibrium thermodynamics) doesn't say too much, because evaporation is a non-equilibrium effect. They're not saying more water vapor is produced for free given the energy, rather that the _rate_ of evaporation is increased — and that is up for grabs.