[hollosi]

From the actual paper ( https://www.science.org/doi/10.1126/sciadv.adu8349 ):

"All measurements were performed at 20° ± 0.2°C maintained by an air circulation system unless otherwise noted. The temperature of the films was controlled using a heating/cooling unit (THMS350V, Linkam Scientific Instruments, Salfords, UK) when necessary."

So the latent heat is conducted away by the cooling apparatus, it's just not explicitly stated, to sound more sensational.

[TJSomething]

Another part from the paper that a lot of people here seem to be ignoring: "Specifically, macroscopic water droplets isothermally form when the NP size is ≤22 nm, RH is >~90%, and ϕPE ranges from 0.05 to 0.35." and "Initial water droplets that are observable under optical microscopy (~1 μm in size) appear within a few seconds after being exposed to 97% RH."

This is really moist air that's only barely short of forming dew. A lot of people are focusing on sensational "violation of physics", when it's an incremental improvement on process that happens naturally.

[TimByte]

I think the interesting bit is less about "breaking physics" and more about how finely tuned the material is to encourage this behavior without external cooling.

[vel0city]

But there was external cooling, or am I reading "The temperature of the films was controlled" incorrectly?

[bm62]

He likely means cooling it below the dew point rather than controlling the temperature in general.

[mppm]

Keeping the temperature constant with a thermostat is not an issue here. That would only explain things if the surface were kept cooler than the surrounding air (below the dew point), but from the description in the paper that does not seem to be the case. They basically claim that macroscopic droplets form spontaneously from an unsaturated vapor. And no, this is not something permitted by the second law of thermodynamics.

[dotancohen]

  > And no, this is not something permitted by the second law of thermodynamics.

If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations—then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation—well these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.

[lolinder]

— Sir Arthur Stanley Eddington, The Nature of the Physical World (1927)

[robertclaus]

While I generally agree that it sounds dubious, this argument depends on whether the entropy of the liquid in the pore is lower than the entropy of the vapor in the air in the pore. I could see a highly hydrophilic capillary restricting a vapor enough to where it has better entropy in a liquid state.

If that's true we just need to balance energy, which the cooler does.

[mppm]

> I could see a highly hydrophilic capillary restricting a vapor enough to where it has better entropy in a liquid state.

My other comment here (and and a reply to a similar question) has more detail [1], but in short: this is true for capillaries and pores, it is not true for "collectable" droplets on a flat surface.

1. https://news.ycombinator.com/item?id=44099078

[ummonk]

Replied to that comment as well but per the article they're not droplets on a flat surface, but rather droplets connected to pores by surface tension.

[bm62]

Practically it just means that the energy to form the droplets is coming from somewhere else, just not via cooling the surface below the dew point. For instance, you could imagine something like squeezing a material that undergoes capillary condensation to get the water out, since you'd pay the requisite energy cost via mechanical work.

[cjbgkagh]

Ah that seems to explain it to me, if instead of presenting it as breaking some physics they should have said what actually makes it useful.

My understanding of it now is that since it can work at a higher temperature in an environment where the ambient temperature is low enough the latent heat can be passively radiated away. Even if using an active heat pump the higher temperatures would allow for a more efficient process. A closed system would eventually reach an equilibrium but there is no need to maintain a closed system.

[ChuckMcM]

I think the work stands out anyway. Unlike adsorption techniques there is zero change to the mechanism which just keeps pulling water from the air. Presumably, they will put a layer of this material on aluminum to conduct the latent heat and have something that just produces water full time, without additional energy input. consider a 'cube' of fins of this stuff sitting in shade with a collection bucket underneath it. It will be interesting when they build something like that how many liters/day it can extract from ambient air and under what conditions.

Devices like that would be essential during 'wet bulb' days where the temperature and water content of the air created dangerous conditions for people. A passive device that takes no energy and just sucks water out of the air? Could be a lifesaver.

[cyberax]

To conduct the latent heat away, the aluminum sheet needs to be below the ambient temperature of the condenser plate.

[ChuckMcM]

You didn't read the paper did you :-). First, it isn't a "condenser" (which is kind of the cool science here) it is more of a molecular sieve that exploits two materials (one that repels, and one that attracts) the molecule in question (water). The water vapor is "forced[1]" together by the nano-structure, which result in a phase change (vapor to water) and that phase change releases heat into the nano-structure (and pushes the liquid water out to the surface) which makes the nano-structure warmer than the ambient temperature. The aluminum conducts that heat and is convectively radiating it into air on surfaces not covered with the nano-structure.

The researchers also noted that the water that was expressed to the surface of the material did not evaporate (as one would expect). There some interesting speculation as to why that is. It wasn't clear whether or not the water would move across the nano-structure if it was affected by gravity (aka dripping) but I can imagine several ways to transport it off the surface so I'm sure the researchers can too.

[1] The description in the paper is that capillary action forces the vapor into the interior of the structure where it collapses into liquid.

[cyberax]

I read it. It sounds like nonsense.

This is basic thermodynamics, you can do however much hydrophobic/hydrophilic nanomaterials, but you won't get condensation unless you somehow conduct away the latent heat. This can be done by storing energy in the material itself (that's how desiccants work), or by providing a temperature gradient (a cooler).

[ChuckMcM]

Okay I think we're saying the same thing, but let me check that..

> This can be done by storing energy in the material itself (that's how desiccants work)

This is exactly where the energy goes. From the paper (in it's Materials and Methods section) -- All measurements were performed at 20° ± 0.2°C maintained by an air circulation system unless otherwise noted. The temperature of the films was controlled using a heating/cooling unit (THMS350V, Linkam Scientific Instruments, Salfords, UK) when necessary.

So the hypothesis is that the heat in the water vapor goes into the nano-pore material, which in their experiment they were actively maintaining at 20 degrees C. So yes, they are actively removing the heat created by the phase change.

One difference with desiccants is that once they are saturated you have to restore them through heating them up, but this stuff doesn't have that property. And while it may sound like nonsense it was reproduced in another lab[1].

Apparently capillary condensation is a thing, its the popping out of the liquid water that was unexpected.

[1] With a material that could potentially defy the laws of physics on their hands, Lee and Patel sent their design off to a collaborator to see if their results were replicable.

[gus_massa]

>>>> consider a 'cube' of fins of this stuff sitting in shade with a collection bucket underneath it.

There is no cube. The droplet's are attached strongly to the surface.

If the droplets drop to a cube, you can replace the cube with a cotton mat and let the water evaporate and get a low temperature mat. And then use the difference of temperature to generate electricity https://en.wikipedia.org/wiki/Thermoelectric_generator and turn on a lamp. And now you are breaking the second law of thermodynamics.

Consider a typical unplugged dehumidifier with Calcium Chloride. It generates water that drops to a cube, but it's salt water that evaporates less than fresh water, so you can't do the trick.

If you use silica gel, the water is trapped inside the material, so there is no cube.

With this new material the droplets are on the surface, but they refuse to fall down.

With an AC you get a cube full fresh water, but it obviously work only while plugged, so there is no magic.

> And while it may sound like nonsense it was reproduced in another lab [1].

They reproduced the visible droplets in the surface of the material. In neither lab they had a cube filing process. The sentence you quoted in [1] is very misleading.

[nullc]

You could coat them with an ultraemissive material and point them at the sky, if its not cloudy it will get significantly below the ambient air temp.

But the paper suggest that it will condense at ambient anyways, because it gets warmer so radiation to ambient is enough for it to work.

[mleonhard]

Looking at the paper, it seems like they put some silicon-dioxide nanoparticles on a substrate, then add a plastic (poly-ethylene) layer on top and melt it (annealing). The spaces between the nanoparticles gets partially filled with plastic. The ratio of plastic to particles is the poly-ethylene volume fraction (ϕPE). They tested different fractions and found that a certain range caused the wetting behavior.

Their experiments suggest that tiny water droplets appear inside the material at 70% RH (relative humidity). If this is true, then I expect there is a way to extract the droplets using very little energy. Ideas:

- make open collection points on the film

- use ultrasound to bounce the droplets around and consolidate them

- make the film on a material that can be saturated with water so the new droplets can easily join the flow

https://en.wikipedia.org/wiki/Volume_fraction

[gopalv]

> So the latent heat is conducted away by the cooling apparatus, it's just not explicitly stated, to sound more sensational.

In theory, if that makes it hotter than ambient air in the process, that would be a good thing - usually we have to cool things down below ambient air to get moisture out.

Not a good thing if you want to measure maximum moisture extraction, but cooling something to ambient temperatures is a much easier task.

[Y_Y]

This point is essential though! As soon as I saw the headline I knew it couldn't be the full story (there's no free osmotic lunch).

Is there a corollary to Betteridge's Law that says that popular science journalism will always overatate the result?

[delusional]

Would this not invalidate the conclusions of the paper? considering they are not just claiming to form water droplets, but that they do so isothermally.

It could still be a useful material, but the science would be bad.