[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.

[ChuckMcM]

Okay, I see where we diverge. The 'cube' was something I was thinking about not in the paper. I'll see if I can describe what I was thinking and you can tell me it breaks the rules :-).

You coat a piece of aluminum with nano-pore material and hang it vertically. Air flows over it and droplets appear on its surface (based on the paper). You also hang a frame of vertical wires (unenergized just small diameter wires, kind of like a screen but without the horizontal members) in front of the sheet by 1/2 the droplet's diameter. The wires don't touch the surface, they are suspended 1/2 droplet away.

Now when a droplet forms, it grows and intersects the wire (which is not hydrophobic) Surface tension puts the droplet around the wire and it slides down to the bottom of the wire frame, impacting any other droplets that had formed below it.

The resulting liquid water drops off the bottom of the wire frame into a catch pan below.

If one of these assemblies generates net water production from RH 70% air then an array of then would generate more water.

What am I missing?

[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.