[mppm]

Unless they have buried some really important caveat somewhere in the paper [1], it really looks like they are making claims that are incompatible with the second law of thermodynamics. They claim that water droplets are condensing on their nanomaterial at constant temperature and less than 100% relative humidity. This is absolutely forbidden by thermodynamics as we understand it. Under these conditions droplets can condense within pores (forming a concave surface), but they can never form a convex droplet on a flat surface.

Their mumbo-jumbo about water being "squeezed out" onto the surface by the hydrophobic component is totally bogus as well. The condensation will just stop earlier, without overflowing. Water condensing in concave pores and being squeezed into convex droplets requires hydrostatic pressure to be positive and negative at the same time.

The possibilities I see are: 1) contaminated surfaces 2) miscalibrated relative humidity or 3) they've neglected to mention a cooling plate that keeps the material below ambient.

1. https://www.science.org/doi/10.1126/sciadv.adu8349

[a1371]

I'm not sure what's forbidden here. You don't need 100% relative humidity to grab water from the air in fact in any wood has a moisture content that in equilibrium is in relation to the air moisture content. The moisture diffuses into every material and evaporates based on where it finds less vapor pressure. That's why you may have dry lips at 40% RH versus moisturized lips at 70% RH.

What you're referring to is condensation and is caused by air oversaturation due to a temperature drop which doesn't seem to be the case here.

Theoretically speaking, you can have a material that somehow absorbs high moisture from the air but has microscale properties that promote creation of droplets then somehow these droplets are separated from the rest of the air (with something like a smart vapor retarder, a passive material) and the water gets harvested.

[mppm]

What you are referring to is called capillary condensation [1]. When you have a hydrophilic surface with thin capillaries or small pores, they can pull water from the air below 100% RH. However, this process requires an enclosed space with a very small radius and the air-water interface is always concave in this case (it's just how capillary forces work).

Forming a convex surface, on the other hand, requires an at least slightly hydrophobic material and produces a positive internal pressure. This is a key difference, because condensation into a hydrophilic pore is favorable in terms of free energy, while condensing onto a hydrophobic surface is unfavorable (unless you have a supersaturated vapor).

> Theoretically speaking, you can have a material that somehow absorbs high moisture from the air but has microscale properties that promote creation of droplets then somehow these droplets are separated from the rest of the air

That "somehow" is what makes the paper's claims impossible. The water condenses spontaneously into the pore because it thereby lowers its free energy. Extruding it onto the surface is then even more unfavorable than direct condensation. Unfortunately, no passive system can achieve this feat, no matter how cleverly nanostructured, as it would go against the arrow of increasing entropy. You need an external energy source to drive that process.

1. https://en.wikipedia.org/wiki/Capillary_condensation

[Swannie]

Thank you, this is a very clear explanation for me.

It filled the critical gaps in my intuition that I didn't have the brain cycles to formulate hypotheses against.

[hinkley]

The reverse problem is also true with such materials:

Water harvesting in pristine lab conditions may break down rapidly in realistic scenarios. Something that’s wet attracts dust and microbes. Dust plus water means more microbes. You’ll have lichen growing on this stuff in no time.

[TimByte]

Also wouldn't be the first time an experiment overlooked a small temperature gradient or calibration issue

[ummonk]

Did you read the article? They're not droplets on a flat surface. They're droplets being held by surface tension to water inside pores.