In reply to TobyA:
> Fair enough, I think what I meant to say was a liquid water droplet. Actually when I wrote that I remember thinking - I'm not using the words right here am I because I guess one water molecule is one water molecule no matter whether it's solid in an ice fall, liquid in water or bouncing around more as a vapour.
It's confusing because Gore's marketing materials tended to be, well, quite 'simplified' from the physics perspective. I think it was their own marketing newspeak that talked about 'molecules' in the first place, which got parroted over by all the non‑scientific outdoor media since.
Yes, a single water molecule is obviously much smaller than any pore in the membrane – whether solid, bunging with others to make ice crystals, liquid, sloshing with others to make a droplet, or vapour, more freely and randomly moving around.
But in the liquid phase, water molecules somewhat more tend to "stick" to each other by intermolecular forces, forming droplets. It's the least energy state, basically. Think surface tension and perfectly circular droplets in zero gravity. Even a liquid surface has surface tension at the liquid‑air interface, that's how some insects are able to walk on water.
The membrane pores, much bigger than individual molecules, are water‑repelling. Think teflon pan or well‑oiled pan. That changes the contact angle and makes it easier for a "droplet" to form. Like in the honey example, the intermolecular forces are stronger, so the honey doesn't flow through the sieve. Or a tea leaf floating on the water surface, even if it normally sinks.
That internal adhesion or surface tension still keeps the liquid water, even if it's a liquid film composed of myriads of individual molecules and not just a few individual droplets, from penetrating the much bigger pores, until enough external pressure (HH) overcomes that.
Water vapour molecules are obviously much more energetic, being vapour. Like steam. Meaning they mill around more easily, alone, not sticking so much together, able to penetrate the much bigger holes in the membrane more easily.
If the holes in the membrane weren't hydrophobic, the much smaller water molecules would just wet them and flow through freely, like water on a sieve. It's the surface tension that prevents that. That's why microporous WPB membranes that are dirty with sweat and not washed enough can leak, like eVent or even Goretex. Think teflon‑coated very fine sieve versus a regular metal sieve. The former would hold the water longer, the latter would leak a lot.
It gets even more complicated because most microporous membranes like Goretex add a thin, solid, hydrophilic (water‑loving) layer. That's to protect the "sieve" from contamination from sweat, ruining its hydrophobic properties. Some do not, and have to be washed a bit more frequently. Obviously, the solid polyurethane doesn't have pores like the ePTFE does, but it can still transport water and vapour through capillary action.
Some other membranes use only this PU layer, without any micropores. Others still have hydrophobic micropores, but made from different polymers than PTFE.
Anyways, any WPB membrane needs some environmental gradient to work. Be it temperature differential, relative humidity, pressure or whatever.
Some work better in some conditions than others and vice versa.
Remember, there is no such thing as 100% waterproof. It's all about balancing the different forces, water can enter even solid rock under enough pressure, after all.
Tent coatings tend to be usually much thicker, so they are not really usually counted as WPB, being just a solid coating on the fibres. But with enough pressure, you can still force water through. Perhaps more easily, since the fibres in a typical woven fabric are much, much further apart than the pores in a microporous membrane. You could force water even through a "waterproof" plastic bag, given enough pressure (it would most probably burst first, of course)
But it's still enough for tent use, and not many people need a fully WPB tent, since that's a lot of £££ for a little benefit (the temperature gradient would be smaller, so it wouldn't even work that well at preventing condensation on the tent walls).
Apologies to any physicists and material scientists out here, as I have definitely made quite a lot of oversimplifications and mistakes, all in the interest of keeping it accessible...
Post edited at 20:35