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I'm looking for a type of plastic membrane that would let water through it from the outside, but wouldn't allow water to escape once it's inside the membrane.

Ideally available in a roll.

Does such a plastic membrane exist?

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    $\begingroup$ This, if exists, would be such a weird membrane. How would the inner side of membrane act as a hydrophile and hydrophobe? $\endgroup$ – M.A.R. ಠ_ಠ Jun 23 '15 at 18:52
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    $\begingroup$ You start by catching as many Maxwell demons as possible... $\endgroup$ – Jon Custer Jun 23 '15 at 22:30
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    $\begingroup$ Could it be possible to use some sort of fancy surface patterning to make such a membrane? Or perhaps take a page out of water transport mechanisms through cell walls? $\endgroup$ – Nicolau Saker Neto Jun 23 '15 at 23:02
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The comments above have gently hinted that it would be a violation of thermodynamics for a passive membrane to force a substance in a single direction. Molecules have an equal probability of moving through the membrane in either direction, unless you apply a force to make them move in a specific direction, such as electro-osmosis or active water transport in plants.

Consider that if such a membrane as you posit existed, you could use it to pump water from a lower level to a higher, and then power a water-wheel: perpetual motion. Sadly, Maxwell's demon is getting a bit arthritic to shuttle water molecules, and the dampness only aggravates its condition.

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  • $\begingroup$ There are plenty of biased pumps- can happen without breaching the laws of thermodynamics- in this case the plastic is a biased sieve so that water molecules on the outside easily enter but can't leave easily the plastic keeps loosing internal energy so it will eventually. fail. $\endgroup$ – user2617804 Jun 24 '15 at 4:40
  • $\begingroup$ True, there are many substances that adsorb and absorb water, such as silica, calcium chloride, and even cellophane. My understanding of the question was perhaps incorrect: I assumed the membrane was to allow water to pass through in only one direction, but if it's merely to absorb water, there are many hygroscopic membranes, such as those used in HVAC, such as paper-board or even liquid membranes (see sciencedirect.com/science/article/pii/S037673880000404X). $\endgroup$ – DrMoishe Pippik Jun 24 '15 at 16:54

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