Water Vapor Permeability

Source -Diffanimals.com

Just like birds of a feather flock together, water is attracted to some surfaces and very little to others. Extracting water, that exists in both vapor and liquid form, from air can, thus, be facilitated either by choosing a naturally-available surface that attracts water droplets and vapor molecules, or by installing a nano substrate, that promotes moisture adsorption and/or absorption, onto the surface. Once water molecules arrive at the surface, one of two situations exist: Either the surface undergoes a transformation (that may be permanent or temporary) or the surface undergoes no transformation. The energy requirements to collect the water from these different surfaces are different. 


Water Vapor Permeability (WVP)

Source - CopyrightFreeImages.com

WVP is the rate of water vapor transmission per unit area per unit of vapor pressure differential.  If a water drop on a leaf stays as a drop the WVP of the leaf is near zero. Such a situation occurs when the attraction between water molecules is very much greater than the attraction between water molecules and the molecules at the surface of the leaf that come into contact with the water molecules. 

Source - psrc.usm.edu

Many novel membranes have been created that are almost impermeable to air but are permeable to water vapor. 


Nafion, a Dupont product, is one such membrane that is commercially available. Such membranes, which do not permit air to pass through them, but permit passage of water vapor, find important applications in power source, pharmaceutical and biotech industries.


The side of the Nafion membrane that comes in contact with water vapor develops a surface concentration in equilibrium with the vapor. With increase in time, the migration of water vapor occurs along the thickness direction of the membrane. 


Initially, a new dry Nafion membrane is impervious to air and large molecules of water and water vapor. However, once the membrane has been exposed to water vapor, the membrane chemically attracts the vapor and absorbs the vapor. The part of the membrane that absorbs water vapor becomes conducting to the vapor, while the rest of the membrane remains dry and non-conducting. Therefore, vapor transport through the
membrane is zero in the initial stages and gains speed once the moisture concentration on the far side of the membrane becomes greater than zero.