How can a membrane resist rain but let water vapor move?

Rainwater and water vapor contain the same H₂O molecules. A breathable waterproof layer therefore cannot work because liquid water has larger individual molecules. Instead, liquid entry and vapor transport depend on structure, interfacial forces and differences between the conditions on either side.

One membrane type has connected microscopic pores in a hydrophobic material. Water does not readily wet their surfaces. Surface tension and the shape of the liquid interface create resistance to entry. Liquid pressure must exceed an entry threshold to force water into these pores. Pore size and surface chemistry help set that threshold.

While the pores remain gas-filled, water vapor can move through their pathways. Another membrane type is dense and hydrophilic, with no continuous open pores. Water molecules are absorbed into the polymer, diffuse through it, and are released at the other surface. Chemical interactions between water and the polymer make this solution-diffusion route possible.

Net transfer follows a difference in chemical potential: a measure of water molecules’ tendency to transfer between environments. For a given temperature, a water-vapor pressure difference provides a useful way to describe that driving contrast. Higher vapor potential inside can produce outward movement; reversing the contrast reverses net movement. Equal potentials give no net transfer even though molecules still move. Liquid resistance and vapor transport thus coexist through two different sets of physical behavior.