GORE-TEX® is a material that consists of a PTFE or Teflon membrane sandwiched between inner and outer laminate materials. The membrane consists of a mass of threads that form micro-pores.
These
pores are sufficiently small that water droplets cannot pass through them
due to their surface tension. When water is vaporized however as it is by
simple body heat, these much smaller groups of water molecules can readily
pass through the micro-pores.
The
micro-pores are much smaller than a rain drop and much larger than water
vapour formed by perspiration. The result is that one side of the material
can be getting the soaking of its life while the other is letting through sweat
as quickly as it is produced so the wearer stays dry and comfortable.