Diffusion

Compensation of concentration differences in solutions or gases due to the heat movement of material particles. Diffusion also occurs through membranes, as far as the physical properties of the particles (size, water shell, electrical charge, water and lipid solubility) and the membrane (pore size, chemical structure, electrical charge) permit. Diffusion is the directional distribution of particles in space. Particles diffuse voluntarily - without having to expend energy - along their concentration gradient. This means: from the place of higher concentration to the place of lower concentration. We can also say that the particles strive to distribute themselves optimally in space or to migrate where there are few, i.e. where there is a lot of space. The diffused quantity per time unit depends on the size of the exchange surface, the length of the diffusion path and the concentration gradient of the respective gas.

https://de.wikipedia.org/wiki/Diffusion _{(Wikipedia CC-by-sa-3.0)}