permeating

Permeating is the process by which a substance penetrates and spreads through another material or boundary. In science and engineering, permeation describes the movement of molecules from one side of a barrier to the other, across films, membranes, tissues, or coatings. The term derives from Latin permeare, meaning to pass through, and encompasses both the extent of penetration and the rate at which it occurs.

Permeation typically involves dissolution of the penetrant in the barrier followed by diffusion to the opposite interface, a mechanism often described by the solubility-diffusion model in nonporous materials. In porous barriers, transport may also occur by pore flow or surface diffusion. The rate of permeation depends on the chemical affinity between penetrant and barrier, the material's free volume, temperature, and the pressure or chemical potential difference across the barrier.

Quantities used to characterize permeation include permeability, permeance, diffusivity, and the partition coefficient. Permeability combines solubility and diffusivity and is often reported for gases in polymers. Other metrics include flux, J, and the selectivity between two penetrants. Experimental measurement typically involves a steady state or time-lag method, with units adapted to the system and material type.

Applications span packaging, where barriers slow oxygen or moisture transmission; membrane separation, gas purification, and fuel cells; protective coatings; and medical devices where controlled transport through films or tissues is required. In biology, permeation describes how substances such as water, oxygen, and nutrients cross cell membranes, sometimes via channels or passive diffusion, contributing to cell physiology and homeostasis.