Difuusioon

Difuusioon (diffusion) is the net movement of particles from regions of higher concentration to regions of lower concentration, driven by random thermal motion. It occurs in gases, liquids, and solids and does not require external energy. The process tends to equalize concentration differences over time and is a fundamental transport mechanism in chemistry, physics, and biology.

Diffusion arises from the random motion of molecules, which creates a net flux down a concentration gradient. In quantitative terms, the diffusive flux J is described by Fick's first law: J = -D ∇C, where D is the diffusion coefficient and ∇C the concentration gradient. Fick's second law governs how concentration evolves: ∂C/∂t = D ∇^2 C. In one dimension this reduces to ∂C/∂t = D ∂^2C/∂x^2. The diffusion coefficient D depends on temperature, solvent viscosity and particle size. The Stokes–Einstein relation D = k_B T / (6 π η r) applies to spherical particles in a fluid.

Diffusion types include simple diffusion, and facilitated diffusion across biological membranes, which uses proteins but does not require metabolic energy. In solids diffusion can proceed via vacancies or interstitial mechanisms. In some systems diffusion is anomalous, with the mean squared displacement not proportional to time due to obstacles or confinement.

Applications span gas exchange in respiration, nutrient transport in organisms, drug delivery, pollution dispersion in air and water, and processes in materials science. Diffusion was first described mathematically by Adolf Fick in 1855, laying the groundwork for modern diffusion theory.