diffusie

Diffusie is the net movement of particles from regions of higher concentration to regions of lower concentration, driven by random thermal motion. It is a fundamental transport mechanism that occurs in gases, liquids and solids, and it can involve atoms, ions, molecules, or even information and influence in a social context.

In passive diffusion, no external energy is required. Facilitated diffusion uses transport proteins or channels to move substances down a concentration gradient, while still without external energy input. In solids, diffusion often proceeds by vacancy or interstitial mechanisms, enabling atoms to exchange positions within a lattice.

Two main laws describe diffusion. Fick's first law relates the diffusive flux to the concentration gradient, J = -D ∇C, where D is the diffusion coefficient. Fick's second law describes how concentration changes in time, ∂C/∂t = D ∇^2 C, assuming D is constant. The diffusion coefficient depends on temperature, the medium, and the diffusing species, and often follows an Arrhenius-type relationship D = D0 exp(-Ea/RT). Diffusion is quantified in units of area per time, typically square meters per second.

Examples include perfume spreading through air, dyes dispersing in water, moisture diffusion through polymers, and ion transport in electrolytes. Applications span material science (diffusion bonding, heat treatments), biology (gas exchange, nutrient transport), environmental science (pollutant dispersion in soils and groundwater), and medicine (controlled drug diffusion through membranes).

Related concepts include osmosis (diffusion of water across membranes), and diffusion can be isotropic or anisotropic depending on the medium. The characteristic diffusion length over time is often approximated by √(2Dt). The concept was formalized by Adolf Eugen Fick in 1855.