Diffusionsfenomen

Diffusionsfenomen refers to the diffusion process, where particles spread from regions of higher concentration to lower concentration due to random thermal motion. This leads to mass transport without bulk flow and occurs in gases, liquids, and solids. Diffusion underpins many natural and technological processes, from gas exchange in biology to material processing in industry.

The movement is described by Fick's laws. Fick's first law states that the diffusive flux J is proportional to the negative gradient of concentration: J = -D ∇c, with D denoting the diffusion coefficient. Fick's second law describes how concentration changes in time: ∂c/∂t = ∇ · (D ∇c). In homogeneous media D is constant, while in heterogeneous systems it can depend on temperature, composition, or position. Diffusion coefficients generally increase with temperature and decrease with increasing molecular size.

The microscopic origin is Brownian motion: random collisions cause particles to explore space, and over time this stochastic motion results in net spreading. In solids, diffusion often proceeds through vacancy or interstitial mechanisms; in liquids and gases it is dominated by molecular collisions. The Einstein relation links diffusion to mobility, and the Stokes–Einstein equation D ≈ k_B T /(6 π η r) provides an estimate for spherical particles in a fluid.

Applications and examples include gas exchange in lungs, nutrient transport in tissues, dye spreading in water, and dopant diffusion in semiconductors. In porous media, diffusion may interact with convection and is modeled with extended or anisotropic forms of Fick’s laws.