Stofdiffusion

Stofdiffusion, or diffusion of substances, is the net movement of particles from regions of higher concentration to regions of lower concentration as a result of random thermal motion. In homogeneous media, diffusion tends to erase concentration gradients over time and, in closed systems, proceeds until uniform distribution is reached.

The process is described quantitatively 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/∂x, where D is the diffusion coefficient. Fick's second law, ∂C/∂t = D ∇^2 C, describes how concentration changes with time in non-steady conditions. The diffusion coefficient depends on particle size, temperature, solvent viscosity, and the medium's structure. In liquids, D is often larger for small molecules and lower for large ones; in gases, diffusion is typically faster.

Several mechanisms can govern diffusion across boundaries. Simple diffusion occurs when molecules freely permeate a medium or membrane; facilitated diffusion uses specific transport proteins to move substances along their concentration gradient without energy consumption.

The diffusion coefficient can be estimated by relations such as the Stokes–Einstein equation for spherical particles in a fluid: D ≈ kB T /(6πηr). Timescales for diffusion scale roughly with the square of the distance, t ~ L^2/D, which explains why diffusion is efficient over short distances but slow over larger ones.

Applications of stofdiffusion include gas exchange in lungs, nutrient transport and waste removal in cells, dye spread in liquids, and the spread of pollutants in air or water. Limitations include slow transport over macroscopic distances and the importance of convection or active transport in many biological and industrial contexts.