This article is about the physical mechanism of diffusion. For alternative meanings, see diffusion (disambiguation).
Diffusion is the spontaneous spreading of something such as particles, heat, or momentum. The phenomenon is readily observed when a drop of colored water is added to clear water, or when smoke from a chimney dissipates into the air. In these cases, diffusion is the result of turbulent fluid motion rather than chemical reactions or the application of external force. In cell biology, diffusion is described as a form of "passive transport", by which substances cross membranes.
Diffusion is one kind of transport phenomenon: compare it, for example, to radiation. All diffusion can be modelled quantitatively using the diffusion equation, the solutions of which go by different names depending on the physical situation. Steady state matter diffusion is governed by Fick's first law. Steady-state thermal diffusion is governed by Fourier's Law. Diffusion of electrons in an electrical field is essentially Ohm's law. In each, a flux (of atoms or energy or electrons) is equal to a physical property (diffusivity or thermal conductivity or electrical conductivity) multiplied by a gradient (concentration gradient or thermal gradient or electric field). The generic diffusion equation is time dependent (i.e. also applies to non-steady-state situations).
In each case, there is only a noticeable diffusion if there is a gradient: for example in thermal diffusion, if the temperature is constant, heat will move as quickly in one direction as in the other, producing no change.
This is the process whereby the random thermally activated hopping of atoms in a solid results in the net transport of atoms. For example, helium atoms inside a balloon can diffuse through the wall of the balloon and escape, resulting in the balloon slowly deflating. Other air molecules (e.g. oxygen, nitrogen) have lower mobilities and thus diffuse more slowly through the balloon wall. There is a concentration gradient in the balloon wall because the balloon was filled up with helium, and thus there is plenty of helium on the inside, but there is relatively little helium on the outside, because helium is not a major component of air. The rate of transport is governed by the diffusivity and the concentration gradient.
Brownian motion occurs when discrete particles diffuse in a liquid medium. The thermal activation is provided by the thermal vibrations in the liquid. Because the particles are in a fluid, they are constantly moving in more or less random directions.
Electric current flows by diffusion in most conductorss. Charge carriers (usually electrons) move randomly in the absence of an electric field. When an electric field is applied, carriers drift preferentially in the field, causing a net current. The rate of transport is governed by the electrical conductivity of the conductor and the electric field.
When photons travel through a material with a high optical depth and a very short mean free path, their behavior is dominated by scattering and the path of any given photon's path is effectively a random walk.
The behavior of a large ensemble of photons in this situation can be described with a diffusion equation.
