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Diffusion is the mixing, in the absence of any chemical reaction or external force, that occurs when two fluids are placed in contact. The term is also used figuratively to describe the spontaneous mixing of abstract entities such as ideas and cultures: see diffusion (anthropology).

In physics, diffusion is one process by which matter, heat or momentum moves from one place to another. 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 the Fourier 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.

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Kinds of Diffusion

Atomic Diffusion

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.

See also Kirkendall effect

Thermal Diffusion

When heat travels through a material with a thermal gradient (for example, heat traveling through the wall of a coffee mug), the rate of transport is governed by the thermal conductivity and the temperature gradient.

Momentum Diffusion

In the case of laminar flow of a liquid flowing past a solid surface, momentum diffuses across the boundary layer near the surface. The gradient in this case is between the liquid in contact with the surface (which isn't moving at all and has zero momentum) and the liquid far away from the wall, which has momentum proportional to the speed at which it is flowing. The rate of transport is governed by the viscosity of the fluid and the momentum gradient.

Electron Diffusion

Electric current flows by diffusion in most conductors. 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.

Brownian Motion

This 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.

Reverse Diffusion

In general, diffusion results in transport down the gradient -- i.e. things move from regions of high concentration to low concentration. However, this is not always the case: during a phase separation, material can diffuse towards regions of higher concentration. This is referred to as reverse diffusion.

See also effusion.

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