Diffusion

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This article is about the physical mechanism of diffusion. For alternative meanings, see diffusion (disambiguation).

Schematic drawing of the effects of diffusion through a cell membrane.

Diffusion is the movement of particles of a substance from an area of high concentration to an area of low concentration, resulting in the uniform distribution of the substance. For example, diffusion of particles causes a net movement of particles from areas of higher concentration to areas of lower concentration until equilibrium is reached. This is simply the statistical outcome of random motion: diffusion is a spontaneous process (more familiarly known as a "passive" form of transport, rather than "active"). Diffusion can affect a variety of different quantities. Examples include diffusion of concentration, heat, or momentum. Diffusion increases entropy, decreasing Gibbs free energy, and therefore is thermodynamically favorable. Diffusion operates within the boundaries of the Second Law of Thermodynamics because it demonstrates nature's tendency to wind down, as evidenced by increasing entropy.^[1]

Diffusion can be described mathematically by the diffusion equation. This equation is derived from Fick's law, which states that the net movement of diffusing substance per unit area of section (the flux) is proportional to the concentration gradient (how steeply the concentration changes in space), and is toward lower concentration. (Thus if the concentration is uniform there will be no net motion.) The constant of proportionality is the diffusion coefficient, which depends on the diffusing species and the material through which diffusion occurs. Fick's law is an assumption that may not hold for a given diffusive system (e.g., the diffusion may depend on concentration in addition to concentration gradient), in which case the motion would not be described by the normal (simple, Fickian) diffusion equation. An analogous statement of Fick's law, for heat instead of concentration, is Fourier's law.

Diffusion can also be described using discrete quantities (the diffusion equation has derivatives and thus applies to continuous quantities). A common model of discrete diffusion is the random walk. A random walk model is connected to the diffusion equation by considering an infinite number of random walkers starting from a non-uniform configuration, where the evolution of the concentration is described by the diffusion equation.

The spreading of any quantity that can be described by the diffusion equation or a random walk model (e.g. momentum, ideas, price) can be called diffusion.

Diffusion is often important in systems experiencing an applied force. In a conducting material, the net motion of electrons in an electrical field quickly reaches a terminal velocity (resulting in a steady current described by Ohm's law) because of the thermal (diffusive) motions of atoms. The Einstein relation relates the diffusion coefficient to the mobility of particles.

In cell biology, diffusion is a main form of transport within cells and across cell membranes.

1 Types of diffusion
2 Diffusion displacement
3 Isotope separation
- 3.1 Diffusion across biological membranes
4 References
5 See also
6 External links

[edit] Types of diffusion

Diffusion is the movement of particles from an area where their concentration is high to an area that always has low concentration.

Diffusion includes not only diffusion of particles, but transport phenomena occurring within thermodynamic systems under the influence of thermal fluctuations (i.e. under the influence of disorder; this excludes transport through a hydrodynamic flow, which is a macroscopic, ordered phenomenon), basically making both sides of the lipid bilayer of the same saturation of chemicals.

Diffusion is the process through which velocity thermodynamic system at local thermodynamic equilibrium returns to global thermodynamic equilibriums, through the homogenization of the values of its intensive parameters.

Atomic diffusion
Light diffusion
Brownian motion, for example of a single particle in a solvent
Collective diffusion, the diffusion of a large number of (possibly interacting) particles
Effusion of a gas through small holes.
Electron diffusion, resulting in electric current
Heat flow
Thermal diffusion
Itō diffusion
Knudsen diffusion
Momentum diffusion, ex. the diffusion of the hydrodynamic velocity field
Osmosis
Photon diffusion
Reverse diffusion
Self-diffusion

[edit] Diffusion displacement

The diffusion displacement can be described by the following formula

$\langle r_{k}^2 \rangle=2\cdot k\cdot D\cdot t$

where $\, k$ is the dimensions of the system and can be one, two or three. $\, D$ is the diffusion coefficient of the particles and $\, t$ is time. For the three-dimensional systems the above equation will be:

$\langle x^2 \rangle + \langle y^2 \rangle + \langle z^2 \rangle = \langle r_{3}^2 \rangle = 6\cdot D\cdot t$

[edit] Isotope separation

Gaseous diffusion
Liquid thermal diffusion

[edit] Diffusion across biological membranes

Facilitated diffusion
Ion diffusion through ion channels
Simple diffusion, not requiring a special protein channel
Diffusion in the respiratory system - in the alveoli of mammalian lungs, due to differences in partial pressures across the alveolar-capillary membrane, oxygen diffuses into the blood and carbon dioxide diffuses out

such as dye in water it diffuses out to change the colour of water in a matter of seconds. An Experiment to Demonstrate Diffusion Requirements:

              1 wide glass tube              2 Corks              Cotton wool soaked in  ammonia solution              4 square sized red litmus paper

Procedure: Cork the two ends of the wide glass tube. Plug the wet cotton wool with one of the corks and hang the litmus papers with a thread within the tube.

Observation: The red litmus papers turn blue.