Diffusion

Molecular diffusion - Wikipedia, the free encyclopedia
For other uses, see Diffusion (disambiguation) ... Diffusion is part of transport phenomena. Of the mass transport mechanisms, molecular diffusion is known as ...
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diffusion: Definition, Synonyms from Answers.com
diffusion n. The process of diffusing or the condition of being diffused. ... Diffusion plays a key role in processes as diverse as permeation through ...
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Diffusion and Osmosis
Their average diffusion rate is expected to depend upon that average velocity, ... Osmosis is a selective diffusion process driven by the internal energy of ...
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Diffusion
Osmosis - the diffusion of water (across a membrane) ... Diffusion and Osmosis are both types of PASSIVE TRANSPORT - that is, no energy ...
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Diffusion - MSN Encarta
Diffusion, in physical science, the flow of energy or matter from a higher concentration to a lower concentration, resulting in a homogeneous...
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This article is about the physical mechanism of diffusion. For alternative meanings, see diffusion (disambiguation).

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration. For example, diffusing molecules will move randomly between areas of high and low concentration but because there are more molecules in the high concentration region, more molecules will leave the high concentration region than the low concentration one. Therefore, there will be a net movement of molecules from high to low concentration. Initially, a concentration gradient—a smooth decrease in concentration from high to low—will form between the two regions. As time progresses, the gradient will grow increasingly shallow until the concentrations are equalized.

Diffusion is a Spontaneous process (more familiarly known as a "Passive transport" form of transport, rather than "Active transport"); it is simply the statistical outcome of random motion. 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.Biddle, Verne, and Gregory Parker. Chemistry: Precision and Design. Pensacola: A Beka Book, 2000. p109.

The diffusion equation provides a mathematical description of diffusion. 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.

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 (kinetic theory) 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.

Types of diffusion The spreading of any quantity that can be described by the diffusion equation or a random walk model (e.g. concentration, heat, momentum, ideas, price) can be called diffusion. Some of the most important examples are listed below.

Atomic 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 current (electricity)
Gaseous diffusion, used for isotope separation
Heat flow
Itō diffusion
Knudsen diffusion
Momentum diffusion, ex. the diffusion of the hydrodynamic velocity field
Osmosis
Photon diffusion
Reverse diffusion
Self-diffusion

Metabolism and respiration rely in part upon diffusion in addition to bulk or active processes. For example, 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. Lungs contain a large surface area to facilitate this gas exchange process.

An experiment to demonstrate diffusion Diffusion is easy to observe, but care must be taken to avoid a mixture of diffusion and other transport processes.

It can be demonstrated with a wide glass tube, two corks, some cotton wool soaked in ammonia solution and some red litmus paper. By corking the two ends of the wide glass tube and plugging the wet cotton wool with one of the corks, and the litmus paper can be hung with a thread within the tube. It will be observed that the red litmus papers turn blue.

This is because the ammonia molecules travel by diffusion from the higher concentration in the cotton wool to the lower concentration in the rest of the glass tube. As the ammonia solution is alkaline, the red litmus papers turn blue. By changing the concentration of ammonia, the rate of color change of the litmus papers can be changed.

References

See also

Bohm diffusion
Brownian motion
Collective diffusion
Diffusion equation
Diffusion equilibrium
Diffusion MRI
Fick's law of diffusion
Isotope separation
Mass transfer
Osmosis
Transport phenomena
Local time (mathematics)

External links

Some pictures that display diffusion and osmosis
An animation describing diffusion.
A tutorial on the theory behind and solution of the Diffusion Equation.

Proboscis | Diffusion
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Diffusion - Wikipedia, the free encyclopedia
Diffusion (see examples below) is part of transport phenomena. It is hard to observe 'pure' diffusion because it is often accompanied by the much quicker convection.

BBC - GCSE Bitesize - Biology | Cell processes | Diffusion
Particles in liquids and gases have kinetic energy. They move about, at speed, in all directions.

BBC - GCSE Bitesize - Biology | Cell processes | Diffusion, osmosis ...
Substances move in and out of cells by means of passive or active transport. Examples of passive transport are diffusion and osmosis (the diffusion of water). Important substances ...

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Definition: diffusion from Online Medical Dictionary
The Online Medical Dictionary is a searchable dictionary of definitions from medicine, science and technology.

Diffusion
Welcome to the web-based Internet resource for international researchers concerned with the "diffusion" of information systems and related technologies", Tom ...

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