diffusion

Information about diffusion

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" form of transport, rather than "active"); 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.[1]

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 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.
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

1. ^ Biddle, Verne, and Gregory Parker. Chemistry: Precision and Design. Pensacola: A Beka Book, 2000. p109.

See also

External links

Diffusion may refer to:
  • Diffusion, the spontaneous spreading of something such as particles, heat, light, or momentum
In the physical sciences:
  • Acoustic diffusion of sound waves
  • Atomic diffusion

..... Click the link for more information.
A spontaneous process is a chemical reaction in which a system releases free energy (most often as heat) and moves to a lower, more thermodynamically stable, energy state.[1][2]
..... Click the link for more information.
Passive transport means moving biochemicals and other atomic or molecular substances across membranes. Unlike active transport, this process does not involve chemical energy.
..... Click the link for more information.
Active transport (sometimes called active uptake) is the mediated transport of biochemicals, and other atomic/molecular substances, across membranes. Unlike passive transport, this process requires the expenditure of cellular energy to move molecules "uphill" against a
..... Click the link for more information.
Ice melting - a classic example of entropy increasing[1] described in 1862 by Rudolf Clausius as an increase in the disgregation of the molecules of the body of ice.
..... Click the link for more information.
In thermodynamics, the Gibbs free energy (IUPAC recommended name: Gibbs energy or Gibbs function) is a thermodynamic potential which measures the "useful" or process-initiating work obtainable from an isothermal, isobaric thermodynamic system.
..... Click the link for more information.
The second law of thermodynamics is an expression of the universal law of increasing entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.
..... Click the link for more information.
The diffusion equation is a partial differential equation which describes density fluctuations in a material undergoing diffusion. It is also used to describe processes exhibiting diffusive-like behaviour, for instance the 'diffusion' of alleles in a population in population
..... Click the link for more information.
Fick's laws of diffusion describe diffusion and can be used to solve for the diffusion coefficient D. They were derived by Adolf Fick in the year 1855.

First law

Fick's first law is used in steady-state diffusion, i.e.
..... Click the link for more information.
flux, both with rigorous mathematical frameworks.
  • In the study of transport phenomena (heat transfer, mass transfer and fluid dynamics), flux is defined as the amount that flows through a unit area per unit time, the volumetric flow rate.

..... Click the link for more information.
Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter, from a region of higher temperature to a region of lower temperature, and hence acts to even out temperature differences.
..... Click the link for more information.
random walk, sometimes called a "drunkard's walk," is a formalization in mathematics, computer science, and physics of the intuitive idea of taking successive steps, each in a random direction.
..... Click the link for more information.
Ohm's law states that, in an electrical circuit, the current passing through a conductor between two points is proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them.
..... Click the link for more information.
In physics (namely, in kinetic theory) the Einstein relation is a previously unexpected connection revealed by Einstein in his 1905 paper on Brownian motion:



linking D, the Diffusion constant, and μp
..... Click the link for more information.
Cell biology (also called cellular biology or formerly cytology, from the Greek kytos, "container") is an academic discipline that studies cells. This includes their physiological properties, their structure, the organelles they contain, interactions with
..... Click the link for more information.
Atomic diffusion is a 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.
..... Click the link for more information.
Brownian motion (named in honor of the botanist Robert Brown) is either the random movement of particles suspended in a fluid or the mathematical model used to describe such random movements, often called a Wiener process.
..... Click the link for more information.
Collective diffusion is the diffusion of a large number of particles, most often within a solvent.

Contrarly to brownian motion, which is the diffusion of a single particle, interactions between particles may have to be considered, unless the particles form an ideal mix with
..... Click the link for more information.

Effusion of gases

  • In chemistry, effusion is the process where individual molecules flow through a hole without collisions between molecules. This will occur if the diameter of the hole is considerably smaller than the mean free path of the molecules[1].

..... Click the link for more information.
Electron

Theoretical estimates of the electron density for the first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: First
..... Click the link for more information.
Electric current is the flow (movement) of electric charge. The SI unit of electric current is the ampere (A), which is equal to a flow of one coulomb of charge per second.

Definition

The amount of electric current (measured in amperes) through some surface, e.g.
..... Click the link for more information.
Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride, UF6, through semi-permeable membranes. This produces a slight separation between the molecules containing uranium-235 and uranium-238.
..... Click the link for more information.
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is the crucial process in the creation of a nuclear weapon.
..... Click the link for more information.
In thermal physics, heat transfer is the passage of thermal energy from a hot to a cold body. When a physical body, e.g. an object or fluid, is at a different temperature than its surroundings or another body, transfer of thermal energy
..... Click the link for more information.
Knudsen diffusion is when the diffusivity is determined by the size of the containing vessel instead of the solvents or solutes. It is generally applicable in a combination of small vessel sizes or very low pressures.
..... Click the link for more information.
Momentum diffusion refers to the diffusion, or spread of momentum between particles (atoms or molecules) of matter, usually in the liquid state.

In the case of the laminar flow of a liquid past a solid surface, momentum diffuses across the boundary layer which forms at the
..... Click the link for more information.
Hydrodynamics, also known as liquid-dynamics in limited academic circles, (literally, "water motion") is fluid dynamics applied to liquids, such as water, alcohol, oil, and blood. However, this distinction from fluid dynamics as a whole is not always fully observed.
..... Click the link for more information.
Osmosis is the net movement of water across a partially permeable membrane from a region of high solvent potential to an area of low solvent potential, up a solute concentration gradient.
..... Click the link for more information.
Photon diffusion refers to a situation where photons travel through a material with a high optical depth and very short mean free path. Their behavior is then dominated by scattering and the path of any given photon is effectively a random walk.
..... Click the link for more information.
Reverse diffusion refers to a situation where the transport of particles (atoms or molecules) in a medium occurs towards regions of lower concentration gradients, opposite to that observed during diffusion.
..... Click the link for more information.

This article is copied from an article on Wikipedia.org - the free encyclopedia created and edited by online user community. The text was not checked or edited by anyone on our staff. Although the vast majority of the wikipedia encyclopedia articles provide accurate and timely information please do not assume the accuracy of any particular article. This article is distributed under the terms of GNU Free Documentation License.