Diffusion

Encyclopedia : D : DI : DIF : Diffusion

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

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

This animation illustrates the process of gaseous diffusion. The colored blocks represent molecules that are experiencing Brownian motion. Initially, the top half is filled with pink blocks, the bottom with green. Over time, the two colors will be approximately evenly distributed between the top and bottom halves.

Diffusion, being the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomenon. Diffusion is the movement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). It is readily observed, for example, when dried food like spaghetti is cooked; water molecules diffuse into the spaghetti strings, making them thicker and more flexible. It is a physical process rather than a chemical reaction, which requires no net energy expenditure. In cell biology, diffusion is often described as a form of passive transport, by which substances cross membranes.

Examples of diffusion

• A balloon filled with helium will deflate a little bit every day, because helium atoms diffuse out of the balloon through its wall
• When spaghetti is cooked, water molecules diffuse into the spaghetti strings, making them thicker and more flexible. Adding salt to the water reduces diffusion by reducing the osmotic pressure.
• Carbon dioxide bubbles in soft drinks start as small nuclei and grow because of the diffusion of carbon dioxide molecules towards them
• Heat diffuses through the walls of a mug filled with hot coffee
• A gas distributes itself over a room by diffusion
• A sugar cube in a glass of water that is not stirred will dissolve slowly and the sugar molecules will distribute over the water by diffusion
• Ink in a beaker of water is an example of diffusion. In the end, the ink particles spread evenly throughout the mass of water

The nature of diffusion

The different forms of diffusion can be modelled quantitatively using the diffusion equation, which goes by different names depending on the physical situation. For instance - steady-state bi-molecular diffusion is governed by Fick's first law, steady-state thermal diffusion is governed by Fourier's law. The diffusion of electrons in an electrical field leads essentially to Ohm's law that is further explained by Einstein relation. The generic diffusion equation is time dependent, and as such applies to non-steady-state situations as well.

In all cases of diffusion, the net flux of the transported quantity (atoms, energy, or electrons) is equal to a physical property (diffusivity, thermal conductivity, electrical conductivity) multiplied by a gradient (a concentration, thermal, electric field gradient). Noticeable transport occurs only 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 heat transport and change in temperature.

Diffusion occurs as a result of the Second Law of Thermodynamics, which states that the entropy or disorder of any closed system that is not in equilibrium must always increase with time (until it reaches equilibrium). Because substances diffuse from regions of higher concentration to regions of lower concentration, they are going from a state of higher order to a state of lower order, in accordance with the Second Law of Thermodynamics. Therefore, diffusion is a spontaneous, natural process, and to reverse diffusion would require the expenditure of energy to counteract the higher order of the system and prevent a violation of the laws of entropy.

Types of diffusion

Diffusion includes not only diffusion of particles, but all transport phenomena occurring within thermodynamic systems under the influence of thermal fluctuations (i.e. under the influence of disorder; this excludes transport through an hydrodynamic flow, which is a macroscopic, ordered phenomena).

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

• 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
• Electron diffusion, resulting in electric current
• Heat flow (thermal diffusion)
• Knudsen diffusion
• Momentum diffusion, ex. the diffusion of the hydrodynamic velocity field
• Osmosis
• Photon diffusion
• Reverse diffusion

Isotope separation

• Gaseous diffusion
• Liquid thermal diffusion

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

See also

• Bohm diffusion
• Brownian motion
• Collective diffusion
• Diffusion equation
• 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]

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