Molecular diffusion

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

File:Simple difussion in cell membrane.svg

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

Diffusion is the net action of matter (particles or molecules), heat, momentum, or light whose end is to minimize a concentration gradient. The process of diffusion, therefore, minimizes thermodynamic Gibbs free energy (though it is not a chemical reaction), and is thus a spontaneous process (more familiarly known as a "passive" form of transport, rather than "active").

The different forms of diffusion can be modeled 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 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 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 net heat transport or change in temperature.

The second law of thermodynamics states that in a spontaneous process, the entropy of the universe increases. Change in entropy of the universe is equal to the sum of the change in entropy of a system and the change in entropy of the surroundings. A system refers to the part of the universe being studied; the surroundings is everything else in the universe. Spontaneous change results in dispersal of energy. Spontaneous processes are not reversible and only occur in one direction. No work is required for diffusion in a closed system. Reversibility is associated with equilibrium. Work can be done on the system to change equilibrium. Energy from the surroundings decrease by the amount of work expended from surroundings. Ultimately, there will be a greater increase in entropy in the surroundings than the decrease of entropy in the system working accordingly with the second law of thermodynamics.

Diffusion can be measured, by the means of concentration gradient. A concentration gradient is the difference between the high concentration and the low concentration. It also determines how fast diffusion occurs.

Diffusion in a continuum may be quantified through the Laplacian. This is widely seen in heat transfer (diffusion of heat), chemistry (diffusion of chemical species), fluid mechanics (diffusion of momentum), and elsewhere.

Types of diffusion

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

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 a hydrodynamic flow, which is a macroscopic, ordered phenomenon).

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.

In the crystal solid state, the occurrence of diffusion is contingent upon the availability of point vacancies throughout the crystal lattice. Diffusing particles migrate from point vacancy to point vacancy. Since the prevalence of point vacancies increases in accordance with the Arrhenius equation, the rate of crystal solid state diffusion increases with temperature.

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

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:

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

by lunga

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

such as dye in water it diffuses out to change the colour of water in a matter of seconds. thanks for the information BY LUNGA

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

• Template:McGrawHillAnimation
• Some pictures that display diffusion and osmosis