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Molecular diffusion
From Wikipedia, the free encyclopedia
Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material. In a phase with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing or a state of equilibrium.
Molecular diffusion is typically described mathematically using two Fick's laws.
== Significance ==Henry Espinoza Is Diffusion Diffusion is part of transport phenomena. Of the mass transport mechanisms, molecular diffusion is known as a slower one. Molecular diffusion is generally superimposed on, and often masked by, other transport phenomena such as convection, which tend to be much faster. However, the slowness of diffusion can be the reason for its importance: diffusion is often encountered in chemistry, physics and biology as a step in a sequence of events, and the velocity of the whole chain of events is that of the slowest step. For example, the rate at which a chemical reaction progresses can be entirely limited by the rate of diffusion of reactants/products to/from the place where the reaction occurs. Diffusion of water is classified as osmosis.
The speed of diffusion can be approximately illustrated as follows (at room temperature)[1]
- in gas: 100 mm per minute
- in liquid: 0.5 mm per minute
- in solid: 0.0001 mm per minute.
,
where γ is the drag coefficient (the inverse of the mobility). The Einstein relation follows directly to be
,
which is the most general expression for the diffusion coefficient. D is the diffusion coefficient (in m^2s^-1) not referring to any microscopic model.
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[edit] Entropy and diffusion
Diffusion increases the entropy of a system. This is nothing else than saying that diffusion is a spontaneous and irreversible process. Something can spread out by diffusing, but it won't spontaneously 'suck back in'.
[edit] In biology
In cell biology, diffusion is a main form of transport for necessary materials such as amino acids through cell membranes.[2]
[edit] Non-equilibrium system
Because diffusion is a transport process of particles, the system in which it takes place is not an equilibrium system (i.e. it is not at rest yet). For this reason thermodynamics and statistical mechanics are of little to no use in describing diffusion. However, there might occur so-called quasi-steady states where the diffusion process does not change in time. As the name suggests, this process is a fake equilibrium since the system is still evolving.
[edit] 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
- Eddy diffusion
- Effusion of a gas through small holes.
- Electronic diffusion, resulting in electric current
- Facilitated diffusion, present in some organisms.
- Gaseous diffusion, used for isotope separation
- Heat equation
- Itō diffusion
- Knudsen diffusion
- Momentum diffusion, ex. the diffusion of the hydrodynamic velocity field
- Osmosis is the diffusion of water through a cell membrane.
- Photon diffusion
- Reverse diffusion
- Rotational diffusion
- Self-diffusion
- Surface diffusion
- Active transport, pumping material across a cell membrane
- Pinocytosis, "cell drinking" intake of small droplets of lipids
- Phagocytosis, carrier proteins are used to transport glucose
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.
[edit] An experiment to demonstrate diffusion
Diffusion is easy to observe, but care must be taken to avoid a mixture of diffusion and other transport phenomena.
It can be demonstrated with a wide glass tube, paper, 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 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.
Another, simpler experiment to show diffusion is to drip a drop or two of food colouring into a glass of water. At first the food colouring will be very dark and concentrated at the spot where it hit the water, but after a while it will drift apart and fill the whole glass with a lighter shade of its colour. This is the dye diffusing in the water.
[edit] References
- ^ E.L. Cussler, "Diffusion. Mass Transfer in Fluid Systems", 2nd edition, Cambridge University Press, 1997, page 1.
- ^ Maton, Anthea; Jean Hopkins, Susan Johnson, David LaHart, Maryanna Quon Warner, Jill D. Wright (1997). Cells Building Blocks of Life. Upper Saddle River, New Jersey: Prentice Hall, 66-67.
[edit] See also
