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Significance
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.
The above numbers should be treated only as an illustration of the slowness of diffusion. Great differences exist in the diffusion speed between particular systems, particularly in the solid state. For example:1
- Hydrogen gas in solid iron at 10 °C - diffusion coefficient of 1.66x10-13 m2/s;
- Aluminum in solid copper at 20 °C - diffusion coefficient of 1.3x10-34 m2/s.
The diffusion speed is proportional to the square root of the diffusion coefficient. Therefore, hydrogen in iron diffuses over 10 orders of magnitude faster than does aluminum in copper.
In biology
In cell biology, diffusion is a main form of transport for necessary materials such as amino acids within cells.2
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.
Tracer and chemical diffusion
Fundamentally, two types of diffusion are distinguished:
- Tracer diffusion, which is a spontaneous mixing of molecules taking place in the absence of concentration (or chemical potential) gradient. This type of diffusion can be followed using isotopic tracers, hence the name. The tracer diffusion is usually assumed to be identical to self-diffusion.
- Chemical diffusion occurs in a presence of concentration (or chemical potential) gradient and it results in net transport of mass. This is the process described by the diffusion equation.
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
- Surface 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 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.
References
- ^ a b 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. pp. 66–67.
See also
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Look up diffusion in Wiktionary, the free dictionary. |
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.
- NetLogo Simulation Model for Educational Use (Java Applet)
- Short movie on brownian motion (includes calculation of the diffusion coefficient)
- Short movie about the diffusion process)
