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Anatomy => Isotopic Tracer => Diffusion
Diffusion
Diffusion, in physical science, the flow of energy or matter from a higher concentration to a lower concentration, resulting in a homogeneous distribution. If one end of a rod is heated or electrically charged, the heat or electricity will diffuse from the hot or charged portion to the cool or uncharged portion. If the bar is made of metal, this diffusion will be rapid for heat and almost instantaneous for electricity; if the bar is made of asbestos, the diffusion will be slow for heat and extremely slow for electricity. Diffusion of matter occurs most rapidly in gases, more slowly in liquids, and most slowly in solids. The spreading of odoriferous molecules (a smell) throughout a room is a common example of gaseous diffusion. A solid may dissolve and diffuse through a liquid, as when a lump of sugar is placed in a cup of water. This process is much slower than the diffusion of a gas; if the water is not stirred, it may take weeks for the solution to become homogeneous. An example of the slowest diffusion process, a solid diffusing into a solid, occurs when gold is plated on copper. The gold will diffuse slowly into the surface of the copper; however, diffusion of an appreciable amount of gold more than a microscopic distance normally requires thousands of years.
All these types of diffusion follow the same laws. In all cases, the rate of diffusion is proportional to the cross-sectional area and to the gradient of concentration, temperature, or charge. Thus, heat will travel four times as fast through a rod 2 cm in diameter as through a rod 1 cm in diameter, and when the temperature gradient is 10° per cm, heat will diffuse twice as fast as when the gradient is only 5° per cm. The rate of diffusion is also proportional to a specific property of the substance, which in the case of heat or electricity is called conductivity; in the case of matter, this property is called diffusivity or diffusion coefficient. The amount of material that diffuses in a certain time, or the distance it traverses, is proportional to the square root of the time; thus, if it takes sugar one week to diffuse through water 1 cm from its starting point, it will take four weeks to diffuse through 2 cm.
As distinguished from stirring, which is a process of mixing masses of material, diffusion is a molecular process, depending solely on the random motions of individual molecules. The rate of diffusion of matter is therefore directly proportional to the average velocity of the molecules. In the case of gases, this average speed is greater for smaller molecules, in proportion to the square root of the molecular weight, and is greatly increased by rise in temperature. Metallic thorium, for example, diffuses rapidly through metallic tungsten at temperatures around 2000° C (3632° F); the operation of certain vacuum tubes is based on this diffusion.
If one molecule is four times as heavy as another, it will, in the case of gases, move half as fast and its rate of diffusion will be half as great. Advantage can be taken of this difference to separate substances of different molecular weights, and in particular to separate different isotopes of the same substance. If a gas containing two isotopes is forced through a fine porous barrier, the lighter isotopes, which have a higher average speed, will pass through the barrier faster than the heavier ones. The gas with the greater concentration of lighter isotopes is then diffused through a series of such barriers for large-scale separation. This technique, known as the gaseous-diffusion process, is widely used in the separation of the fissionable uranium isotope U-235 from the nonfissionable U-238. In another isotope-separation technique, called the thermal-diffusion process, the separation depends upon thermal effects exhibited by some gases; if such gases are enclosed in a chamber subjected to a temperature gradient, the heavier isotopes tend to concentrate in the cool region.
Diffusion processes are of great biological importance. For example, digestion is essentially a process of chemically changing food so that it will be able to pass, by diffusion, through the intestinal wall into the bloodstream. Shock, a condition that frequently follows surgery or injury, is a state in which the blood fluids have diffused excessively through the blood-vessel walls into the body tissues. Treatment of shock consists of injecting chemicals, usually in the form of blood, plasma, or plasma expanders, into the remaining blood fluid to compensate for the loss by diffusion and to alter pressure in the blood vessels, thus obviating further loss .
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