Physics At the macroscopic level, diffusion is a universal process that leads to the elimination of concentration gradients in solids, liquids and gases. At the microscopic level, diffusion arises from the random motion of the atoms or molecules, activated by local fluctuations in the internal energy. The macroscopic laws that describe diffusion are known as Fick’s laws, after their formulator, the German physiologist Adolf Eugen Fick (1829–1901).
First law: Consider a plane of unit area of cross-section drawn in the medium, normal to the x-direction, across which a concentration gradient ∂c/∂x exists. The net flux of matter in the x-direction, Jx, through the plane, from regions of high concentration to regions of low concentration, is given by
Jx = – D (∂c/∂x).
This equation defines a quantity D, known as the diffusion coefficient, which turns out to be characteristic of the system and independent of ∂c/∂x and area.
Second law: If D is a constant
∂c/∂t = D(∂2c/∂x2).
This is the fundamental law upon which all experimental studies of diffusion depend.
J Thewlis, ed., Encyclopaedic Dictionary of Physics (New York, Oxford and London, 1962)
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