Classical diffusion theories

Diffusion is the physical process by which atoms and molecules are transported from one position of a system to another. Atoms or molecules in solids occupy definite equilibrium positions. They, however, will migrate if their concentrations are disturbed. The atoms will then diffuse from the region of higher concentration to that of lower concentration. Such a condition exists in solids containing mechanical imperfections and internal stresses. The rate of diffusion increases very rapidly as the temperature is increased. When the temperature becomes very high, thermal agitation can become violent and pull the atoms and molecules apart resulting in melting or disintegration of the solid. Concentration gradient can also be developed by nonuniform distribution of moisture. The process of moisture transfer is fundamentally equivalent to that of heat transfer and each process may or may not be treated independently depending on the physical conditions.