What is diffusion?
Diffusion is a process in which molecules or atoms of two (or more different substances) mix together because of their kinetic energy (thermal agitation). Diffusion can take place between the molecules different gases, different liquids and even between the molecules of different solids (on very short distances). In the last case, the two solids that are initially in contact weld together after a long time. The rate of diffusion is proportional to the difference of the absolute concentrations of the two substances.
`(d(rho(x,t)))/dt = D*(d^2(rho(x,t)))/dx^2`
where `rho(x,t)` is the density of the given substance.
The factor of proportionality is named the diffusion constant `D` and it is proportional to the temperature of the medium in which the diffusion takes place.
`D = mu*k_B*T`
where `mu` is the molecules mobility of the given substance and `k_B` is the Boltzmann constant.
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