Question

Fick's second law of diffusion is valid for non steady state diffusion.

Answer 1

Steady state diffusion is clearely a very special case,abd in general you have to deal with the concentration C being a function of both time t and position x:C=C(x,t).Again we will confine ourselves to one-dimensional problems.In most general situation,C would be a fumction of all coordinates and time C=C(x,y,z,t).

The equation that describes the one-dimensional non-steady-state case is known as Fick’s Second Law:

∂C /∂T = D∂ 2C/∂x2.

Fick’s Second Law is in essence an expression of the continuity of particles (or continuity of mass): It amounts to stating mathematically that the time rate of change in concentration in a small volume element is due to the sum total of particle fluxes into and out of the volume element.