Steady-State Diffusion

The flux of diffusing atoms, J, is expressed either in number of atoms per unit area and per unit time (e.g., atoms/m2-second) or in terms of mass flux (e.g., kg/m2-second).Steady state diffusion means that J does not depend on time. In this case, Fick’s first law holds that the flux along

_{direction x is:}

J = – D dC/dx

Where dC/dx is  the gradient  of the concentration  C,  and  D is  the diffusion  constant. The concentration gradient is often called the driving force in diffusion (but it is not a force in the mechanistic  sense). The  minus sign  in  the equation  means  that  diffusion  is down  the concentration gradient.