<P> If the diffusion coefficient is not a constant, but depends upon the coordinate or concentration, Fick's second law yields </P> <Dl> <Dd> ∂ φ ∂ t = ∇ ⋅ (D ∇ φ) (\ displaystyle (\ frac (\ partial \ varphi) (\ partial t)) = \ nabla \ cdot (\, D \, \ nabla \, \ varphi \,) \, \!). </Dd> </Dl> <Dd> ∂ φ ∂ t = ∇ ⋅ (D ∇ φ) (\ displaystyle (\ frac (\ partial \ varphi) (\ partial t)) = \ nabla \ cdot (\, D \, \ nabla \, \ varphi \,) \, \!). </Dd> <P> An important example is the case where φ is at a steady state, i.e. the concentration does not change by time, so that the left part of the above equation is identically zero . In one dimension with constant D, the solution for the concentration will be a linear change of concentrations along x . In two or more dimensions we obtain </P>

When there is a difference in concentration of a substance across a space it is said to