<Dl> <Dd> ∂ n ∂ t = − ∇ ⋅ J + W, (\ displaystyle (\ frac (\ partial n) (\ partial t)) = - \ nabla \ cdot \ mathbf (J) + W \,,) </Dd> </Dl> <Dd> ∂ n ∂ t = − ∇ ⋅ J + W, (\ displaystyle (\ frac (\ partial n) (\ partial t)) = - \ nabla \ cdot \ mathbf (J) + W \,,) </Dd> <P> where W (\ displaystyle W) is intensity of any local source of this quantity (the rate of a chemical reaction, for example). For the diffusion equation, the no - flux boundary conditions can be formulated as (J (x), ν (x)) = 0 (\ displaystyle (\ mathbf (J) (x), \ nu (x)) = 0) on the boundary, where ν (\ displaystyle \ nu) is the normal to the boundary at point x (\ displaystyle x). </P> <P> Fick's first law: the diffusion flux is proportional to the negative of the concentration gradient: </P>

What is one condition for diffusion to take place