what is one condition for diffusion to take place
Diffusion 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} .