Fick's First Equation

$J_x=-D\frac{\partial C}{\partial x}$

• The flux (per unit (area * time)) in a certain direction $x$
• $D$ is the diffusive constant
• $C$ is concentration in the fluid
• The intuition is that if concentration is increasing rightward for instance, then flux will direct flow oppositely in a manner proportional to the diffusive constant

$\vec{J}=-D\nabla C$

• 3D version