Fick's First Law

A constitutive relation for diffusive transport stating that the particle flux J is proportional to the negative concentration gradient: J = −D ∇C, where D is the diffusion coefficient.

In a continuum and under steady‑state conditions with negligible bulk convection, the diffusive flux J_i of species i (amount per area per time) is proportional to the negative gradient of its concentration c_i: J_i = -D_i·∇c_i (in one dimension J_i = -D_i·dc_i/dx). D_i is the molecular diffusion coefficient (assumed constant in the simplest form).