where the average relative speed <vr> is a function of temperature and other fixed parameters, σ is the collision cross-section area, CA and CB are the number concentrations of A and B, and kC is the collision rate constant. In the diffusive collision case (Fig. ), Smoluchowski proposed in 1916 to use the diffusive flux (from Fick's laws of diffusion) to calculate the collision frequency between A and B. Assuming at the boundary condition with a steady-state flux, the collision frequency was predicted to be 14