Introduction

Diffusion is the process by which molecules, ions, or other small particles spontaneously mix, moving from regions of relatively high concentration into regions of lower concentration.

Fick’s first law: The molar flux (mol/area.time) due to diffusion of a constituent \(A\) relative to the average velocity of all constituents, is given by Fick’s first law as \[\text{Molar flux} = -D_{AB}\frac{\partial C_A}{\partial z}\]

Fick’s first law is applicable to steady state system.

Fick’s second law: \[\frac{\partial C_A}{\partial t} = D_{AB}\frac{\partial^2 C_A}{\partial z^2}\] This describes the transient diffusion phenomenon.

This tells us that rate of change of concentration at a specific time and position is proportional to the second derivative of concentration profile.