Difference Between Knudsen and Molecular Diffusion

The key difference between Knudsen and molecular diffusion is that Knudsen diffusion involves the collision of gas molecules with pore walls, whereas molecular diffusion involves the movement of molecules from one system to another according to the concentration gradient.

Diffusion refers to the movement of molecules (specifically gas molecules) through a system. This process can be found in two types: Knudsen diffusion and molecular diffusion.

CONTENTS

1. Overview and Key Difference
2. What is Knudsen Diffusion
3. What is Molecular Diffusion
4. Side by Side Comparison – Knudsen vs Molecular Diffusion in Tabular Form
5. Summary

What is Knudsen Diffusion?

Knudsen diffusion is the diffusion that occurs when the scale length of a system is comparable to or smaller than the mean free path of the particle involved. This term is used mainly in physics and chemistry, and it was named after the scientist Martin Knudsen.

When considering the movement (more specifically, diffusion) of the gas molecules through very tiny capillary pores, if the mean free path of the diffusing gas molecules is larger than the pore diameter, then that means the density of that gas is very low, and gas molecules tend to collide with the pore walls compared to the collisions between the molecules. This process is named the Knudsen diffusion or Knudsen flow.

Additionally, we can define the Knudsen number, which is a good measure of the relative significance of Knudsen diffusion. If this number is greater than 1, it means the Knudsen diffusion is important for that system. Practically, this number is applicable only to gases. This is because the mean free path of molecules in liquid or solid states are very small.

What is Molecular Diffusion?

Diffusion is the movement of molecules from a region of high concentration to a low concentration via a concentration gradient. These movements occur in the same solution. The factors that affect the concentration gradient affects the diffusion as well.

This motion is terminated when the concentrations of the two regions become equal at every point. This means this motion occurs until the concentration gradient disappears. Then the molecules spread everywhere inside the solution.

The rate of the movement of the molecules through diffusion is a function of temperature, the viscosity of the gas (or fluid) and particle size. Usually, molecular diffusion describes the net flux of molecules from a region of high concentration to a low concentration. When considering the two systems, A1 and A2, which are at the same temperature and are capable of exchanging molecules between them, a change in the potential energy in either of these systems can create an energy flow from one system to the other (from A1 to A2 or vice versa) since any system naturally prefers low energy and high entropy states. This creates a state of molecular diffusion.

What is the Difference Between Knudsen and Molecular Diffusion?

There are two types of diffusion as Knudsen diffusion and molecular diffusion. The key difference between Knudsen and molecular diffusion is that Knudsen diffusion involves the collision of gas molecules with pore walls, whereas molecular diffusion involves the movement of molecules from one system to another according to the concentration gradient.

Summary – Knudsen vs Molecular Diffusion

There are two types of diffusion as Knudsen diffusion and molecular diffusion. The key difference between Knudsen and molecular diffusion is that Knudsen diffusion involves the collision of gas molecules with pore walls, whereas molecular diffusion involves the movement of molecules from one system to another according to the concentration gradient.

Reference:

“Knudsen Diffusion.” An Overview | ScienceDirect Topics, Available here.

Image Courtesy:

1. “Knudsen diffusion” By KleverI – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Diffusion self1” By Rosentod – Own work (Public Domain) via Commons Wikimedia