Fickian Diffusion vs. Non Fickian Diffusion — What's the Difference?

Fickian Diffusion and Non Fickian Diffusion represent distinct types of molecular dispersion mechanisms in materials. Fickian Diffusion is based on Fick's laws, which describe the diffusion process as being driven solely by concentration gradients. This linear relation signifies that the rate of diffusion is directly proportional to the concentration gradient, making it a straightforward process to understand.

On the contrary, Non Fickian Diffusion, sometimes termed anomalous diffusion, does not strictly follow Fick's laws. This type of diffusion arises in systems where the diffusion rate isn't just determined by concentration gradients. Factors such as polymer relaxation or other molecular phenomena can influence the diffusion process, making it deviate from the linear relationship described by Fick's laws.

In practice, Fickian Diffusion is observed in simpler systems, like the diffusion of a solute in a solvent. The concentration-driven process ensures that the solute molecules spread out to equalize concentration throughout the solvent. Non Fickian Diffusion, however, is commonly seen in complex systems, especially in polymer science. Here, the diffusion of a solvent into a polymer may not be just concentration-driven; the relaxation or movement of polymer chains can also play a role.

To summarize, while Fickian Diffusion provides a foundational understanding of diffusion in many systems based on concentration gradients, Non Fickian Diffusion introduces complexities, accounting for additional factors influencing the diffusion process. Recognizing the difference between these two types of diffusion is crucial in material science, especially when designing or analyzing materials where molecular dispersion plays a significant role.