Fick's law of Diffusion

Fick's law of Diffusion describes diffusion and the law is used to solve diffusion coefficient. Adolf Fick derived the law in the year 1855. There are two laws of diffusion derived by Fick.

Fick’s First Law of Diffusion

The first law of diffusion relates the diffusive flux and concentration. This law explains that the flux goes from regions of high concentration to regions of low concentration with a magnitude proportional to the concentration gradient.

J = -D (C ∂ / ∂x)

'J' is the diffusion flux per unit area per unit time. It measures the amount of substance flowing through a small area during small time interval. ‘D ‘is the diffusion coefficient. C is the concentration in dimensions. ‘x’ is the position.

Fick’s first law accurately describes diffusion.

Fick’s second law of diffusion

Second law of diffusion calculates how the diffusion cause change in concentration. The law can be expressed as follows:

∂C/ ∂t = D (∂²C/ ∂x²)

C = concentration in dimensions, t = time, d = diffusion coefficient, x = position

 To use the second law of diffusion it is important to assume that the crystal is isotropic. If the concentration is large assumption may break down. When the concentration is large, diffusivity becomes a function.

Applications of Laws of Diffusion

These laws are commonly used to transport processes in foods, neurons, biopolymers, pharmaceuticals, porous soils, population dynamics, semiconductor doping processes etc.  Researches in polymer science and food science have shown that it is very important to have a general approach for the transportation of components in materials that undergo glass transition. The first law of diffusion gives rise to the following formula:

Flux = -P.A (c₂-c₁) where P is the permeability, A is the surface area where diffusion takes place and (c₂-c₁) is the difference in the concentration of gas across the membrane. Fick’s first law of diffusion is also very important in radiation transfer equations. Combing Fick's law and Graham’s law the exchange rate of gas across a fluid membrane can be determined easily. When two miscible liquids come into contact with each other and cause diffusion, the average concentration evolves Fick’s law. IC fabrication technologies, model processes like CVD, thermal oxidation, wet oxidation, doping etc. Use Fick’s diffusion laws.  Thus the law is applicable in almost all fields of science.

Questions:

• What are the two laws of diffusion introduced by Fick?
• What are the applications of Fick’s laws?