Ficks Laws of Diffusion

An analogous situation can be envisioned if the medium is stationary (or a fluid in laminar flow in the x direction) and the temperature difference (7) — T0) is replaced by the concentration difference (Cj — C0) of some species that is soluble in the fluid (e.g., a top plate of pure salt in contact with water). If the soluble species (e.g., the salt) is A, it will diffuse through the medium (B) from high concentration (Cj) to low concentration (Co). If the flux of A in the y direction is denoted by nAy, then the transport law is given by 

Consider the system shown in Fig. 11-1. A thin partition separates the two gases A and B. When the partition is removed, the two gases diffuse through each other until equilibrium is established and the concentration of the gases is uniform throughout the box. The diffusion rate is given by Fick s law of diffusion, which states that the mass flux of a constituent per unit area is proportional to the concentration gradient. Thus 

An expression similar to Eq. (11-1) could also be written for the diffusion of constituent A in either the y or z direction. 

Notice the similarity between Eq. (11-1) and the Fourier law of heat conduction. 

The heat-conduction equation describes the transport of energy, the viscous-shear equation describes the transport of momentum across fluid layers, and the diffusion law describes the transport of mass. 

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If the equilibrium at the crystal face is established rapidly so as not to be rate-limiting, the crystallization process would be controlled by solute transport, which is proportional to the solute concentration gradient across the stagnant lay r-(Cs). The rate of crystallization could be modeled using Ficks law of diffusion and is given by... 

The release of steroids from silicone implants follows Ficks law of diffusion with the flux of diffusion being affected by a diffusion coefficient, membrane surface area, thickness of the membrane and the concentration gradient across the diffusion path in the membrane. 

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