Diffusion and Fick's law

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Posts: 2
Joined: Fri Sep 19, 2014 7:18 pm

Diffusion and Fick's law

Post by Scotsgofree » Sat Oct 25, 2014 3:10 pm

Fick's law has it that the rate of diffusion of substance A into substance B within a system increases proportionally as the area of the exchange surface increases and as the concentration gradient increases, and inversely as the distance of the diffusion pathway increases. All this is tied together using a diffusion constant for substance A into B, which has units for length^2 and time^-1 and is a negative number to keep the diffusion flux value positive. So Fick's law is all about the amount of substance A diffusing into substance B per unit area per unit time in mol/(m^2.s). This could also be thought as relating to the difference in pressure of substance A between two arbitrary surfaces in the system during the time when substance A diffuses into substance B, so why isn't the temperature of the system a factor when considering the rate of diffusion?

Posts: 4
Joined: Tue Oct 28, 2014 4:54 pm

Re: Diffusion and Fick's law

Post by noncentric » Wed Oct 29, 2014 11:48 am

The diffusivities (diffusion "constants") are implicit functions of temperature. You don't worry about temperature dependence so much because you almost always use Fick's Laws under isothermal conditions (or at least assume it is isothermal enough).