Which force governs diffusion

1. Mar 26, 2003

Homer

Hello,
In a biological course in the university, I asked my professor Which force governs diffusion and He didn't know.
Note that i don't mean Fick's first or second law, Einstein's law of diffusion, the Brownian Motion and not even the The Second Law of Thermodynamics, simply what "compels" particles to move and thus to balance concentrations.
For example, falling objects towards Earth is governed by the universal law of Gravitation
Any ideas?
Thanks

2. Mar 26, 2003

J-Man

I presume you mean solutions, like salt in water.

I believe heat, or thermal energy, may be one of the greatest contributors to diffusion in a solution. Thermal energy is what makes atoms and molecules vibrate, it also allows cooler parts of a solution to fall and warmer parts to rise. After that I would guess that electric attraction and repulsion play a part. For the water example, each water molecule is polarized, that is one half is more positive and the other half is more negative, this enables the individual water molecules to "stick together"; it also allows other charged molecules to stick to water, but only one (or two if you have 2 differently charged molecules) per molecule, so that there are only so many places to "stick". Another contributor to diffusion, though not a force in itself, is the geometry of how the molecules stick together. Some geometric configurations are more densely packed, so it's harder to get an atom or molecule to fit inside or in-between, while other configurations are less densely packed so it's easier for an atom or molecule to fit inside or in-between.

3. Mar 26, 2003

Homer

Hey,
As I understand, thermal energy is the driving force behind Brownian Motion, however, for itself i don't see how it would explain what drives the diffusion phenomena especially when you consider a multi-gas environment where each gas tries to "achive" equilibrium "with its own kind" so to speak.
The electro-magnetic force is an interesting Idea.
Thanks.
Bye

4. Mar 26, 2003

nbo10

All systems try to go into the lowest energy state.

JMD

5. Mar 27, 2003

sir-pinski

If you reduce the problem to the basics then you find it is all down to the particles following a random walk. Essentially there is no driving force to diffusion only the force which causes random walks which is system specific e.g. particle collisions, electromagnetic etc.

6. Mar 28, 2003

Homer

Hey,
Thanks for the responses.
I have a follow-up question then.
If random walk and Thermal energy do indeed govern diffusion, why is it that each gas tend to equalize its partial concentration only with regard to itself.
E.g. If we intermix two systems of oxygen and helium, each gas will achive equilibrium only with its own kind (so to speak).
Thanks.