(ceramics) random walk approach to gases, liquids, or solids...

asdf1

For the random walk approach to gases, liquids, or solids, why isn't there a gradient? The atoms don't jump by themselves, right? They should have to feel forces to jump...

Astronuc

Atoms vibrate - that is what temperature is - the kinetic energy of atoms. Think - Brownian motion.

And there is a gradient - the temperature gradient or concentration gradient. One can observe a concentration gradient by taking a drop of ink and dropping it in a liquid like water, and watching the ink disperse.

In the case of solids, the atoms are more or less fixed in position - that's what makes a solid solid. In liquids, the atoms/molecules are subject to interatomic/intermolecular forces, but the individual atoms/molecules can migrate. In gases, there is distance between the atoms/molecules and the interatomic/intermolecular forces are very low if existent.

Now in solids, there can be diffusion, but is very slow - orders of magnitude less than in liquids and gases. Hydrogen can diffuse in many metals. There is self-diffusion of atoms in a solid.

Think of the process of precipitation hardening of a metal.

asdf1

Ok, I got it! Thank you very much!