Why doesn't gravity prevent diffusion across a barrier?

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The concept of diffusion is confusing me, as it seems to contradict gravity.

x = particles of matter with a mass of 1
| = a barrier particle x can permeate

Situation A:
xxxxx|x


(1)

Result of Situation A (tested approximately)
xxx|xxx


The explanation of this is that the particles are simply too tightly packed and want to expand.


(2)However, Newtons law of gravity would show that the 5x's would attract the 1x with a force 5 times greater than the 1x would attract the 5x's(I think). Therefore, in order to equal out(3x's per side), wouldn't the 1x need to pull twice as hard on the 5x's? Doesn't Newtons law of gravity show that the result of that experiment would tend towards the side with 5x's at least keeping all of it's particles, if not gaining the 1x?
 
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The force of gravity between atoms is way, way, way less than the collision forces driving them apart.

You can safely neglect any effect of gravity.