I had Oreos and milk a while ago and left my half-full cup of milk out on the counter. Afterwards I noticed that the crumbs had surfaced in a circular coin-sized glob, and just now I looked again to see all of the crumbs seem to have compressed into a more dense glob at the very center of the cup.
Why does this happen?
There are two separate questions here:
why do the crumbs clump together?
why does the clump form in the centre of the cup?
The answer to (1) is discussed in my answer to Why does a cork float to the side of a glass?. Assuming the crumbs are relatively hydrophilic, so the contact angle of water on them is less than $\pi/2$, the curvature of the water meniscus between two crumbs will pull them together. The result is that all the crumbs are pulled into a single aggregate.
The answer to (2) is harder. I would have guessed that the lowest energy configuration was for the aggregate to move to the edge of the cup, just as the cork moves to the edge of the glass in the question I linked above. Had you stirred the milk before dunking the Oreo's? If the milk was still rotating the crumbs would have moved to the centre of the dimple simply because it's lower than the edges. I'm sure the aggregate would eventually move to the centre of the cup, but once it had formed at the centre this motion would be slow unless you disturbed the cup.
I believe it has something to do with the shape of the glass/cup. Try an irregular-shaped cup or pour milk into a massive massive bowl - try different shapes and sizes and see if the effect is the same.
I think in a circular cup there will be force exerted on the milk by the biscuit, but because the glass is round, that force is transferred around the cup/glass. When the particles reach the center (I'm making this up as I go) the forces exherted by the milk due to the boundary of the circular cup cancel out and cause the biscuit to remain mainly in the middle of the glass.
This is one hypothesis.
