I poured a small amount of fridge-temperature cream into my just-poured same-fridge-temperature cold coffee. (Not enough time for the coffee to meaningfully increase in temperature. Lower-rate Brownian motion and regular post-pour chaotic motion still very much in effect, but with no mechanical stirring motion. My preferred cups are circular have a truncated conical tapering which obviously necessarily introduces some circular motion.) Every time I do this, the same two things happen:
1. Right away, there is a stratification effect. The cream punches straight down through the coffee and forms its own density layer, with minimal integration. (Clearly more than zero integration, but without intentional mixing, they remain quite obviously separate.) The height of the layer always seems more than the amount of cream would justify. (This must be a result of the layer taking on the additional volume of coffee post-integration.) Drink result: It's like two-two-two tasty coffee drinks in one! A nice cold bitter on top and a smooth café con crema finish. This is good, and should not be a surprise.
2. Almost simultaneously, at the punch-through location, despite the intentional lack of stirring, a swirly thing happens at the very top of the coffee-air interface. The cream stays on top, and proceeds to very gently expand in a spiral galaxy pattern, staying somewhat together. This is of course, perpendicular to the direction of the cream punching through. There is virtually no integration with the coffee layer, other than what must necessarily be happening at the invisible-to-the-naked-eye horizontal interface between the cream and the coffee. No doubt the very low temperature difference must contribute to the low rate of mixing. (Both coffee and cream are fairly miscible liquids.) Even if left alone for a few minutes, there's not much change.
The similarity to spiral galaxies finally struck me today. (Probably because I have work to do.) Are some galaxy formation mechanics a result of a more-or-less linear matter stream that suddenly collided with a bit of chaotically whirling matter, resulting in clearly separate strata in the dimension of the matter stream, leaving only the surface interaction visible? The centre of such a galaxy would be a since-closed path to a second level of organization with deep integration between the original matter stream and whatever it is that comprises the non-empty parts of space.
There are a lot of beautiful images out there that show the galactic* coffee effect far better than I could photograph. I'll leave it to you to find your own faves.
*Language nerds, this word choice was for you.
