In the comments to the post of weird fluids, Jared A mentioned seeing a demonstration in which a fluid was first stirred so that two colors became mixed and then, when the direction of stirring was reversed, the liquid became ‘unmixed’ so to speak.
Intrigued, I looked into it and found this video that demonstrates something similar to what he had likely seen.
How is this possible? It illustrates an interesting point about reversibility and irreversibility in the laws of physics. At the microscopic level, the laws of physics are reversible so that reversing the cause will reverse the effect. But when many particles are involved, as is the case with fluids at the macroscopic level, the interactions between the molecules become so complex that you cannot reverse the motions of all the constituent particles and so unmixing is not possible in any practical sense although theoretically not forbidden.
But under certain conditions, such as by constraining fluid of a certain viscosity between two cylinders so that you have close to a two-dimensional fluid, normal chaotic perturbations can be minimized and unmixing achieved. This was first demonstrated as far back as in 1966.
In the journal Nature, Troy Shinbrot discusses a 2005 paper titled Chaos and threshold for irreversibility in sheared suspensions that studies the phenomenon in some detail to determine the conditions under which this occurs.
Incidentally, one has to be always careful with such videos and look for signs that the effect was not obtained by simply running the video backwards, as done by the Mythbusters guys ‘solving’ Rubik’s cube blindfolded and with their feet. There are people who can actually do this, but the Mythbusters were pulling our legs.