I came across this video of Taylor-Couette Flow on YouTube. Originally I was looking for a visualization of the wavy Taylor vortices induced by the angular motion of the inner cylinder.
However, I found something strange (as you can see at the end of the video), the experimenter at the beginning injects three different dyes in a viscous liquid:
And then he begins to stir slowly in a specific direction until the three dyes are fully diffused into the main liquid:
Finally, he stopped the stirring and began to stir in the opposite direction returning the three dyes to its original spots (well sort of!):
So, how is this reversibility even possible? shouldn't the diffusion of dyes into the liquid be irreversible?
Best Answer
This is what's happening in the video. I've drawn just a single drop, and for convenience I've ignored the curvature of the plates (it's harder to draw curves!):
It looks as if the (red in this example) ink drop is being mixed with the fluid, but actually it's just being stretched out into a thin sheet. When you turn the cylinder back again the sheet is pushed back into a drop.