Beautiful. I saw one of these videos many moons ago and it was somewhat instrumental in making me study fluid dynamics for my doctorate work. The contraption is a Couette cylinder. It creates an almost constant shear force across the gap between the inside and outside cylinders. It's an approximation of two infinite plates moving relative to one another. A geometry that fluid physicists like to look at. The liquids used have to be Newtonian and the angular velocity has to be small such that the Reynolds number, Re, is low. Re is a function directly proportional to fluid density, fluid velocity, and a characteristic length scale (in this case the gap width) and inversely proportional to viscosity. So if your fluid is viscous enough and you rotate it slowly enough the Re number is close to zero and the nonlinear part of the Navier Stokes equations which governs flow can be ignored. What's left is a linear equation. When linear, the equation is thus reversible. The slight 'smudging' you get is mainly from diffusion.

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oh MAN that is bad ass. It stretches believability... what's the substance that it's in?

Probably a glycerine and water mixture. At least that's why I used in my phd experiments. But we added polymers to it to make it non-Newtonian (xanthan gum) which makes it behave really weird. I should see if I can find a video of non-Newtonian liquids.

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