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I'm seeking the name of or reference for an experiment I once saw in a college physics class. At the beginning of one class the instructor repeatedly wound a wiper that spread a blot of some type of ink all over the interior of a glass jar. Then during the lecture (which I admittedly don't remember very well) he must have explained something about the second law of thermodynamics or entropy and that once a large system gets all mixed up, there's really no chance for it to return to its original state. Then he concluded the class by winding the wiper in the opposite direction, and clearly to his delight our jaws all dropped---the film of ink, which had been spread all over the interior of the jar, reappeared in the original blot.

Surely, there are folks on this site who demonstrate this every semester. What on earth is this experiment?

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  • $\begingroup$ This is a fun experiment because it works for thermodynamics lessons and fluid dynamics lessons (not that fluids and thermo aren't linked of course, but it's easy to hand-wave it away). $\endgroup$ – tpg2114 Dec 18 '12 at 19:46
  • $\begingroup$ See this reversible laminar flow video from this Phys.SE question and this Phys.SE answer. $\endgroup$ – Qmechanic Dec 18 '12 at 20:59
  • $\begingroup$ @Qmechanic: That's great! It's interesting that this video has more than a million views. $\endgroup$ – Rus May Dec 19 '12 at 16:05
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I don't know if there is a formal name for it, but my favorite search engine likes to call it a Reverse Entropy Machine.

The main fluid is glycerin and the dye is food coloring. You can see an example of the setup here.

You can also watch a video that describes it along with some lecture notes where it is called Kinematic Reversibility.

It works because the motion of the inner cylinder is relatively slow and the glycerin is very viscous so the flow is laminar. Molecular diffusion, an irreversible process, is negligible over these time scales (although if you let it sit a really really long time it would smear). So the entire process itself is reversible and the ink blob can reform from it's distorted shape.

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  • $\begingroup$ Thanks so much for the references. They were exactly what I was hoping for. $\endgroup$ – Rus May Dec 19 '12 at 15:59

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