@swagpussc @wonderofscience

This is laminar flow, which in a very simplified manner of speaking, means "no mixing". Laminar, as you might sus out, means layers (think laminating a paper, and then laminating it again, or laminated flooring). The layers don't mix with each other.

This is opposed to *turbulent* flow, in which layers do get irreversibly mixed up. If the demonstration turned that handle too fast, it would go turbulent, and you wouldn't be able to reverse it.

@kravietz @wonderofscience

Entropy only goes up in a universal system. It is very possible to make a specific subsystem isentropic (or even reduce its entropy).

@linnefaulk @wonderofscience Stokes flow is a well known phenomenon, although in this day and age things can be readily faked.

Here is a similar figure on Wikepedia.
https://upload.wikimedia.org/wikipedia/commons/d/d2/Time_reversible_flow_demonstration_in_a_Taylor-Couette_system.png