Does this video show two fluids becoming “unmixed”?

A video shared by the Twitter account Amazing physics on October 27, 2021, showed a person squirting two dots of ink, one green and one red, into a clear tub of liquid. This person then grabbed a crank to slowly mix the two colored dots with the clear liquid before reversing the crank and “unmixing” them back to their original location.

Although the video appears to defy thermodynamics, it is authentic and it is a concept that Harvard University describe as an inverse fluid mixture.

The cylinder-shaped tank is filled with a thick, viscous fluid such as glycerin while lines of colored fluid are injected into the tank parallel to the axis of the cylinder.

“When the inner cylinder is slowly turned (by means of a crank) relative to the outer cylinder, the lines of colored fluid mix with the rest of the fluid. Several revolutions make the mixture completely clear and “blended”. If one now reverses the direction of rotation, the colored flowing lines reappear by ‘unmixing’ after the same number of rotations in the opposite direction,” reads a presentation published by the Harvard Natural Sciences Lecture Demonstration.

Because slow, steady mixing does not create turbulence, colors can be “unmixed” through a concept known as laminar flow. Unlike turbulent flow, where a fluid experiences irregular mixing, laminar flow allows a liquid to move smoothly.

To visualize the concept, think of a river where the top layer of water bounces off obstacles like rocks or whirlpools, causing it to mix and fold in on itself, thus experiencing what is called mixing. turbulent. Below this top layer is the laminar flow, or the part of the water that follows a straight path and does not hit obstacles or mix.

“Diffusion processes are, of course, much slower than the demonstration time scale. As one cylinder rotates, the other can simply think of fluid layers being displaced without involving turbulence, with the boundary layer next to the inner rotating cylinder being displaced the most and the layer adjacent to the outer cylinder the least. Counter-rotation puts those layers back in place,” Harvard wrote.

Sources

“https://Twitter.Com/Amazing_physics/Status/1453587546278092802.” Twitter, https://twitter.com/amazing_physics/status/1453587546278092802. Accessed October 29, 2021.

« Laminar flow | Physics.” Encyclopedia Britannica, https://www.britannica.com/science/laminar-flow. Accessed October 29, 2021.

Reversible fluid mixing. https://sciencedemonstrations.fas.harvard.edu/presentations/reversible-fluid-mixing. Accessed October 29, 2021.



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