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January 21, 2011


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Check-out this video for an explanation roughly in terms of reversible deformation under at very low Reynolds numbers. Cool demonstration!

Oh, and yes, I believe that you are correct that no actual mixing of the dyes occurs.

Thanks, Johnny. Good link!

This is a standard demonstration in any Fluid Dynamics I course in engineering school. Incompressible Newtonian Stokes flow is completely time-reversible. This isn't an illusion; the colored blobs aren't placed at different layers. They "mix" just as you are seeing. However, when the mixture is purely non-turbulent laminar flow, time-reversing the flow solves the same equations as the original Stokes flow - the molecules move back along their original path to their origins.

This demonstration was first performed in 1960:
J.P. Heller, Am J Phys 28, 348-353 (1960). "An Unmixing Demonstration"

Thanks for the explanation, Paul! I'm relieved to learn it's not magic after all. :)

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