Because it's Friday: Entropy, schmentropy

Here's a neat little science experiment: put three drops of dye into a cylinder of liquid, mix up the colours by turning the handle, and then reverse the process:

 

Pretty mind-boggling, eh? Now, I'm sure this isn't a trick in the sense that there's no duplicity here, but I think it *is* an illusion in that the three colours of dye never actually mix because they're inserted at different depths (from the wall) in the cylinder. I'm guessing that the dyes are smeared out in the laminar flow of concentric cylinders of liquid, but the camera is looking through three "smears" of dye that don't aren't actually mixed. If the dyes *do* mix, I can only assume someone repealed the second law of thermodynamics without telling me. But then again, fluid dynamics was never my strong suit: to this day I still believe that the Bernoulli effect is what lets airplanes fly. Any experts out there care to explain what's going on?

Comments

[Johnny Logic]

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

[Johnny Logic]

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

[David Smith]

Thanks, Johnny. Good link!

[Paul Bleicher]

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"

[David Smith]

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