Jaw-dropping “unmixing” demo appears to reverse entropy

Education Science

I identify as a scientist, but I gotta admit: When I saw this video from Steve Spangler Science, my first impulse was to jump back from the computer, cross myself, and douse the screen with holy water. It reminded me of a line from John Carpenter’s underappreciated 1987 horror movie, Prince of Darkness:

And we assume time is an arrow because it is as a clock…Cause precedes effect – fruit rots, water flows downstream. We’re born, we age, we die. The reverse NEVER happens…

Unless, apparently, you’re dealing with a system operating under conditions of laminar flow. Obviously, there is no real “violation” of the second law of thermodynamics, here, but because almost all of our intuitions about how liquids are going to behave are formed under conditions of turbulent flow, it sure does seem like it. [Thanks, Alan Dove!]


14 thoughts on “Jaw-dropping “unmixing” demo appears to reverse entropy

  1. Anthony says:

    …however I don’t laminar flow has anything to do with it. I’m no physicist but I recall second law of thermo states the entropy of a closed system does not increase on it’s own. Once the liquids are mixed, you are adding energy into the system by reversing the crank and therefore it is not a closed system. Perhaps a real physicist can comment?

  2. NeuroPulse says:

    I liked it more when Kari Byron of Mythbusters did it :-)

  3. Pelrun says:

    I’m reminded of those photos of pedophiles who used Photoshop’s Twirl filter to hide their identities, thinking it was a one-way transform… only for the authorities to use the inverse and restore the original images.

  4. Sam Ley says:

    It is definitely a laminar flow issue – while you are right that it isn’t a closed system, in this case, thermodynamics isn’t what we are dealing with.

    Fundamentally the reason this works is that the colors never really “mixed”. We think of most “mixing” as a process by which you get some uniform distribution of stuff, and usually that distribution is handled by very chaotic processes, like stirring, heat, or diffusion. Undoing that kind of mixing is an apparent violation of thermodynamics.

    In this case, the final “mixed” state was really just one where the dye spots had been stretched out into concentric spirals. They only look mixed because of the light passing through them. Like his analogy, it is like spreading a deck of cards out on the table, then sliding them back into a pile again. Because the corn syrup is viscous, and because the flow is laminar (a very non entropic type of flow, where the liquid up against the container doesn’t move at all, the liquid in a bit moves a little, and the liquid at the center moves the fastest), this isn’t a chaotic mixing process, just a mechanical one. The dye puddle has stretched out into a very long spiral, and then is just compressed again.

    Of course it isn’t perfectly laminar (nothing is) – if you gave it 20 or 30 cranks, or just left it to sit for a while after cranking (letting diffusion between the very thin “layers” of dye), then it would actually become mixed, and you wouldn’t be able to undo it.

    If you want another exploration of impossibly thin layers making something that appears homogenous, but then doesn’t behave the way you’d expect, read up on how puff pastry dough is made and used! Deliciously microscopic.

  5. Anthony says:

    @Sam: Ahh, I have not thought about it from that aspect before. I did notice that the spots did not “unmix” perfectly when reverse cranked and figured it wouldn’t work after repeated attempts. I’ll have to check out the pastry dough. Thanks for the input!

  6. theophrastus says:

    That’s a delightful demonstration – thank you!

    But here’s something, perhaps, worth contemplating: if the viscosity of his medium were less then he’d only be able to crank just a little and then back to something like he started with. That is, the viscosity is countering the entropy. If he had cranked 37 times it wouldn’t get back to anything like he started with it.

    It may even be the case that his viscosing agent sets up long aligned microchannels that really protect the laminar flow

  7. matt adkins says:

    This one’s been around for ages…Physicist David Bohm often used this experiment as a metaphor to explain the concepts of implicate & explicate order…always interesting, though…

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I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and makezine.com. My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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