Bacteria-powered micromachines

Discuss

19 Responses to “Bacteria-powered micromachines”

  1. atteSmythe says:

    I like how the video looks like an old-timey cartoon. I half expected Steamboat Willie to show up!

  2. misterfricative says:

    This is very frustrating because the press releases seem to have hit the streets before the actual PNAS paper has become available, so there’s no way to really know what’s going on.

    However, two things. First I think the ratchet-like profiles of the gears is irrelevant. I suspect that if you flipped these wheels over, you’d see exactly the same thing.

    Second, I think what’s actually being demonstrated here is a biological control system. What makes the wheels turn is “a nonequilibrium source of random velocity kicks”, and the clever part is that instead of this being driven physically (eg by a heat differential, or by a beam of photons or whatever) it’s being driven by a biological ‘activity differential’. Which is to say that the bacteria to the left of the wheels are more active than the bacteria on the right. And this activity is in turn being controlled by the oxygen concentration.

    If there’s anything more to it than this, then I shall be [pleasantly] surprised.

    • Anonymous says:

      It looks like the ratchet-like shape is what keeps the forces from canceling. Bacteria hitting the radial side are pushing in a direction that will make the wheel turn. But bacteria hitting the sloped sides are partly pushing directly inwards, and so are creating less torque.

  3. Anonymous says:

    The note says the bacteria are working together to turn the gears. Hmmm. Is that right? Are these trained bacteria? I guess I have to read the damn thing again. Wish I had bacteria to do the reading for me.

  4. Anonymous says:

    Addendum to the last post about the ratchet shape:

    Actually, I think I just explained that very badly, because what I said sounds like this would happen using Brownian motion in a frictionless setting. It wouldn’t. The more important point is streamlining, so that it’s easier to turn one way than the other.

    • misterfricative says:

      I agree that it looks that way, but I think that’s not in fact the case. If it were in fact easier for the wheel to turn one way and not the other, then you’d essentially have a Brownian ratchet — which is impossible because it would break the second law.

      The device in the OP works (I think) because it’s a Brownian motor. In this case I infer that the motor is powered by a gradient of active bacteria on the left and less active bacteria on the right. I could be wrong, but as far as I can see, it would actually make no difference if you reversed the gradient: the net rotational force would be the same, the resistive forces would also be the same — and the wheels would spin the other way at the same rotational velocity.

      I would hope that the [irrelevant?] asymmetry of the wheels is addressed in the paper itself; however, it’s still not showing up in PNAS searches.

      • Anonymous says:

        I’m afraid I’m a little confused by what you’re saying. Your link for Brownian motor says an important part is that moving one way is very difficult. This is where I think stream-lining matters: there would be more resistance to rotating the opposite direction. That wouldn’t happen with a mirror-symmetric gear.

        It also makes it sound like you can rely on transient gradients generated by chance motion, since they’ll all turn the gear the same direction. This is what the video looks like to me – clouds of bacteria but with no consistent gradient, which I’d guess would be hard to maintain on this scale.

        Given those two things, that’s a lot like what I was envisioning. I don’t think it’s the same as the ratchet. According to your link, the important thing there is that the ratchet is in thermal equilibrium with the medium. But here, back-sliding is prevented by the fluid and forwards motion is caused by the bacteria, which are swimming so should have a different “temperature”.

        But I might have misunderstood the language, since this is the first time I’ve heard these names. Thanks for pointing them out to me!

        • misterfricative says:

          I should first make clear that I’m certainly no expert, and I’m struggling to understand this as much as you are.

          Also, in the Brownian motor link, I’m not at all sure what they mean by that ‘hurricane’ sentence. The Science paper (reference 1 on the Wikipedia page) has a much clearer explanation of what happens at this scale (see below).

          Having said that, I also think I was wrong in what I said before: assuming that the gradient would produce something approximating a linear flow, then the shape of the vanes would matter. It would, as you said, be more efficient in the direction shown in the video; reversing the gradient — in effect reversing the flow so that it hits the angled slope — would result in less net torque being transferred to the wheels. (Although I still think they’d rotate; just not as fast)

          But I’m still not sure if any of this means that it’s intrinsically easier for the wheels to rotate in one direction and not the other. Consider this: if one of these wheels were in an environment with random thermal noise, the net rotation would be zero, right? But it would jiggle clockwise and anticlockwise a bit because of random thermal noise. If the wheel could rotate more easily in one direction, then after a while it would end up spinning in that direction. And since that doesn’t happen, I have to conclude that it must spin equally easily in either direction. (There may well be errors in my reasoning, in which case please advise!)

          And yes, the video looks exactly like the clouds of bacteria are in random motion: there’s no visible evidence of a gradient or a flow except for the fact that the wheels actually turn. But if there’s really no gradient then I can’t see how this works at all. What are we even looking at here? Is the video shot in real time? Are those smudges actually the coherent clouds of bacteria or just some kind of artifact? Where’s the source of the oxygen? Damn it, I need to see the paper! These premature press releases and Youtube videos(!!) are very frustrating.

          Meanwhile, in case you can’t access the Science paper, here are three pretty good paragraphs from the introduction: (with apologies for the copypasta)

          A small particle in a liquid is subject to random collisions with solvent molecules. The resulting erratic movement, or Brownian motion, has been described theoretically by Einstein (1) and independently by Langevin (2). Langevin hypothesized that the forces on the particle due to the solvent can be split into two components: (i) a fluctuating force that changes direction and magnitude frequently compared to any other time scale of the system and averages to zero over time, and (ii) a viscous drag force that always slows the motions induced by the fluctuation term. These two forces are not independent: The amplitude of the fluctuating force is governed by the viscosity of the solution and by temperature, so the fluctuation is often termed thermal noise.

          At equilibrium, the effect of thermal noise is symmetric, even in an anisotropic medium. The second law of thermodynamics requires this: Structural features alone, no matter how cleverly designed, cannot bias Brownian motion (3, 4). To illustrate this point, Feynman discussed the possibility of using thermal noise in conjunction with anisotropy to drive a motor in the context of a “ratchet and pawl” device shrunk to microscopic size (4). He showed that when all components of such a device are treated consistently, net motion is not achieved in an isothermal system, despite the anisotropy of the ratchet’s teeth. However, a thermal gradient in synergy with Brownian motion can cause directed motion of a ratchet and can be used to do work. As a practical matter, large thermal gradients are essentially impossible to maintain over small distances. Particularly in biology and chemistry, the thermal gradients necessary to drive significant motion are not realistic.

          It might seem then that, despite its pervasive nature, Brownian motion cannot be used to any advantage in separating or moving particles, either in natural systems (such as biological ion pumps and biomolecular motors) or by artificial devices. Recent work has focused, however, on the possibility of an energy source other than a thermal gradient to power a microscopic motor. If energy is supplied by external fluctuations (5-8) or a nonequilibrium chemical reaction (9, 10), Brownian motion can be biased if the medium is anisotropic, even in an isothermal system. Thus, directed motion is possible without gravitational force, macroscopic electric fields, or long-range spatial gradients of chemicals.

          • chenille says:

            Consider this: if one of these wheels were in an environment with random thermal noise, the net rotation would be zero, right? But it would jiggle clockwise and anticlockwise a bit because of random thermal noise. If the wheel could rotate more easily in one direction, then after a while it would end up spinning in that direction. And since that doesn’t happen, I have to conclude that it must spin equally easily in either direction. (There may well be errors in my reasoning, in which case please advise!)

            Well it certainly doesn’t happen, but I think it’s important to consider why. The resistance to turning is going to come from particles colliding with the gear, and so its energy is going to be transferred to the medium as heat. If that’s what was powering it in the first place, it makes sense they could cancel, even if the gear would exert more resistance one way against being turned manually.

            The case with a ratchet is a little more complicated, but I think since it’s in thermal equilibrium it ends up the same, with the medium taking the heat. Since there’s no difference in temperature, there’s no possibility of work. That’s why you need some external energy gradient to run it.

            The gear in the video is different, because the turning is coming from bacteria and the braking from the fluid. These aren’t in equilibrium because the bacteria are swimming, so there’s no reason the forces have to balance. Instead, it’s more like blindfolded people wandering into a revolving door. The fluid can act the ratchet because it’s not the source of Brownian motion.

            That’s my guess, anyways, as to why it’s possible for the shape to matter. That the asymmetry is what determines the motion I can now tell you for certain: the paper has just come online.

            - Same as anon 12-16; I figured so long as we’re talking, an account might not hurt.

          • misterfricative says:

            Thanks for the link! I just read the paper — and you’re absolutely right. The asymmetry of the gear teeth is indeed essential.

            So color me pleasantly surprised! I still don’t see how this doesn’t break the 2nd law (or conversely, why it wouldn’t work at an even smaller scale if the bacteria weren’t even present). I think your revolving door analogy is very apropos, but I still can’t quite get my head around it. I guess I have some reading to do…

  5. SamSam says:

    So is this extracting energy out of brownian motion? (I know it’s not extracting anything yet, but it could in theory turn a generator).

    Could such a thing work at molecular scales, using the brownian motion in gasses?

  6. Pantograph says:

    So why is this not a perpetual motion machine? My intuition is that it would not work with inanimate particles in Brownian motion – the forces in different directions should cancel out. Are those bacteria doing something more complex than bumping randomly onto the surfaces of the gears?

    Either that or Maxwell’s demon has been busy lately.

    • Anonymous says:

      So why is this not a perpetual motion machine?

      The bacteria are swimming, and so using energy from their environment. This is essentially a chemical motor with a biological component.

      It’s not impossible to take advantage of random motion. As an easy example, if a revolving door can only turn one way, random blindfolded people running into it would make it do so. But it’s not perpetual.

  7. charlesj says:

    So why is this not a perpetual motion machine? My intuition is that it would not work with inanimate particles in Brownian motion – the forces in different directions should cancel out. Are those bacteria doing something more complex than bumping randomly onto the surfaces of the gears?

    • SamSam says:

      I think it’s just the shape of the gears. If they hit the tooth on the wrong side, it won’t really do much. They’d either be pushing towards the center of the wheel, or they’d kind of slide off.

      I think it could work with brownian motion, but the gears would have to be much smaller. The tooth shape probably wouldn’t do anything at that large scale.

      As for perpetual motion, well in the case of the bacteria it’s clear that they’re getting their own energy from somewhere — the sun eventually. But if it were the random motion of atoms? Obviously it would still be getting energy from the sun’s heat, but given that it would theoretically be able to keep going until all the energy in the universe was spent, I’d say it could be considered a perpetual motion machine.

    • Anonymous says:

      A perpetual motion machine would be impossible via brownian motion because the atoms need heat energy to power the wheels, energy that would be diminished by all the collisions

  8. dculberson says:

    So… is that real time video or has it been sped up?

  9. hillbillygeek says:

    It’s bacterial slavery, I tell you!! Someone call the SEIU!
    Turn the volume up all the way and you can hear a little voice shouting ‘Stroke!’ ‘Stroke!’ ‘Stroke!’

  10. lysdexia says:

    The final surviving bacterium will for a time be chained to the Petri Dish of Woe, but will eventually infect and destroy Thulsa Doom and, in the fullness of time, rule Aquilonia.

Leave a Reply