listen to part of a lecture in a physics class.

Listen to part of a lecture in a physics class.

All right, let's start. Last time we started talking about random motion, and I said, I mentioned that people have tried to harness this motion to do useful work, and well, there are problems with that idea. Uh, so today we'll look at one experiment that approach things a little differently. Hopefully you remember from, uh, from last week, the idea of Brownian motion. Just a review-- Brownie motion refers to the random movement of microscopic particles when they are suspended in a fluid. Remember those tiny bits of pollen and dust suspended in water that we looked at last week? Those particles were so small that collisions with the surrounding molecules caused them to move. And I didn't mention this last week, but this type of motion has inspired physicists to think of ways to turn this random motion into useful motion in a fixed direction. And way that they hypothesized this could be done was with a theoretical machine called a Brownian ratchet.

The way this theoretical model of this Brownie ratchet works is, well, there'd be a wheel attached to a gear. Now, this gear is special, and it can turn in only one direction. It has a break mechanism that stops it from turning back the other way. So Brownian motion will nudge the wheel at random as the microscopic particles bump up against it, but because of the break, the wheel will only turn in one direction, and that's how the random movement of particles could be controlled, harnessed and made useful theoretically.

Unfortunately, we’ve long known that the Brownie ratchet wouldn't work the way we wanted to. I won't get into the full explanation, but remember that the same random motion of particles that moves the wheel can also move the break, so the break mechanism gets nudged around, causing it to fail, letting the gear slip back in the wrong direction. But enough, the machine is useless. To take a bigger picture view, there are actually theoretical reasons why such a model is unlikely to work, but you should have all anticipated that, right? We've studied the very basic law of physics that states that if a system is in perfect internal equilibrium, and movement of all particles in the system is completely random. It's impossible to extract useful work, a directed motion, for example, from the system. And that's what the Brownian ratchet tries to do.

Recently, though, researchers have begun experimenting with a living source of motion that they think might be useful. I'm talking tiny swimming bacteria. Now, while the motion of these bacteria appears random, it's different from the motion of particles from Brownian motion in a couple of ways. First, instead of bouncing back when they hit an obstacle, the bacteria keep swimming in the same direction. They also move together in swarms, almost like schools of fish, so that movement carries more momentum. So in this experiment, scientists suspended 90 gears, about the size of a grain of sand, in a thin liquid film filled with these bacteria. The shape of these gears is important. The teeth on the gear edges, the lengths of the sides of the teeth were not equal. They were different, with one side more angled than the other. So even though the bacteria were moving in all directions, the ones pushing in the desired direction got trapped against the corners where those small double arrows are, and so pushed on the gear for extended periods of time. In the other direction, shown by the small single arrow, they quickly slid off so the gears wouldn't turn that way. The researchers could control the movement of the bacteria by changing the amount of oxygen in the liquid. With less oxygen, the bacteria slowed down, and with more they swam at full speed. And this really worked. From this largely directionless bacterial motion, the researchers were able to get the gears to spin in one direction.

Pretty elegant and exciting experiment, I know. But there are a lot of limit. First, the bacteria aren’t particles. They are organisms, they need nutrients, they produce waste, and they'll eventually die. Also, there's no way to guarantee consistent motion in the right direction. There’s nothing to keep the bacteria from swimming away from the gear. And finally, the gears in this experiment were free spinning. To extract work, the gears would need to propel some actual device, and we are not exactly there yet.