Physicochemistry: Shut the hatch behind the demon

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Physicochemistry: Shut the hatch behind the demon
Physicochemistry: Shut the hatch behind the demon

Shut up behind the demon

How do salespeople get the store empty just before the store closes? Exactly: You only let customers out, but no new ones in. Molecules work the same way if you know where they are. Although the laws of thermodynamics should keep such demons out of practical life. In fact, thermodynamics is the most democratic of all scientific disciplines. After all, it rarely reveals what constraints and temptations the many particles of a system were exposed to, and only presents the researcher with the result of the choice. Statistics that are somehow valid, but strictly speaking do not reveal any laws of nature. And yet it betrays us some iron rules that nature apparently follows. For example, the second law of thermodynamics states that no order is created by itself. A quick look at the desk or in the children's room will confirm this.

But the Scottish physicist and thermodynamicist James Clerk Maxwell didn't want to trust statistics that easily. In a thought experiment, he challenged the second law with a demon that creates order where there should be chaos. This Maxwellian demon resides in a typically thermodynamically unadorned system consisting of a two-chamber gas-filled box. Between the chambers there is a little door where the demon is up to mischief. If a particle from the left half happens to fly towards the door, it quickly opens and lets it through. On the other hand, if a particle from the right wants to change sides, the flap remains closed. In no time at all, the demon collects all the particles in the right chamber and empties the left - order that runs counter to thermodynamic perception.

On closer inspection, Maxwell's demon is, of course, a toothless house ghoul that can't harm any thermodynamic subordinate clauses. Because its order requires a proper effort: The demon needs the information when a particle comes from where and has to react accordingly. Consequently, nothing arranges itself here by itself, but the second law only applies to systems without external constraints – so-called closed systems.

Nevertheless, with information and a bit of energy, the demon manages to bring a system far from equilibrium. And that's something that makes machines work. Reason enough for David Leigh from the University of Edinburgh and his research group to create a humble demon to prove its mischief on a chemical toy. The molecule, called rotaxane, consists of a large ring running over a long axis. At its left and right ends there are two equally strong holding points for the ring. On the extended axis, the ring randomly wanders back and forth between these points and is statistically and therefore thermodynamically seen equally often at each of these points.

But the chemists, meanwhile, built a joint into the axle, which plays the role of the door in the original demon image. And not well-behaved in the middle, but much further on the left end. If the joint is kinked, it will block the ring from moving. So far nothing has changed in terms of balance, because the movements are extremely fast and 50 percent of the rings are still on the left and 50 percent on the right. The demon only strikes when the kink gets a switch and there are two finger molecules that can actuate it. One of these fingers swims in the solution and switches the joint to "impermeable kink", the second finger sits firmly on the ring and can in principle set the joint to "permeable straight". In principle, because he has to press for a relatively long time, and he only has this patience when the ring is anchored on a breakpoint.

Controlled by light, the game is set in motion - and lo and behold: the rings neatly accumulate on the right sides of their axes. If they are on the left stop point, their finger reaches to the switch, they press the joint straight and whiz to the right. Seen from the stop on the right, however, the distance to the counter is too great. While the finger in the solution can easily bend the axis, the ring cannot remove the obstacle - and is stuck on the right.

A victory for the demon, bought again by the information where the particle is. In contrast to other experiments, in which the rings were also driven from one side to the other, the holding points in the Edinburgh experiment remain the same. This could be an important advantage for future molecular machines, because such a system can be arbitrarily removed from the equilibrium state. Even adverse energy gradients could be overcome in this way - an ability that makes biological machines so effective. With Leigh's Rotaxane, the natural demon may be slowly making its way into our artificial nanorobots.

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