Molecules Stumbling Uphill
Molecular particles usually migrate in the direction of lower resistance. Unless they start to influence each other - then things get more complicated. Nevertheless, they can be controlled in a targeted manner.
Voluntarily spilled milk does not return to the bottle. In principle, it could, because all of its molecules are constantly in motion, driven by the ambient heat. In fact, some even manage to escape from the puddle association. By sheer luck, they get so much shoved support from their neighbors that they can defy gravity and squeeze into a momentary gap between the air molecules. In this way, the milk gradually evaporates. If she then chose the way back through the bottleneck in the air, the mishap would be undone. But the probability that trillions of trillions of particles will go in the right direction is small. Pretty low. Let's say practically impossible.
Moving coincidence alone is of no use if we want to create order. Mechanisms are needed that show the whole thing a direction. Who turn a changing back and forth into a resolute back and forth. And they actually exist – you might even have one in your tool box. As a "ratchet" or "ratchet", these marvels of hidden mechanics save us the hassle of reaching for the screwdriver: while we turn the handle or lever to the left and right, a spring and an asymmetrical gear wheel inside ensure that it goes in the wrong direction only rattles - because the spring jumps over the flat flanks of the teeth - and turns into the right one - then the spring has gotten stuck on a steep flank.
In addition to do-it-yourselfers, this ratchet principle is also of interest to scientists, whose nanoscale objects should also reach a specific goal without using any other motor than chance. Biologists, for example, for whom it is still largely a mystery how living cells juggle with the molecular over-diversity within themselves. Or physical chemists whose reactions depend on diffusing supplies. And engineers who think about stray vortices in superconductors. In short, random walks and their guidance have a meaning at the molecular level that is inverse to their spatial dimensions.
On the steering issue, scientists led by Victor Moshchalkov from the University of Leuven in the Netherlands have now achieved a partial success: they can determine which direction the flow is net by varying the number of particles. They took the first steps with pencil and computer. In a one-dimensional model, they designed an asymmetric ratcheting potential with two types of depressions - which roughly corresponds to a toothed rack whose depressions each contain two depressions, one of which is a little bit deeper than the other. Bit by bit, the researchers added particles to this landscape that repelled each other, just like charges of the same name do in reality, for example. A mathematical simulation of the molecular dynamics then showed amazing effects.
For a single particle, everything went as expected. Since the shallower of the two sub-troughs acted like a staircase, the particle escaped more easily from the depression on this side, which is why it migrated in that direction on average. But as soon as there were two particles in the cavity, the net direction reversed. Now both lower troughs were occupied. If the particle fell from the shallower to the deeper barrier - which was the energetically simplest transition - it pushed away the particle already there with its repulsive force, so that on average one of them jumped over the steep flank more easily than over the lower barrier. The simulation was the same for three, five and other odd numbers of particles: the bottom line is that the particles migrated against the "natural" direction that the ratchet potential specifies. For even numbers, however, they behaved as we would intuitively expect.
The researchers showed in an experiment with superconductors that their theoretical results also hold up in reality. To do this, they provided a sample with regular holes of two different sizes, which served as potential wells. The role of the particles was taken over by small vortices carrying a magnetic moment. The random movements were driven by an oscillating electric field. As predicted, a stress resulted whose sign depended on the number of vortices. Coincidence had suddenly become calculable.
Whether and to what extent natural systems make use of this control mechanism remains to be clarified. It is conceivable, for example, that ion channels in biological membranes modulate the amount of charged particles passing through in order to determine the direction of flow. The transport processes on the filamentous structures inside the cell, known as microtubules, may also benefit in a similar way from the controlled randomness. Only when it comes to spilled milk, no gun box, no matter how large, will continue to be able to replace the wiping cloth.