A little friction also stops
Smooth soles are slippery, cleated profiles ensure a firm grip. In our world of the big, we have a choice between sled and hook. But at the atomic level of nanotechnology, every surface is rough and granular. Without friction we are missing something. In a world full of ideal black ice, we could not sit on a chair, stand on our own two feet and certainly not determine in which direction we are going. Like novices on in-line skates or ice skates, we would be at the mercy of the subtle play of forces. How good that there is friction. But it has to be the right amount. Surrounded by rough emery paper, we couldn't open a drawer if every door were locked and the abrasion would wear through the bottom of our pants in no time at all. It's practical that we live in a reality of he althy mediocrity.
The little things are not doing so well. On a scale of billionths of a meter, the inside loses importance in favor of the surface. Almost every atom and molecule here is in contact with the outside world and has to see how it gets along with its neighbors. Where there is hardly any mass, it is not a question of the massive swing whether you get stuck on a plane, but a fact of how much the ground sticks to your feet. Friction is one of the main issues here. That wouldn't be so bad for us macroscopists if we didn't advance into this frictional microcosm with our much-praised, long-awaited nanotechnology. And there they are, the worries about liability: nano-components stick to each other where they should move and rub against each other when things should actually be running smoothly. If nanotechnology wants to move something in the literal sense, it will have to get the friction under control. But this is exactly where the problem lies.
Two teams of scientists have now developed methods in the laboratory for mechanically or electrically making the bumpy nanoscale a little smoother. The group headed by Anisoara Socoliuc from the University of Basel chose an atomic force microscope as a model, and dragged its atom-thin tip over a smooth surface of common s alt [1]. However, this ground was only really smooth from our normal view - for the tip of the microscope it was more of a hilly plain. During the rough gliding, it initially got stuck in every indentation between the sodium and chlorine atoms. The bracket stretched like an arc until finally there was so much tension that it yanked the tip against resistance-into the next trough, where the game began again.
The researchers countered this strong friction with a method that each of us has already tried out intuitively whenever something got stuck in its shell - shaking it properly! Although it was of course more elegant in the laboratory than at the desk, in the workshop or on the campsite, the oscillator on the tip holder of the force microscope served the same purpose as a manual shaker of the packaging: the tip was moved up and down for a short time and emerged sufficiently far from the atomic valley that it managed to jump over the obstacle with significantly lower tension forces. It glided over the surface correspondingly more smoothly.
Jeong Park from the Lawrence Berkeley National Laboratory and his colleagues also achieved comparable success [2]. However, they focused more on the situation in semiconductors, whose electrical properties are particularly easy to manipulate. The tip of their atomic force microscope therefore had to make its way over a silicon sample that consisted of alternating regions of electron excess and deficiency. Surprisingly, there were differences in the friction between these areas, especially when a voltage was applied between the tip and the sample at the same time - then the areas with a lack of electrons exerted twice as much frictional resistance as those with an excess of electrons. A very interesting result for which the scientists have not yet found a conclusive explanation.
So there is something for everyone in these experiments: Shaking and shaking reduces friction, while electrical voltage poses new puzzles. That's what it's like when you leave your usual macroscopic world. But what don't you do to be surrounded by nano-tiny helpers one day.
