Quantum friction explains the amazing flow of water


Every time you are going to wash the dishes, how easily the water slides on the dirty plate depends on how uneven the surface of the plate is. However, at the nanoscale, where surface objects can be hundreds of thousands of times smaller than the average width of human hair, water can experience friction even on surfaces that seem perfectly smooth. Consider, for example, the mysterious case of carbon nanotubes: experiments have shown that, contrary to common sense, the narrower these miniature tubes, the less friction water “feels” in them, and the faster the final flow. This is the exact opposite of how plumbing works in our familiar macro-scale world.

Now, after decades of confusion, physicists from the Instituto Fleetyran in New York and the French National Center for Scientific Research (CNRS) in Paris have explained this amazing observation as a product of quantum mechanics. Their work published in v Nature in February shows that it is not so much the surface of the walls of nanotubes that is important as what moves the electrons inside it. This understanding can lead to improved applications supplemented by nanotubes such as water purification.

“20 years ago there was an experiment with water flowing through a membrane of carbon nanotubes, which was quite alarming. It showed extremely fast water flows, ”said Leader Boke, a physicist at CNRS and co-author of the new study. “We started an experimental program to measure the flow of water inside one nanotube and eventually got the same result. But we could not explain it. ” According to Boke, after all the commonly used theoretical methods failed, he turned to Nikita Kavokin, a collaborator and first author of the study, who is now a graduate student at the Fleetheir Institute, hoping that Kavokin could use often strange quantum rules. mechanics to explain the amazing experiment. “We had some idea that the ingredient was missing and we might need to include some quantum effects, but we had no idea how far we would have to go,” Kavokin recalls.

Ultimately, to find the long-sought answer, they had to scale beyond the nanoscale. Instead of simply considering how a water molecule rubs against a carbon atom, the team developed equations that describe the subatomic interactions of electrons in water molecules and carbon atoms. From this point of view, the wall of the carbon nanotube transforms from a smooth, static surface into a dynamic pool of electrons that can bend and pulsate when broken. This different perspective has allowed researchers to identify electrons as the main culprit of friction between a carbon nanotube and water. Water molecules have slightly different charges at each end, so when they flow along the walls of nanotubes, the electrons in those walls respond to a slight electric shock from the molecules, moving synchronously. Thus, friction still occurs even in a perfectly smooth nanotube when electrons dance in unison to the melody set by water molecules. Narrow carbon nanotubes simply have fewer carbon atoms and thus fewer electrons that cause water to pull. In wider nanotubes, where more electrons can join the choreography, the effect is enhanced and water flows less easily.

This electronic quantum friction has previously been studied for metal surfaces, but it has always been difficult to calculate it in detail, says Wenjie Dou, a physicist at Westlake University in China who was not involved in the study. “We are counting on it [form] electronic friction will always be very intense, ”he says, adding that many of the favorite mathematical techniques and approximations of physicists in this case fail. Marie-Laure Boke, a CNRS chemist and co-author of the study, notes that computational attacks on the problem are also not very helpful. “Modern modeling tools were not enough to explain the experiments,” she says. “It’s the most interesting thing about it: we had to go beyond the modern.”

Part of the difficulty is how many different quantum interactions need to be tracked to simulate the effect that arose; even the most powerful computer and the most intelligently programmed software cannot simulate how every water molecule interacts with any other water molecule, how every electron interacts with every other electron and how all water molecules interact with the whole community of all electrons, all at once, explains Christoph Schran, a chemist from Cambridge University who was also not involved in the study. To cope with these difficulties, the research team had to use complex mathematical methods that are not typical of fluid studies.

Although this is a breakthrough in theoretical physics, research has clear implications in the real world. For example, understanding fluxes through carbon nanotubes can improve water desalination processes, says Jeffrey Sokolov, a physicist at Northeastern University who was not involved in the study. “Water experiences strong friction when it passes through filters. So knowing you have these [carbon] nanotubes with very low friction, this can be a very good way to desalinate, ”he explains. He also wants to see more experiments exploring the basics of quantum friction detailed in this theoretical study. “We need to do more experiments, and maybe this work will encourage people to do them,” he said.

Shran agrees. “This new friction mechanism is definitely very interesting and exciting,” he says. “But what I think is missing is an accurate reference measurement.” For example, quantifying how changes in friction based on the interaction of water with one and more layers of carbon atoms can significantly support a new theory that predicts that more electrons in multilayer carbon will increase friction.

The research team is already on this path and dreaming of what’s next. They hope to eventually test their theory with fluids other than water, and nanotubes made up of elements other than carbon. In such cases, the molecules in the liquid and the electrons in the walls of the nanotubes will follow different models of interaction, which may lead to a change in the degree of quantum friction. Leader Bokeh says it is even possible to control the amount of friction experienced by a fluid by building nanotubes with precise attention to the behavior of electrons.

The new study lays the groundwork for years of comprehensive research by experimental physicists and theoretical physicists and, according to Kavokin, also shows a fundamental shift in how physicists should think about friction. “Physicists have long thought this is different on a nanoscale, but this difference was not so obvious to find and describe,” he says. “They dreamed of some quantum behavior that occurs on this scale, and now we’ve shown how it happens.”

Quantum friction explains the amazing flow of water

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