A carbon nanotube is an extraordinarily tiny pipe whose walls are as thick as an atom. The reason these pipes are so small is also due to the solid graphene, a honeycomb-like single-atom thickness of carbon atom sheets.
But for more than a decade, scientists have been puzzled by the mysterious way water flows through tiny channels of carbon nanotubes. The flow of water in such miniature pipes does not seem to match all theories of fluid mechanics.
Paradoxically, for example, water seems to flow more easily through narrower, multilayer carbon nanotubes, and of all carbon nanotubes, the friction that exists during the movement of water is equally difficult to explain.
Recently, in a new theoretical study, scientists have innovatively tried the "mix-and-match" of fluid mechanics and quantum mechanics, and finally found the answer to this puzzle, that is, quantum friction. The proposed explanation in the study suggests for the first time the existence of quantum effects at the boundary between solids and liquids.
Water flow in carbon nanotubes
Since 2005, scientists have measured the speed and characteristics of water's movement in carbon nanotubes. Since these pipes are so small, the water flow velocity is only one billionth of a liter per second.
However, it is generally believed that the resistance of the liquid to flow in the carbon nanotubes is very small, because the pipe walls of the graphene are completely smooth, which reduces the resistance to the water molecules passing through. Graphene also doesn't "grab" molecules on the surface like many other materials, and those that are captured also slow down the fluid.
However, more experiments have led to confusing results. For example, a 2016 experiment showed that friction in multilayer carbon nanotubes depends on the radius of the pipe, and the wider the nanotubes, the friction effect will increase.
According to the previous theory, this does not make sense at all, because no matter how big the pipe is, the pipe wall should be equally smooth. These strange phenomena have led to many debates in this field, scientists have developed many complex friction models to try to give reasonable explanations, and it has become one of the key problems in nanoscale flow research.
The combination of fluid mechanics and quantum mechanics
Due to the failure of existing hydrodynamic theories, in this new study, the team took a closer look at the properties of graphene walls.
Such an approach is actually quite unusual for studying fluids. Because in "general" fluid mechanics, the pipe wall is just a "wall", and many times there is no need to care about what it is made of. But the team has realized that at the nanoscale, this has become very important.
A key factor is that some of the electrons in graphene can move freely through the material. In addition, these electrons can interact electromagnetically with water molecules. This is because each water molecule has a slightly positively charged end and a slightly negatively charged end, in which the oxygen atom pulls on the electron cloud somewhat more than the hydrogen atom.
In the researchers' explanation, electrons in the graphene wall move with the water molecules that flow through them. But electrons tend to lag slightly, slowing down the speed of the molecules. This effect is called electron or quantum friction. Originally, it was thought to occur only in the interaction between two solids or a single particle and a solid.
Schematic diagram of the water flow mechanism in carbon nanotubes. | Image credit: Nikita Kavokine
However, when it comes to liquids, the situation becomes more complicated because in liquids, many molecules interact together. Electrons and water molecules jitter due to their thermal energy. If they happen to jitter at the same frequency, a resonance effect occurs, increasing quantum friction.
This resonance effect is greatest in having neatly arranged multilayer carbon nanotubes, because the movement of electrons between layers is synchronized with the movement of water molecules.
For narrower nanotubes, geometric constraints lead to misalignment between layers. This atom-level dislocation hinders the movement of electrons, reducing friction and allowing water to flow faster through narrower pipes. This explains the phenomena observed before.
From theory to experiment
The researchers believe that this newly discovered interaction between liquids and solids has not been noticed until now for two main reasons. First of all, the friction caused by this effect is very slight, and it is almost negligible for materials with rough surfaces. Second, the basis of this effect is that it takes some time for electrons to adapt to moving water molecules. But molecular simulations were unable to detect this friction because they used the Born-Oppenheimer approximation in the simulations, which assumed that electrons would immediately adapt to the motion of nearby atoms.
For now, the new study is still theoretical, so researchers still need experiments to confirm their ideas and explore more counterintuitive findings. They also argue that the results also tell us about the need to improve existing simulations.
This theoretical finding could have implications for potential applications of carbon nanotubes, such as filtering salt from seawater or using the difference in salinity between brine and freshwater to generate energy.
#创作团队:
Compile: M ka
Typography: Wenwen
#参考来源:
https://www.simonsfoundation.org/2022/02/02/quantum-friction-slows-water-flow-through-carbon-nanotubes-resolving-long-standing-fluid-dynamics-mystery/
https://www.chemistryworld.com/news/new-phenomenon-quantum-friction-explains-waters-bizarre-properties/4015163.article
#图片来源:
Cover image: Pixabay
Photo: Maggie Chiang/Simons Foundation