Very strange laws apply when water flows through a carbon nanotube. As a team led by Nikita Kavokine from the Sorbonne Université in Paris reports, a previously unknown quantum friction occurs in them, the effects of which are in radical contradiction to everyday experience. Substances outside the tube influence the friction inside. And instead of flowing more easily through a large opening than a smaller one, water behaves completely opposite with certain carbon nanotubes: the larger the opening, the greater the resistance. As the working group now writes in »Nature«, the cause is a quantum mechanical effect. Electron states in the carbon nanotubes resonate with collective oscillations of the water molecules, thereby slowing them down.
The strange behavior that has puzzled experts for years occurs in multi-walled carbon nanotubes, in which increasingly larger tubes are wrapped around one another like onion skins. Experiments over the past few years have shown that when water flows through a carbon nanotube, the friction is extremely low. Because the wall is very smooth and quite stiff, the molecules lose little energy through collisions. However, the same experiments had also shown that friction increases when the diameter of the tubes increases – a result that seems completely absurd at first glance. In fact, however, the extremely low classical friction is part of the explanation, because it means that weak quantum effects have noticeable effects.
To explain these effects, Kavokine’s team developed a quantum theory of the interface between solids and liquids. She describes the interactions between quantum states in the materials involved and explains, among other things, why the friction between water and graphite is higher than between water and a single graphene layer. As the working group writes, such effects come about because plasmons, vibrational states of freely moving electrons, in graphite have energies similar to the collective vibrational states of the neighboring water molecules. This is why they can couple with each other and exchange energy – which is noticeable as friction.