Another example of friction at very small scales
R. David Whitby | TLT Worldwide July 2022
Researchers look into quantum friction to explain why carbon nanotubes don’t follow a certain rule.
It has been noticed during the last few years that the flow of water through narrow carbon nanotubes is faster than through wider ones. Until now, no one has been able to explain why.
The standard understanding of friction is that when two things move past each other, a greater degree of contact between them means that more force is needed to maintain the movement. Therefore, with narrower tubes, the flow of a liquid through the tube should require more force to overcome the friction. Carbon nanotubes do not seem to follow this rule when it comes to water flowing through them.
A recent paper in the journal Nature
1 by Nikita Kavokine and colleagues seeks to explain that the phenomenon is the result of quantum friction.
Physicists have been fascinated for several years by the possibility that two materials moving past each other might experience a lateral frictional force that arises from quantum fluctuations in a vacuum. A number of groups have calculated that quantum friction could arise from the Casimir force between the plates. This force was first postulated by Dutch physicist Hendrik Casimir for electromagnetic systems in 1948. In quantum field theory, the Casimir effect is a physical force acting on the macroscopic boundaries of a confined space, which arises from the quantum fluctuations of the field. However, calculations done by other physicists suggest that the Casimir effect does not exist.
Carbon nanotubes are made of thin layers of graphite rolled into tubes just a few nanometers in diameter. It also has been found that water flows faster through nanotubes that have thinner walls. Kavokine and his colleagues contend that, because water molecules at room temperature move in different directions, their uneven distribution means they are electrically charged on a microscopic level. Consequently, freely moving electrons in the carbon nanotubes can interact with these microscopic fluctuations in the water to create quantum friction.
Flat sheets of graphite and graphene (both of which are forms of solid carbon) typically comprise hundreds of layers that have a precise alignment. However, according to Kavokine and his colleagues, curving the layers distorts this alignment, hindering the ability of electrons in the nanotubes to move freely. Narrower carbon nanotubes have more highly curved graphite layers, so fewer electrons are able to move freely and interact with the water molecules, and the resulting quantum friction is lower. For nanotubes with thicker walls, there are a greater number of electrons that can move between layers to push and pull on the water molecules, which is why these nanotubes cause more quantum friction than thinner-walled tubes.
The researchers have demonstrated a marked difference in quantum friction between water–graphene and water–graphite interfaces, due to the coupling of water Debye collective modes with a thermally excited plasmon specific to graphite.
1 This suggests an explanation for the radius-dependent slippage of water in carbon nanotubes, in terms of the electronic excitations of the nanotubes.
1 They believe their findings could be used for quantum engineering of hydrodynamic flows. They postulate that quantum friction provides the first reasonable explanation of what is happening with water flowing through carbon nanotubes.
If validated, the research results might enable material chemists and engineers to optimize flows of liquids through tiny channels. This could prove very useful in processes such as water purification by reverse osmosis, which uses a partially permeable membrane to separate unwanted materials from the water. An applied pressure is used to overcome osmotic pressure in the membrane, to force the water molecules through while retaining the larger types of dissolved and suspended chemicals and biological species in the water.
Current reverse osmosis systems need to use considerable amounts of energy to apply sufficient pressure to force water molecules through the membranes. It would be very advantageous if the membranes could be made of materials that used quantum friction to reduce the amount of energy required.
REFERENCE
1.
Kavokine, N., et al. (2022), “Fluctuation-induced quantum friction in nanoscale water flows,”
Nature, 602, pp. 84-90.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.