Experiments reveal that water can ‘talk’ to the electrons in graphene
(Nanowerk News) For the last 20 years, scientists have been puzzled by how water behaves near the surface of carbon. It may flow faster than expected from conventional flow theory or form odd arrangements such as square ice.
Now, an international team of researchers from the Max Plank Institute for Polymer Research of Mainz (Germany), the Catalan Institute of Nanoscience and Nanotechnology (ICN2, Spain), and the University of Manchester (UK), report in a study published in Natural Nanotechnology (“The cooling of electrons in graphene is enhanced by plasmon-hydron resonance”) that water can interact directly with carbon electrons: a very unusual quantum phenomenon in fluid dynamics.
Liquids, such as water, are made up of small molecules that move randomly and constantly collide with one another. Solids, on the other hand, are made of neatly arranged atoms bathed in a cloud of electrons. The solid and liquid worlds are assumed to interact only through collisions of the liquid molecules with solid atoms: the liquid molecules do not “see” the solid’s electrons.
However, just over a year ago, a paradigm-shifting theoretical study proposed that at the water-carbon interface, the liquid molecules and the solid’s electrons push and pull on each other, slowing the flow of the liquid: this new effect is called quantum friction. . However, the theoretical proposal lacks experimental verification.
“We have now used lasers to see quantum friction at work,” explained study lead author Dr Nikita Kavokine, a researcher at the Max Planck Institute in Mainz and the Flatiron Institute in New York. The team studied samples of graphene – a single layer of carbon atoms arranged in a honeycomb pattern. They used ultra-short red laser pulses (with a duration of just a millionth of a billionth of a second) to instantly heat up graphene’s electron cloud. They then monitored its cooling with terahertz laser pulses, which are sensitive to the temperature of graphene’s electrons. This technique is called optical pump spectroscopy – terahertz probe (OPTP).
To their surprise, the electron cloud cooled faster when graphene was immersed in water, while immersing graphene in ethanol made no difference to the cooling rate. “This is another indication that the water-carbon pair is special, but we still have to understand what’s really going on,” said Kavokine. A possible explanation is that the hot electrons push and pull on the water molecules to release some of their heat: in other words, they cool down through quantum friction. The researchers studied the theory, and did: the air-graphene quantum friction could explain the experimental data.
“It is very interesting to see that graphene’s carrier dynamics continue to surprise us with unexpected mechanisms, this time involving solid-liquid interactions with molecules of none other than the ubiquitous water,” commented Prof. Klaas-Jan Tielrooij from ICN2 (Spain) and TU Eindhoven (Netherlands). What makes water special here is that its vibrations, called hydrons, are synchronous with the vibrations of graphene electrons, called plasmons, so that the graphene-water heat transfer is enhanced through an effect known as resonance.
The experiments thus confirmed the basic mechanism of solid-liquid quantum friction. This will have implications for filtration and desalination processes, where quantum friction can be used to adjust the permeability of nanoporous membranes. “Our findings are not only of interest to physicists, but also have potential implications for electrocatalysis and photocatalysis at solid-liquid interfaces,” said Xiaoqing Yu, PhD student at the Max Planck Institute in Mainz and first author of the work.
The discovery came down to bringing together experimental systems, measurement tools, and theoretical frameworks that rarely go hand in hand. The main challenge now is gaining control over the water-electron interaction. “Our dream is to enable and disable quantum friction on demand,” says Kavokine. “In this way, we can design more intelligent water filtration processes, or maybe even liquid-based computers.”
