Water Interacts with Graphene’s Electrons, Scientists Reveal

Scientists Confirm Theory of Quantum Friction in Water-Graphene Interface

In a groundbreaking discovery, researchers have demonstrated the ability of water to communicate with electrons in graphene, shedding light on an extraordinary phenomenon known as quantum friction. By employing advanced ultrafast techniques, the team provided experimental evidence that supports earlier theoretical predictions regarding the interaction between liquid water and graphene—a single layer of carbon atoms.

The implications of this research are far-reaching, potentially revolutionizing water purification, desalination processes, and even liquid-based computing systems.

Over the past two decades, scientists have been perplexed by the behavior of water in proximity to carbon surfaces. Water flow near these surfaces was found to exceed expectations based on conventional flow theories, and peculiar arrangements like square ice formations were observed.

Now, an international team comprising researchers from the Max Planck Institute for Polymer Research (Germany), the Catalan Institute of Nanoscience and Nanotechnology (ICN2, Spain), and the University of Manchester (England) has published a study in Nature Nanotechnology on June 22, 2023, revealing that water can directly interact with the electrons in carbon, a remarkable quantum phenomenon that defies traditional fluid dynamics.

While a liquid consists of randomly moving molecules, a solid is composed of orderly arranged atoms enveloped in an electron cloud. Generally, the interaction between the liquid and solid realms occurs solely through collisions between the liquid molecules and the solid’s atoms, with the liquid molecules unable to perceive the solid’s electrons.

However, a paradigm-shifting theoretical study proposed just over a year ago that at the water-carbon interface, the molecules of the liquid and the electrons of the solid exert forces on each other, resulting in a slowdown of the liquid flow—an effect coined quantum friction. Until now, this theoretical proposition lacked experimental validation.

Lead author Dr. Nikita Kavokine, a researcher at the Max Planck Institute in Mainz and the Flatiron Institute in New York, explains, “We have now observed quantum friction in action using lasers.”

The team focused on graphene—a single layer of carbon atoms arranged in a honeycomb pattern—and employed ultrashort red laser pulses, lasting merely a fraction of a billionth of a second, to instantaneously heat the electron cloud of the graphene. The subsequent cooling of the electrons was monitored using terahertz laser pulses, which are sensitive to the temperature of the graphene electrons. This method, known as optical pump-terahertz probe (OPTP) spectroscopy, allowed the team to make a surprising discovery: the electron cloud cooled more rapidly when the graphene was immersed in water, while the presence of ethanol made no difference to the cooling rate.

“This finding suggested that the water-carbon interaction possesses unique characteristics, although we still needed to unravel the underlying mechanisms,” Kavokine remarks.

One possible explanation proposed was that the hot electrons influence the water molecules, causing them to release some of their heat, resulting in cooling through quantum friction. In-depth investigation of the theory confirmed that quantum friction between water and graphene could indeed account for the observed experimental data.

Professor Klaas-Jan Tielrooij from ICN2 (Spain) and TU Eindhoven (The Netherlands) comments, “It’s captivating to witness how the carrier dynamics of graphene consistently surprise us with unexpected mechanisms, this time involving solid-liquid interactions with the omnipresent water molecules.”

What sets water apart in this scenario is the synchronization between its vibrations, known as hydrons, and the vibrations of graphene’s electrons, referred to as plasmons. This synchronization enhances the transfer of heat between graphene and water, a phenomenon known as resonance.

By confirming the fundamental mechanism of solid-liquid quantum friction, these experiments will impact filtration and desalination processes,

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More about quantum friction

• Nature Nanotechnology: Electron cooling in graphene enhanced by plasmon-hydron resonance
• Max Planck Institute for Polymer Research: Homepage
• Catalan Institute of Nanoscience and Nanotechnology (ICN2): Homepage
• University of Manchester: Homepage