Scientists Just Discovered That Light Can Create Friction—No Contact Required

4 min readHere’s what you’ll learn when you read this story:Usually, friction requires two surfaces rubbing against each other. But researchers have now demonstrated that friction can also arise in the quantum realm without any physical contact at all.When scientists submerged graphene nanotubes in water and exposed them to light, they found that brighter, hotter illumination caused the tubes to slow down rather than speed up—the opposite of what thermal energy normally does to particles in a liquid. The culprit was quantum friction: fluctuating charges generated by light on the graphene surface interacted with surrounding water molecules, creating a drag force that increased with illumination.Nemesis of everything from tires to surfboards, but a hero when it comes to preventing skids or lighting matches, friction is the resistance that slows down movement when surfaces rub against each other. This is the force behind grippy shoe soles and “magic” infomercial cloths that polish to a mirror shine. Friction is made possible by physical contact—at least under normal circumstances. But what happens when you remove contact from the equation entirely?Our understanding of friction is being redefined by a weird quantum phenomenon. In 2022, researchers discovered that the positive charge of water molecules, which fluctuates as it flows through a carbon nanotube, interacts with the negatively charged electrons in the tube’s walls. The result was an infinitesimal amount of friction at the interface of water and carbon—arising not from surface roughness, but from electromagnetic interactions. Then last year, a research team at the Lanzhou Institute of Chemical Physics in China observed that folding ultrathin graphene sheets altered the structure of electrons by exciting them, causing friction at the folded edges to deviate from the usual linear pattern.Wanting to build on these findings, physical chemist Sebastian Kruss of Ruhr-University Bochum in Germany investigated what happened to graphene nanotubes when they were submerged in water and exposed to light of increasing brightness. The result defied expectations—but not in the way you might think. Higher levels of thermal energy usually cause particles to diffuse through a liquid faster, because diffusion is driven by kinetic energy: more heat means more molecular motion, happening sooner and at higher velocities. Yet when Kruss and his team tracked the nanotubes, they found the opposite. Instead of speeding up under brighter, hotter light, the tubes slowed down. Diffusion decreased because an unexpected source of friction was working against it.“Friction is expected to increase when the surface response function of the substrate overlaps with the low-frequency spectrum of the solvent…this is demonstrated by anomalies in hydrodynamic friction at water–carbon interface and by the rapid cooling of hot electrons in graphene in water,” he said in a study recently published in Nature.So what was creating this contactless drag? The answer lies in graphene’s extraordinary properties. Super-strong despite being incredibly thin, graphene has around 200 times the strength of steel but the thickness of only one atom, making it 100,000 times thinner than the typical human hair. Being a single sheet of carbon atoms arranged in a perfect lattice gives it an incredible smoothness, free of the bumps and imperfections found on most materials—even those that appear flawless to the naked eye. That smoothness rules out conventional friction from surface roughness. Instead, the source of the drag was electronic: it was as if the tubes, which glowed under illumination, were moving through something thicker than water.Here’s why. The light was turning electrons into excitons—pairs consisting of an electron that has been excited to a higher energy state and the positively charged “hole” left behind in its former position. The negative charge of the electron and the positive charge of the hole cancel each other out, so excitons carry no net charge. But they can still transfer energy and emit light, which explains the fluorescent glow of the tubes. Crucially, the formation of excitons on the graphene surface created fluctuating electrical charges within the material. Those fluctuations transferred momentum to H₂O molecules at the graphene-water interface, producing drag. More illumination meant more excitons, which meant more friction and more deceleration, all without any surfaces physically rubbing together.The researchers confirmed this mechanism using terahertz (THz) spectroscopy and found that eliminating excitons removed the friction entirely. The discovery opens the door to practical applications: by manipulating quantum friction with light, scientists could potentially speed up or slow down chemical reactions on demand. Even more tantalizing, nanorobots designed to explore previously unreachable places, from inside the human body to the deepest crevices of the Mariana Trench, might one day be steered simply by adjusting the light that triggers this effect.“Light-induced slowing down of [motion] can be physically or chemically manipulated to affect the movement of a nanomaterial in an aqueous solution,” said Kruss. “We can anticipate future approaches to control the movement of microswimmers or nanorobots...the findings also raise the question of whether the effect could be present in other systems with high exciton mobility/charge fluctuations.”Elizabeth Rayne is a creature who writes. Her work has appeared in Popular Mechanics, Ars Technica, SYFY WIRE, Space.com, Live Science, Den of Geek, Forbidden Futures and Collective Tales. She lurks right outside New York City with her parrot, Lestat. When not writing, she can be found drawing, playing the piano or shapeshifting.