A research team in Germany and India have reported a new experiment showing that light can slow down the diffusion of carbon nanotubes suspended in water by creating an unexpected kind of “quantum friction.” The effect is stronger when more light is applied, and the work was published last month, in Nature.
Researchers shined light on carbon nanotubes in water and found that the nanotubes spread more slowly through the solution than they did without illumination. The article frames this as a rare case where friction is not caused by ordinary mechanical contact, but by quantum effects at the interface between the nanotubes and the liquid.
The proposed mechanism is tied to excitons, which are short-lived excited states created when the nanotubes absorb light. In carbon nanotubes, those excitons are mobile, so they travel along the tube and carry fluctuating charges with them. Those charges interact with nearby water molecules and create extra drag, which increases friction and slows motion.
This is the clearest experimental evidence yet that friction can emerge not only from surfaces rubbing together, but also from quantum behavior at a material-liquid interface. It also highlights that the friction can be controlled by light, which makes the phenomenon especially interesting for both basic physics and future manipulation of nanoscale systems.
This is not important because of an immediate consumer application but because it reveals a new physical effect at very small scales. For scientists, the result adds to the growing picture of how quantum mechanics can influence transport, drag, and diffusion in complex environments. The main takeaway is that light is not just a source of energy here; it can actively change how matter moves in liquid.
The published findings are in Nature.
Image: Kistwal, T., Kanhaiya, K., Buchmann, A. et al. Light-induced quantum friction of carbon nanotubes in water. Nature 654, 941–947 (2026).
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