Water flows freely through carbon nanotubes

Despite the frenzy of research into carbon nanotubes (CNTs) over the past few decades (see, for example, the article by Thomas Ebbesen, Physics Today, June 1996, page 26 ), there isn’t much experimental evidence for one of the tiny structures’ most talked-about superpowers: the ability to funnel water with nearly zero friction. The problem has been achieving the sensitivity to measure water transport rates as feeble as a femtoliter a second. Now Lydéric Bocquet and his colleagues at École Normale Supérieure in Paris have confirmed the slipperiness of CNTs by directly measuring water flow through individual nanotubes whose bores ranged from 15 nm to 50 nm. The researchers stuck a multiwalled CNT inside a small pipette and essentially turned the nanotube into the needle of a syringe. Pressure applied inside the pipette caused water to flow through the CNT and into a tank of water. Rather than tracking the water as it flowed through the tube, Bocquet and his team analyzed the motion of suspended polystyrene nanobeads in the tank to deduce the strength of the jet emerging from the CNT (see the image, which shows the response at various pressures). The results verify that CNTs allow water to flow extremely efficiently. Bocquet’s team also confirmed its 2010 prediction that the flow rate would increase as the tube’s radius decreased, although the dependence turned out to be roughly quadratic rather than quartic. The biggest surprise came when the researchers replaced the CNTs with nanotubes of boron nitride. Although the BN tubes are nearly structurally identical to their carbon counterparts (see the article by Marvin Cohen and Alex Zettl, Physics Today, November 2010, page 34 ), they proved far more resistant to water flow. The finding seems to suggest that electronic properties—CNTs are conductors; boron nitride nanotubes are insulators—play a role in hydrodynamics at very small scales. Bocquet and his team plan to investigate that possibility as they explore the nanotubes’ potential for applications such as water distillation and filtration. (E. Secchi et al., Nature 537, 210, 2016, doi:10.1038/nature19315 .)