Illustration of the experimental setup showing a CNT attached to the tip of a glass capillary tube.
Illustration of the experimental setup showing a CNT attached to the tip of a glass capillary tube.

Water flowing through the narrowest carbon nanotubes (CNTs) experiences less friction, a team of researchers has confirmed [Secchi et al., Nature (2016), doi: 10.1038/nature19315].

Scientists have long believed that water flow in CNTs is almost frictionless – but because of the challenges in making flow measurements at such small scales, definitive proof was hard to find. Now researchers from the PSL Research University in France and Brown University have devised an experimental setup that allows the water flow through CNTs to be determined.

Alessandro Siria and Lydéric Bocquet realized that while the actual water flow through a CNT is too small – a few femtoliters per second – to be measured directly, it is possible to monitor the effect of a water jet emerging from it. The researchers created a nanofluidic device consisting of two reservoirs separated by a watertight membrane. Water is introduced through a fine glass capillary tube with a single nanotube attached to the tip. The nanotube tip pierces the watertight membrane and directs a jet of water into a reservoir containing polystyrene particles. The particles are large enough to be seen with an optical microscope, so that their motion in response to the water emerging from the CNT can be measured.

The researchers put CNTs of different diameters and another promising nanomaterial, boron nitride, into the nanofluidic device. When considering fluid flow, ‘slip length’ is used to indicate the slipperiness of a surface and how much friction it exerts. Siria and Bocquetfound that nanotube diameter has a profound affect on slip length.

“Water flow occurs nearly without friction inside carbon nanotubes,” report Bocquet and Siria. “And the smaller the tubes, the less friction there is. The flow is accordingly much faster than in other nanochannels.”

Boron nitride nanotubes are rather sticky compared with their carbon equivalent, but why is not yet exactly clear. The researchers suggest the difference in behavior arises from atomic-scale variations between the two materials – particularly their electronic structure.

The clear correlation between hydrodynamic flow behavior and the electronic structure of nanotubes is, believes Yuan Chen of the University of Sydney, the researchers’ most significant finding.

“This could open up the possibility of modulating flow behaviors by designing nano-channels with tunable electronic structures,” he says.

Bocquet and Siria agree, adding: “This is a particularly exciting period for the exploration of fluid transport atthe nanoscale, because we now have the tools to investigate properties that were out of reach up to now.” The results could be surprising, say the researchers, because the flow of fluids at the nanoscale is still “a mysterious world full of unexpected and strange behavior”.

This article was originally published in Nano Today (2016), doi: 10.1016/j.nantod.2016.10.011