Frictionless when flat

Furthermore, they observed that the highest helium permeability measured in graphite nanochannels was independent of the channel length, indicating that there is no momentum loss in the channels and that frictionless flow has been achieved. [...]more experiments with other gases in graphite and h-BN nanochannels are needed to further unravel the influence of molecular diameter and de Broglie wavelength on specular reflection, given that larger diameters typically correspond to smaller de Broglie wavelengths. [...]a quantitative comparison of gas transport through rectangular nanochannels and through circular nanotubes made ofthe same material is needed to evaluate the effect of channel curvature. [...]frictionless gas transport through channels that are asymmetrically constricted10 could enable gas-flow rectification11, a process that might allow the development of new pumps, valves and other devices for controlling gas flow. ?