Gas permeation through nanoscale pores is ubiquitous in nature and has an important role in many technologies1,2. Because the pore size is typically smaller than the mean free path of gas molecules, the flow of the gas molecules is conventionally described by Knudsen theory, which assumes diffuse reflection (random-angle scattering) at confining walls3,4,5,6,7. This assumption holds surprisingly well in experiments, with only a few cases of partially specular (mirror-like) reflection known5,8,9,10,11. Here we report gas transport through ångström-scale channels with atomically flat walls12,13 and show that surface scattering can be either diffuse or specular, depending on the fine details of the atomic landscape of the surface, and that quantum effects contribute to the specularity at room temperature. The channels, made from graphene or boron nitride, allow helium gas flow that is orders of magnitude faster than expected from theory. This is explained by specular surface scattering, which leads to ballistic transport and frictionless gas flow. Similar channels, but with molybdenum disulfide walls, exhibit much slower permeation that remains well described by Knudsen diffusion. We attribute the difference to the larger atomic corrugations at molybdenum disulfide surfaces, which are similar in height to the size of the atoms being transported and their de Broglie wavelength. The importance of this matter-wave contribution is corroborated by the observation of a reversed isotope effect, whereby the mass flow of hydrogen is notably higher than that of deuterium, in contrast to the relation expected for classical flows. Our results provide insights into the atomistic details of molecular permeation, which previously could be accessed only in simulations10,14, and demonstrate the possibility of studying gas transport under controlled confinement comparable in size to the quantum-mechanical size of atoms.
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This work was supported by the European Research Council, Lloyd’s Register Foundation, the EU Graphene Flagship and the Royal Society. B.R. acknowledges a Leverhulme Early Career Fellowship, a L’Oréal Fellowship for Women in Science and EPSRC grant EP/R013063/1. F.C.W. acknowledges support from the National Natural Science Foundation of China (11772319 and 11572307) and the Shanghai Supercomputer Center. S.J.H. and A.P.R. were funded by the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreements ERC-2016-STG-EvoluTEM-715502 and DISCOVERER-2017 737183), the US Defence Threat Reduction Agency (HDTRA1-12-1-0013) and the EPSRC (EP/P009050/1 and EP/K016946/1).
Nature thanks L. Bocquet and the other anonymous reviewer(s) for their contribution to the peer review of this work.