In one-atom-thin membranes, ultrafast gas flows through tiny holes – validation of a century-old equation of fluid dynamics.

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Researchers at the University of Manchester’s National Graphene Institute and the University of Pennsylvania identify ultrafast gas flows in 2D membranes through atomic holes and validate a centuries-old equation of fluid dynamics.

In a study published in Science Advances, researchers at the University of Manchester’s National Graphene Institute and the University of Pennsylvania have identified ultrafast gas flows through tiny holes in one-atom-thin membranes.

The work – together with another Penn study on the manufacture of such nanoporous membranes – holds promise for numerous applications, from purification of water and gas to monitoring of air quality and production of energy.

The renowned Danish physicist Martin Knudsen formulated theories to describe gas flows in the early 20th century.

Knudsen’s definitions of gas flows were challenged by emerging new structures with narrower pores, but they remained accurate and it was not clear at what point they would fail to decrease in size.

The Manchester team – led by Professor Radha Boya, in collaboration with Professor Marija Drndić’s University of Pennsylvania team – has shown for the first time that the description of Knudsen appears to be at the outermost atomic limit.

The science of two-dimensional (2D) materials is progressing rapidly, and the development of one-atom-thick membranes is now routine for researchers. A method to drill one atom-wide hole in a monolayer of tungsten disulfide was developed by Professor Drndić’s group in Pennsylvania. An important question, however, remained: how to verify that the atomic holes without seeing them by hand are continuous and conductive.

Until now, inspecting the holes in a high-resolution electron microscope was the only way to confirm that they were present and of the correct size.

The team of Professor Boya developed a method for calculating gas flow through atomic holes and using that flow as a tool for quantifying hole density in turn. She said, “Although there is no doubt that seeing is believing, science has been quite limited because we could only see atomic pores in a fancy microscope. Here we have devices that not only allow us to measure the gas currents, but also use the currents as a guide to estimate how many atomic holes were in the membrane to begin with.”

J. Thiruraman, the co-first author of the research, said, “To be able to achieve that atomic scale experimentally, and to have the imaging of that structure with precision so that you can be more confident that it’s a pore of that size and shape, was a challenge.”

“Professor Drndić added, “Between finding something in the lab and creating a usable membrane, there is a lot of device physics.

That came with the growth of technology as well as our own approach, and what’s new here is to incorporate that into a system that you can actually take out, carry across the ocean if you like [to Manchester], and measure.

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“Manually checking the formation of atomic holes over large areas on a membrane is tedious and probably impractical. Here we use a simple principle: the amount of gas the membrane allows to pass is a measure of how holey it is.”It is tedious and probably impractical to manually check the formation of atomic holes on a membrane over large areas. We use a simple principle here: the amount of gas the membrane allows to pass is a measure of how holey it is.

Several orders of magnitude greater than fluxes previously observed in angstrom-sized pores in the literature are the gas fluxes achieved.

A one-to-one correlation of atomic aperture densities by transmission electron microscopy imaging (measured locally) and of gas fluxes (measured on a large scale) was combined by this study and published by the team. S. Dar, a co-author from Manchester, added, “Surprisingly, there is no/minimal energy barrier to flow through such tiny holes.”

“We now have a robust method to confirm the formation of atomic holes over large areas of gas flow, which is an essential step for pursuing their prospective applications in various fields, including molecular separation, detection and monitoring of gases at ultralow concentrations.” Professor Boya added.

Reference: “Gas flow through atomic-scale apertures” by Jothi Priyanka Thiruraman, Sidra Abbas Dar, Paul Masih Das, Nasim Hassani, Mehdi Neek-Amal, Ashok Keerthi, Marija Drndic and Boya Radha, 18 December 2020, Science Advances.DOI: 10.1126/sciadv.abc7927
This work was conducted as part of an international collaboration and includes experime

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