Ballistic molecular transport through two dimensional channels
Publication Date: 20 June, 2018
Atomic-scale ping pong
In nanoscale channels with perfectly flat-walls, gas flow has been found to be frictionless, challenging the classical Knudsen gas flow theory. In classical physics, ballistic gas flow can only occur under ideal condition, i.e., specular reflection in which molecules rebound in a way that their angle of incidence and reflection are the same. In reality, gas molecules diffusely reflect in all directions. Can diffuse reflection be switched to specular to achieve frictionless gas transport? Now researchers at The University of Manchester, with collaborators in China and Iran, have demonstrated that for perfectly atomically flat channels made from 2D materials the gas flow can be either diffusive or specular depending on the fine details of the surface atomic landscape. For example, boron nitride or graphene are much flatter than the metal dichalcogenides, allowing specular behaviour giving helium gas flows that are orders of magnitude faster than expected from theory. This quantum effect observed at room temperature is corroborated by reversed isotope effect, whereby the mass flow of hydrogen is higher than that of deuterium, in contrast to the relation expected for classical flows. These results could have an impact on many technologies, e.g., in shale gas extraction and in membrane-based gas separation methods and pollution control.
- Knudsen gas flow theory: describes how gas flows through a narrow tube in free molecular flow. When the mean free path of the molecules in the gas is larger than or equal to the diameter of the tube, the molecules will interact more often with the walls of the tube than with each other.
- Ballistic flow: flow without any resistance or friction.