Nanoconfined fluids
(NCFs), which are confined in nanospaces, exhibit
distinctive nanoscale effects, including surface effects, small-size
effects, quantum effects, and others. The continuous medium hypothesis
in fluid mechanics is not valid in this context because of the comparable
characteristic length of spaces and molecular mean free path, and
accordingly, the classical continuum theories developed for the bulk
fluids usually cannot describe the mass and energy transport of NCFs.
In this Perspective, we summarize the nanoscale effects on the thermodynamics,
mass transport, flow dynamics, heat transfer, phase change, and energy
transport of NCFs and highlight the related representative works.
The applications of NCFs in the fields of membrane separation, oil
and gas production, energy harvesting and storage, and biological
engineering are especially indicated. Currently, the theoretical description
framework of NCFs is still missing, and it is expected that this framework
can be established by adopting the classical continuum theories with
the consideration of nanoscale effects.
The accurate determination of fluid viscosity based on the microscopic information of molecules is very crucial for the prediction of nanoscale flow. Despite the challenge of this problem, researchers have done a lot of meaningful work and developed several distinctive methods. However, one of the common approaches to calculate the fluid viscosity is using the Green–Kubo formula by considering all the fluid molecules in nanospace, inevitably causing the involvement of the frictional interaction between fluid and the wall into the fluid viscosity. This practice is certainly not appropriate because viscosity is essentially related only to the interactions among fluid molecules. Here, we clarify that the wall friction should be decoupled from fluid viscosity by distinguishing the frictional region and the viscous region for the accurate prediction of nanoscale flow. By comparing the fluid viscosities calculated from the Green–Kubo formula in the whole region and viscous region and the viscosity obtained from the velocity profile through the Hagen–Poiseuille equation, it is found that only the calculated viscosity in the viscous region agrees well with the viscosity from the velocity profile. To demonstrate the applicability of this clarification, the Lennard-Jones fluid and water confined between Lennard-Jones, graphene, and silica walls, even with different fluid–wall interactions, are extensively tested. This work clearly defines the viscosity of fluids at nanoscales from the inherent nature of physics, aiming at the accurate prediction of nanoscale flow from the classical continuum hydrodynamic theory.
Mitigating leaks through permeable defects by stacking graphene layers would greatly reduce the molecular permeance through porous graphene membranes for gas separation. We propose a multilayer graphene membrane with conical nanopores which instead presents an ultrahigh molecular permeance even higher than those of single-layer graphene membranes. Comparison with existing experimental data also shows that such membranes present an excellent separation performance in the aspect of molecular permeance. The highly permeable conical nanopore is particularly promising for the strongly adsorbed gases on the graphene surface, such as CO 2 and H 2 S. The underlying mechanisms are revealed by using molecular dynamics simulations, including (1) a large permeable area in the penetration side and (2) low permeation resistance caused by molecular bouncing in the nanopore for finding a possibility of permeation. The proposed conical nanopore can not only improve the molecular permeance through the defect-free and easy-fabricated multilayer graphene membranes but also provide a good example for the applications involving molecular permeation through nanopores.
We establish a theoretical model to describe the surface molecular permeation through two-dimensional graphene nanopores based on the surface diffusion equation and Fick’s law. The model is established by considering...
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