Structure and transport of confined liquid in nanochannels

Abstract

In recent years, the mass transport through nanochannels has received widespread attention from the academic and engineering communities, as well as industry, not only due to its cutting-edge significance in physical mechanics of surfaces and interfaces, but also because of its crucial application prospects in the fields of energy, environment, and health. At the nanometer scale, the interface effect and intermolecular interactions are prominent. The free movement of liquid molecules is confined, and its structure also exhibits layering, ordering, and other characteristics. Due to this fact, the transport of confined liquid yields distinguishable results from those at the macroscale. Both the governing equations and boundary conditions for nanoflow are worth further investigation. The understanding on the structure and transport of confined liquid also plays an important role in the applications of microfluidic and nanofluidic technologies. As the length scale of nanodevices decreases continuously, the surface and interface effects become significant. The structure and dynamic behaviours of confined liquid are dramatically different from that of the bulk phase. At the nanoscale, several physical mechanisms are intercoupling; various intermolecular and surface forces compete with each other. There are many characteristic length scales which fall in the range from a few angstroms to several nanometers. This review article focuses on the structure and transport of confined liquids, and systematically reviews the recent research progress in this field. Starting from the ordered structures of the confined water, we discussed not only the layered structure perpendicular to the confining walls, but also various two dimensional ice phases parallel to the walls. Emphasis was placed on the hot topics of water transport and ion sieving through various kinds of nanochannels, especially for capillaries existing in graphene oxide membranes and nanocapillaries fabricated by van der Waals assembly using two-dimensional materials. The source of the complexity of the ordered structure and transport process of confined liquids was analyzed. We also discussed that slip boundary conditions and driving forces in modeling such process require extra attention and in-depth investigations. It was revealed that the mechanism of liquid flow through nanochannels is dominated by intermolecular forces. Although there are still many unknown questions to be further explored, we tried to point out the possible directions of the organic combination of theory, simulation and experimental research, as well as the interdisciplinary integration.