Thickness Dependent Nanofluidic Transport in Nanopores and Nanochannels

Due to the high performance water transport through ultrathin membranes, nanopores and nanochannels have drawn a great deal of attention in a variety of applications, such as water desalination, power generation and biosensing. Classically, the transport rate scales inversely with the thickness. However, transport in carbon-based nanopores and nanochannels far exceeds the classical transport governed by the Hagen-Poiseuille (HP) equation, suggesting large transport enhancement factors with respect to the permeation predicted by HP. Here, using molecular dynamics simulations we characterize the thickness dependence by studying the hydrodynamical properties of pores and channels in graphene and finite-length CNTs. Transport in graphene and short CNTs is shown to be dominated by a high interfacial friction and viscosity at the pore/channel entrance. A corrected Hagen-Poiseuille (CHP) model, based on viscosity and friction from Green-Kubo relations, successfully predicts the non-equilibrium pressure driven flows for different sizes of channels. The previously reported enhancement factors (of the order of 1000) approach unity when the permeations are normalized by that of the CHP. The results of our study will help better understand nanoscale flows in nanopores and nanochannels.