Flow And Transport Properties Of Fluids In Nanochannels

With the growing interest in microelectromechanical systems (MEMS), fluid flows confined in micro/nanochannels have attracted particular attention over last two decades. It is desirable to understand the motion and transport properties of fluid flows in nanochannls. The present dissertation investigates the numerical modeling, flow properties and boundary conditions of fluid flows confined in parallel-plate nanochannels. The fluid transport phenomena and mechanism of carbon nanotubes (CNTs) are explored. In addition, gas separation by CNTs is studied by evaluating the tradeoff between permeability and selectivity. All the atomistic calculations are performed based on molecular dynamics (MD) simulations. The research may offer some insight and clues to further explore the mechanism and modeling of fluid flows in nanochannels.Poiseuille flow is a fundamental question in macroscopic hydrodynamics. The central issue of simulations for Poiseuille flow at microscopic level mainly concentrates on the method to generate a constant pressure gradient within the confined fluids. A "channel moving" pressure-driven model is proposed to generate a constant pressure gradient within fluids confined in nanochannels. Classical MD simulations are carried out to explore the pressure-driven flow in parallel-plate nanochannels with the channel width ranging from 2.611 to 5.595nm. Considering the slip boundary conditions, relationships among the pressure gradient, mean flow velocity and the channel width are investigated to couple the atomistic regime to continuum. The simulation results show that the mean flow velocity almost linearly increases with the increase of the pressure gradient. The slope of the linear relationship between the pressure gradient and the mean flow velocity is nonlinearly decreased with increasing the channel width. Discrepancies between the simulation results and the continuum increase with the decrease of the channel width.The fluid flows confined in nanochannels usually possess relatively larger Knudsen numbers. The breakdown of the continuum at nanoscale dimensions leads to uncertainties of the governing constitutive laws. It is desirable to look for an alternative approach for conducting continuum models suitable for fluid flows confined in nanochannels. Based on the drawback analysis of the classical second-order velocity-slip boundary condition, a corrected second-order velocity-slip boundary condition is proposed to solve the Navier-Stokes (N-S) equations for fluid flows confined in parallel-plate nanochannels. The corrected slip boundary condition is not only dependent on the Knudsen number and the tangential momentum accommodation coefficient (TMAC), but also dependent on the relative position of the slip surface at the Knudsen layer according to the density distribution across the channel. For fluid flows in the slip-flow regime with the Knudsen number ranging 0.1～0.3, a Couette cell is first investigated using MD simulations to verify Newtonian flow behaviors by examining the constitutive relationship between shear stress and strain rate. Then, MD simulations of Poiseuille flows are performed to investigate the shear stress and velocity profiles. By comparing the shear stress and velocity profiles of Poiseuille flows predicted by the N-S equations with the corrected slip boundary condition with that from MD simulations, we found that the flow behaviors in our models can be effectively captured. Furthermore, the range of TMAC obtained from the corrected slip boundary condition is in good agreement with experimental measurements while the TMAC obtained from the classical slip boundary condition is invalid. Moreover, the influences of the strength of the solid-fluid coupling, the fluid temperature and the density of the solid wall on the velocity slip at the fluid boundary are investigated. For weak solid-fluid coupling strength, high temperature of the confined fluid and high density of the solid wall, the large velocity slip at the fluid boundary can be obviously observed. This study provides an effective velocity-slip boundary condition and extends the applicable scope of the N-S equation for fluid flows confined in nanochannels.CNTs, which can be seen as a special type of nanochannel, have a significant application in nanotechnology as nanopipes conveying fluids. In order for the liquids with high surface tension to enter the core of the CNTs, either a sufficiently large external pressure is applied or the surface tension is reduced. In this paper, a pressure control model for transport of liquid mercury through CNTs using classical MD simulations is presented. Wetting of single-walled carbon nanotubes (SWCNTs) by mercury occurs above a threshold pressure of liquid mercury. The liquid mercury can be transported through CNTs with the continuous increase of pressure. The most important is that SWCNTs can transport liquid mercury when a periodical pressure is applied on the liquid. The major advantage of this approach is that CNTs can transport fluids without increasing the pressure of the liquid infinitely. Transport efficiency of double-walled carbon nanotubes (DWCNTs) is a little lower than that of SWCNTs while DWCNTs transport liquid more steadily than SWCNTs. Also, electrowetting of DWCNTs by mercury is studied using classical MD simulations. Wetting of DWCNTs by mercury occurs above a threshold size of inner tube when the voltage is applied on the outer tube, but no wetting phenomenon appears when the voltage is applied on the inner tube. The filling rate increases greatly with enlarging the diameter of the inner tube. The space between the two tube walls of DWCNTs can not be filled by liquid mercury in our simulations.Gas separation, as a special form of flow transport, is also an important application of CNTs. A kinked CNT model for gas separation is presented in this paper. Transport of pure nitrogen, oxygen and their mixture in SWCNTs with a kink formed by bending is studied using MD simulations. The results show that a kinked SWCNT results in transport resistance to nitrogen while allowing oxygen to pass even though the two gases have very similar molecular sizes. The permeability decreases while the selectivity increases with increasing the bending angle of SWCNTs. So it is very convenient to obtain the required purity and permeability of the oxygen by adjusting the bending angle of SWCNTs. The tradeoff between permeability and selectivity is evaluated by linear weighting method to attain an optimum bending angle of a SWCNT for gas separation. The emphasis is that the kink model can be used to improve the permeability of oxygen by changing the diameter of the SWCNT with the optimum bending angle while keeping a high purity of oxygen in gas separation process. Furthermore, the influences of gas temperature, gas pressure, component ratio of gas mixture and pushing velocity on gas separation are investigated. Relatively high gas temperature and gas pressure, and low pushing velocity are propitious to separate oxygen from gas mixture. The purity of oxygen can be kept higher than 80% when the component ratio of nitrogen is lower than 3/4.