Simulation Studies On Viscosity, Diffusion And Shear Flow Behaviors Of Water Inside Carbon-based Nanoscale Channels

The properties and behaviors of materials at the nanoscale have attracted much attention in recent years. As one of the most common fluids in nature, the water possesses many extraordinary characteristics at the nanoscale compared with that at the micro-or macroscale, such as the ordered molecular configuration, the high transport rate, the special diffusion behavior and so on. These nanoscale characteristics make the water have widely promising applications in molecule delivery, nanoequipment cooling and so on. Hence, it is very significant to investigate the unexpected changes and rules of the water at the nanoscale. By using the molecular dynamics (MD) method, this paper focuses on the transport properties and flow behaviors of the water inside the carbon-based nanochannels, including the viscosity of the water confined in carbon nanotubes (CNTs), the diffusion behavior of the water confined in CNTs and the shear flow of the water driven by the graphene sheets. The detailed contents are presented as follows:Firstly, the viscosity of the water inside the CNTs is investigated. The existing methods in the MD simulations are difficult to calculate the viscosity of fluids inside the channels of extremely small size. In this paper, to study the size and temperature effects on the viscosity of the water inside the CNTs, a semi-empirical "Eyring-MD" method is proposed on the basis of the classical Eyring theory and the MD simulations. The numerical examples demonstrate the correctness and the efficiency of the proposed method. Based on the "Eyring-MD" method, the viscosity of the water inside the CNTs of diameter within8～54A is calculated. The computational results indicate that the viscosity of the water inside SWCNTs is lower than that of the bulk water, which increases nonlinearly with enlarging diameter of SWCNTs and gradually approaches the viscosity of the bulk water. Then, through considering some new numerical experiments, the coefficients in the "Eyring-MD" method are supplemented and corrected. The temperature dependence of the viscosity is studied by the modified "Eyring-MD" method. The results reveal that size-dependent trend of the viscosity of the water inside the CNTs is almost independent on the temperature. For the CNTs of the same diameter, the relative viscosity increases with increasing the temperature. Furthermore, except for the (8,8) and the (9,9) CNTs, the amounts of the hydrogen bonds of the water confined in the CNTs exhibit similar profiles with the curves of the viscosity. There is an increment in the amounts of the hydrogen bond of the water inside the (8,8) and the (9,9) CNTs. According to the computational results, we provide a formula for the size-and temperature-dependent viscosity.Secondly, the diffusion behavior of the water inside the CNTs is investigated. A natural, real and efficient MD model is constructed to study the diffusion behavior of the water molecules inside the CNTs. The computational results suggest that the motions of water molecules inside the CNTs of diameter smaller than12.2A follow a two-stage diffusion mechanism. Initially, the water diffusion exhibits some unconventional diffusion mechanisms, and thereafter it transits to the single-file type inside the (6,6) CNT and shifts to the normal type (Fickian diffusion mechanism) inside the larger CNTs. As the temperature increases, the crossover between the two stages becomes obscure. As for the CNTs of diameter larger than12.2A, the diffusion of the confined water occurs through the Fickian mechanism, which is identical to that of the bulk water. Except for the (8,8) and the (9,9) CNTs, the diffusion coefficient of the confined water increases with increasing the diameter up to about40A, and then gradually approaches that of bulk water. A sharp reduction can be observed in the diffusion coefficient of the water inside the (8,8) and the (9,9) CNTs. As the temperature increases, the diffusion coefficient of the confined water gradually increases, but their variation trends with the diameter are almost unchanged. Furthermore, it is suggested that the extraordinary diffusion mechanism of the water molecules inside the CNTs is attributed to the cooperation of the size confinement and the surface property, and the nonmonotonic variation of the diffusion coefficient is ascribed to the competition of the size confinement and the surface property. Based on the qualitative examination of these two effects, we provide a formula for the diffusion coefficient with consideration of the size and the temperature effects.Finally, through the MD simulations, the shear flow of water driven by the graphene sheets is investigated. To explore the influence of the electricity on the flow behavior of the polar water molecules, the graphene sheets with the adsorbed charges are further considered based on the simulations on the pure graphene sheets. The computational results reveal that the pure graphene sheets can result in a linear velocity profile in the water. But there is enormous velocity dissipation between the wall and the water. It can also be seen that the internal slip exists between the molecular layers near the solid boundary. The graphene sheets with the adsorbed charges can drive the shear flow of the water more effectively, which can greatly reduce the velocity dissipation between the wall and the water. As the charge increases, the coupling strength between the tube wall and the water molecules is enhanced, and the velocity gradient of the water molecules gradually increases. For the graphene sheets with the same electric quantity but different electric properties, the flow behaviors are almost identical but the driven mechanisms are completely different. The slip lengths of the water molecules on the two types of the graphene sheets are also calculated. The results reveal that the slip length slightly increases at the low shear rate, and then sharply increase after the shear rate reaches the threshold value. For the pure graphene sheets, the slip length ranges from500A to2500A; for the graphene sheets with0.10e per carbon atoms, the slip length ranges from50A to350A. The present results demonstrate that the introduction of the electricity can effectively suppress the slip phenomenon. According to the computational results, the relationships between the slip length and the shear rate in the two cases are given.