Flow enhancement in nanotubes of different materials and lengths

Abstract

The high water flow rates observed in carbon nanotubes (CNTs) have previously been attributed to the unfavorable energetic interaction between the liquid and the graphitic walls of the CNTs. This paper reports molecular dynamics simulations of water flow in carbon, boron nitride, and silicon carbide nanotubes that show the effect of the solid-liquid interactions on the fluid flow. Alongside an analytical model, these results show that the flow enhancement depends on the tube's geometric characteristics and the solid-liquid interactions.

Hybrid molecular-continuum simulations of water flow through carbon nanotube membranes of realistic thickness

“…The high permeability arises from the smooth internal structure of nanotubes, producing low-friction fluid transport. Substantial reductions in water-surface friction have been demonstrated with decreasing tube radius (<10 nm) (Falk et al 2010), which explains the many orders of magnitude flow enhancement over continuum fluid theory that is observed in both experiment (Holt et al 2006;Majumder et al 2005;Majumder and Corry 2011) and simulation (Thomas and McGaughey 2009;Kannam et al 2012;Walther et al 2013;Ritos et al 2014).…”

“…D r = 1.16 − 0.33 = 0.83 nm. The second assumption is that the viscosity of bulk water (1 mPa s) is used in all calculations, although it is known that the water viscosity inside such a narrow CNT differs significantly (Thomas and McGaughey 2008;Ritos et al 2014). The bulk water density (ρ = 997.5 kg/m 3 ) is also used in any transformations from volumetric to mass flow rate, and vice versa.…”

“…2a) are simulated using the mdFoam molecular dynamics solver. This software has been validated over a broad range of applications (Macpherson and Reese 2008;Ritos et al 2013;Dongari et al 2011) including flow of water through carbon nanotubes Ritos et al 2014;. In all our simulations we use the TIP4P/2005 water model (Abascal and Vega 2005;Vega and Abascal 2011) with a Lennard-Jones potential for water-carbon interactions that has the following intermolecular parameters: σ CO = 0.319 nm and ǫ CO = 0.427 kJ mol −1 .…”

On the structure and rejection of ions by a polyamide membrane in pressure-driven molecular dynamics simulations

“…This approach has been used successfully to model water transport through nanotubes [33][34][35]. A modified approach has been proposed recently by Ritos et al who used a non-uniform force to account for the different geometries of the reservoirs and that of the nanotube [36]. Another approach has been developed by Wang et al who have shown that it was possible to carry out NEMD simulations by adding two movable walls (acting as pistons) in the simulation box [37,38].…”

Molecular dynamics pre-simulations for nanoscale computational fluid dynamics

“…This water model consists of four interacting sites: one oxygen atom (O) with no charge but which is the centre of the LJ potential, two hydrogen sites (H) each with a fixed point charge of q H ¼ 0:5564 e, and a massless site (M) with charge q M ¼ À1:1128 e. All oxygen atoms interact using the LJ potential, Eq. (7) with OÀO ¼ 0:7749 Â 10 À21 J and r OÀO ¼ 3:1589 Â 10 À10 m. Water-carbon interactions also use the LJ potential between carbon and oxygen atoms with r CÀO ¼ 3:19 Â 10 À10 m and CÀO ¼ 0:709302 Â 10 À21 J as in Ritos et al (2014). These values reproduce the macroscopic contact angle of a water droplet on a graphitic surface, using the methodology of Werder et al (2003).…”