Water flow and thermal transport through carbon nanotubes

The atomic-level mechanisms of water flow and thermal transport through carbon nanotubes (CNTs) are examined using molecular dynamics (MD) simulation. To begin, the density and distribution of water molecules inside CNTs are examined using equilibrium MD simulation. For CNTs with diameters smaller than 10 nm, surface curvature effects are found to change the average density of the confined water and reduce the energetic coupling between the water and the carbon surface. Next, the relationship between water flow rate and pressure gradient for CNTs with diameters between 0.83 nm and 4.98 nm is examined using non-equilibrium MD simulation. The flow rate enhancement, defined as the ratio of the observed flow rate to that predicted from the no-slip Poiseuille relation, is calculated for each CNT. By calculating the variation of water viscosity and slip length as a function of CNT diameter, it is found that flow through CNTs with diameters greater than 1.4 nm can be understood in the context of continuum fluid mechanics. When the CNT diameter is less than 1.4 nm, however, water molecules assemble into long-range one-dimensional structures that have neither a well-defined slip length nor a well-defined viscosity. Within this subcontinuum regime, the Poiseuille relation is not applicable and transport is instead related to the CNT-dependent inter-molecular water structure.;The thermal conductivities of empty and water-filled CNTs are then predicted using (i) a direct application of the Fourier law and (ii) an application of the spectral energy density. By using the Fourier law approach, the transition from ballistic (subcontinuum) to fully-diffusive (continuum) phonon transport thermal conductivity is observed with increasing CNT length. The interactions with water molecules are found to scatter low-frequency phonon modes, causing a 20%--35% reduction in the fully-diffusive CNT thermal conductivities. A new procedure for calculating phonon properties directly from the velocity of the atoms in a crystal using the spectral energy density is then presented. This procedure, which can be applied to any periodic or non-periodic system, is used to predict the mode-by-mode contributions to the thermal conductivity of empty and water-filled CNTs. The thermal conductivity predicted using the spectral energy density is consistent with that predicted using the Fourier law approach. The number of atoms and simulation run-time required to calculate the spectral energy density, however, are both at least one order-of-magnitude smaller.