Fluid interactions with carbon nanotubes

• Main Author: Longhurst, Matthew James
• Published: Imperial College London 2007
• Subjects: 620.106
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.486910

Using a purpose built molecular dynamics (MD) code. we simulate a range of infinite and finite length H-terminated carbon nanotubes in vacuo. We find that the radial breathing mode (RBM) of the finite nanotubes approaches that of the infinite nanotubes for nanotubes greater than 5 nm in length. We investigate the effect on the RBM frequency' of immersion in water and find that external wetting is responsible for an upshift in the RBM of around 4-10 wave numbers. and internal wetting approximately 2-6 wave numbers. The upshift is comprised of two components: increased hydrostatic pressure on the nanotube due to curvature effects. and the dynamic coupling of the RBM with a shell of adsorbed fluid: In contrast to much of the current literature, we find that the latter of the two effects . is dominant. The upshift can be modelled analytically by considering the adsorbed fluid as an infinitesimally thin shell which interacts with the nanotube via-a continuum Lennard-Jones potential. Using MD, the RBM of carbon nanotubes in fluids can be accurately reproduced by replacing the fluid molecules with a mean field harmonic shell potential. greatly reducing simulation times. The pressure dependence of shifts in the vibrational modes of individual carbon nanotubes is strongly affected by the nature of the pressure transmitting medium as a result of adsorption at the nanotube sUrfade. Using analytical methods, as well as MD, we observe an as yet unreported low frequency breathing mode for the adsorbed fluid at around 50 cm-1 , as well as diameterdependent upshifts in the RBM frequency with pressure, suggesting metallic nanotubes may wet more than semiconducting ones. Finally. we describe a methodology for the continuous pumping of fluid through carbon nanotubes. Fluid is imbibed from a reservoir at 300 K. heated. and subsequently ejected from the hot end. Very high pressures are developed in the smaller nanotubes due to strong capillary forces, suggesting thAir use as nanoscale reaction vessels. A theoretical framework is developed allowing us to predict pumping fluxes over a range of nanotube diameters and temperatures.