| The behavior of fluids at the nanoscale is currently of interest, as nanotechnology promises tools for the precise delivery and analysis of minute volumes of liquids. Because of their reported properties, ease of manipulation and hollow cavity, carbon nanotubes have attracted attention for fluidic delivery. Thus far, experimental studies on wetting of carbon nanotubes have been limited, and other work revolves around theoretical modeling. Consequently, there is a considerable lack of experimental data on the subject.;In this work, environmental scanning electron microscopy was employed for visualization of the interaction of carbon nanotubes with fluids such as water, liquid crystals, and biological solutions. Carbon nanotubes of diverse surface structure and chemistry were evaluated, and hydrophobic and hydrophilic behavior was observed. Nanotubes made by catalytic chemical vapor deposition showed highly hydrophobic behavior by the formation of spherical water droplets on nanotube agglomerates. This behavior was caused by roughness of the agglomerates as well as pinning of the droplets on defects and functional groups along the walls of the nanotubes. On highly graphitic carbon nanotube agglomerates, no droplet formation was observed due to the lack of defects and terminations on the surface. Formation of "barrel" and "clam-shell" shaped droplets on individual nanotubes was also observed.;Capillary condensation inside carbon nanotubes made by non-catalytic chemical vapor deposition was also observed in the environmental scanning electron microscope. Condensation was marked by the formation of a thin film of water on the inner walls of the nanotubes, followed by growth of a meniscus of low contact angle, indicating the hydrophilic nature of the nanotubes. Spontaneous elastic deformation of the nanotubes was observed upon the formation of the film due to surface tension effects. By annealing, the carbon nanotubes were graphitized, and highly hydrophobic behavior was discernible by the lack of condensation of water inside the graphitized nanotubes. Filling of these carbon nanotubes with non-aqueous fluids including liquid crystals, and biological macromolecules, such as proteins and protein solutions, demonstrated the ability of carbon nanotubes to hold and transport a variety of substances. Finally, the ability of the carbon nanotubes to penetrate cells was also confirmed. |
