Atomistic Simulations of Thermophoretic Motion of water Nanodroplets in Carbon Nanotubes

Open-ended nanotubes offer unique possibilities as fluid conduits with applications ranging from molecule separation devices in biocatalysis to encapsulation media for drug storage and delivery. Liquids and solids in nanochannels may be driven by electrophoresis, osmosis, gradients in the surface tension (Marangoni effect), pressure gradients, and thermophoresis. Hence, electrophoresis has been used for driving electrically charged particles in nanosystems and gradients in the surface tension have been exploited to drive flow through carbon nanotubes (CNTs) immersed into a lipid membrane Pressure gradients imposed in nanopipes have been used to generate controlled flows for nanoscale applications9, and to enhance electrophoretic motion across carbon nanotube membranes The use of thermal gradients to induce mass transport is known as thermophoresis, the Soret effect or thermodiffusion. The first observation of thermodiffusion was reported by Ludwig13 in 1856, who found differences in samples taken from different parts of a solution in which the temperature was not uniform. A systematic investigation of the phenomena was subsequently conducted by Soret in 1879-81, and by in 1921-1925 for thermodiffusion in gases. Ibbs found that the coefficient of thermal diffusion is more sensitive than any of the other transport coefficients to the nature of the intermolecular forces.1 Thus, a complete understanding of the thermal diffusion could provide a powerful means of investigation of forces between molecules. Although the theoretical explanation of thermodiffusion for molecules in liquids is still under debate, the investigation of its practical usability is motivated by potential applications in nanotechnology. Hence, thermodiffusion was recently used as the driving mechanism in artificially fabricated nanomotors, and thermodiffusion is expected to allow microscale manipulation and control of flow in nanofluidic devices. In a recent theoretical study, thermophoresis was shown to induce motion of solid gold nanoparticles confined inside carbon nanotubes. In the present investigation, we study thermophoretic motion of liquid water nanodroplets confined inside carbon nanotubes.