Vibration and orientation of diatomic molecules flowing through small carbon nanotubes.

With their unique long cylindrical shape, carbon nanotubes may one-day form nozzles for nano-scale printing or flow into a chamber. Since the scale of the flowing molecules is similar to the diameter of the nanotubes, molecular vibration, orientation and density become influenced by the confinement during flow. We have studied the flow of diatomic molecules through carbon nanotube nozzles using non-equilibrium molecular dynamics simulations, in an effort to gain a greater understanding about the fundamental properties of such molecules in such a setting. The frequency of vibration of the molecules is shown to be dependent on the density inside the nanotubes and follow the same relation as an experimental micro-scale density-frequency study suggests, although only for nanotubes above a certain diameter. Meanwhile no relation is found between the frequency of vibration and the flow rate. The effect of nanotube diameter on the orientation of the molecules is also examined in detail, showing the transition between axial and radial orientation, with "pull" and "push" effects determining the orientation.