A numerical demonstration of discontinuous directional motion at the nanometre scale

Classical molecular dynamics simulations have been employed to investigate the behaviour of a relatively complex assembly of carbon nanotubes embedded into a CF4 liquid phase submitted to heating and cooling cycles. The assembly is formed by two concentric capsule-like carbon nanotubes, the smaller of which located in the interior of the larger one, which has a partially open end. The carbon nanotubes have radii such that the distance between their cylindrical walls, though keeping attrition negligible, does not permit the passage of chemical species from one side of the inner capsule to the other. CH4 molecules in liquid phase are confined between the capped end of the outer nanotube and the inner capsule. On the other side, CF4 molecules can access the volume available between the inner capsule and the outer nanotube through its open end. When temperature is raised, the confined CH4 molecules undergo the liquid-gas transition. The consequent volume expansion pushes the inner capsule in the direction of the open end of the outer nanotube, which determines the rapid ejection of the CF4 molecules. This generates a thrust that allows the motion of the whole assembly in the direction opposite to ejection. When temperature is decreased and CH4 molecules return in the liquid phase, the system slowly recovers the initial configuration. The whole cycle of operation is fully reversible and allows the assembly to directionally move into the liquid phase.