Two recent articles show very surprising mechanisms for transporting nanoparticles through nanotubes. In the first, Xu and Chen at Columbia embed a single water molecule inside a fullerene cage, which was placed inside a carbon nanotube. An applied electrical field parallel to the tube causes the fullerene to migrate, even though the cage and its entrapped water molecule are electrically neutral. This phenomenon seems to be due to conservation of energy and momentum: the initially randomly oriented water molecule largely aligns with the electrical field. Leftover momentum provides the kick needed to move the whole cage forward. There’s a nice summary here.
In the second, the Cohen group at Berkeley show that an iron nanoparticle migrates through a carbon nanotube with an applied electrical field. In this case, electromigration within the nanoparticle induces something like a plastic flow of the particle through the tube.