This article over at Ars Technica has a nice overview of some recent work showing that when water is forced through a nanotube of appropriate size, the polar nature of the water molecule lines them up so as to create a voltage along the tube.
They show that such a tube can be used as a power cell:
A DFT/MD mutual iterative method was employed to give insights into the mechanism of voltage generation based on water-filled single-walled carbon nanotubes (SWCNTs). Our calculations showed that a constant voltage difference of several mV would generate between the two ends of a carbon nanotube, due to interactions between the water dipole chains and charge carriers in the tube. Our work validates this structure of a water-filled SWCNT as a promising candidate for a synthetic nanoscale power cell, as well as a practical nanopower harvesting device at the atomic level.
One assumes that by applying a voltage it could be used as a pump as well, obviating all sorts of nanoscale machinery in some cases.