With the right diameter (8 angstroms) nano-tubes can influence the water inside to solidify into a square tube, transitioning into a type of ice. This can lead to new forms of nano-channels devices, like nano-scale capacitors or syringes.
Researchers at the Rice University used molecular models to demonstrate their theory that weak van der Waals forces between the inner surface of the nanotube and the water molecules are strong enough to snap the oxygen and hydrogen atoms into place. According to them if the nanotube is too small and can fit only one water molecule, it cannot be used. If it's too large, the water keeps its amorphous shape. But at about 8 angstroms, the nanotubes' van der Waals force starts to push water molecules into organized square shapes.