Carbon nanotubes, unfortunately, are like people. Some are short, others are long. Some can be used in semiconductors; others can't because they don't conduct electricity at all. This wide variety of tubes, however, will come out in the same batch. Using nanotubes inside medical or electrical devices is dependent on figuring out how to sort the ones you want from the big pile produced in the laboratory crucible or growing them in place on silicon wafers.
Researchers at the National Institute of Standards and Technology say they have figured out a way to sort short ones from long ones. When suspended in viscous fluids, short ones will gravitate toward the walls of the container while longer ones congregate in the interior. The group analyzed suspensions ranging in viscosity from syrup-like to watery under different mixing conditions.
Sorting short from long is not the only problem. Chirality, a measure of the arrangement of the carbon atoms, determines whether a nanotube will act like a metal, silicon or a non-conductive material has yet to be solved. Still, NIST says the recent breakthrough could be important for developing new types of composites.