Carbon nanotubes, those tiny hollow carbon whiskers nanometers wide
but microns or longer in length, have intriguing optical, electrical,
thermal, and mechanical properties. Perhaps the earliest big practical
use for nanotubes will be as an additive in many composite materials,
both liquid and solid. NIST physicist Erik Hobbie gauges nanotube flow
properties by suspending them in a liquid polymer solvent between two
parallel plates and then subjecting the fluid to shear force by moving
one of the plates.
In general getting the long nanotubes lined up is
like herding cats; they get tangled very easily. But at low concentration
and high enough shear, the tubes do line up, as if the mixture were
a “nematic” liquid crystal, a liquid in which rod-shaped polymer molecules
are aligned with each other. Lower the amount of shear or raise the
nanotube concentration and the tangles begin. Increase the concentration
further and the tangling gets more elaborate; the nanotubes form bands
(visible to the human eye) parallel to the plates and perpendicular
to the flow direction. At even higher concentrations (around 3%) the
aggregation becomes so great that fluid flow comes to a halt.
In this tangled state the web of interconnections between nanotubes
takes on a fractal-like geometry. Knowing this geometry well will be
of use in numerous upcoming industrial processes involving carbon nanotubes.
Hobbie reported his results at last week’s meeting of the Society of
Rheology in Lubbock, Texas. (Paper MF9,