Rheo-optical studies of carbon nanotube suspensions

We use a polarization-modulation technique to investigate the optical anisotropy of multi- and single-wall carbon nanotubes suspended in a variety of solvents under simple shear flow. Measurements of birefringence and dichroism are performed as a function of shear rate, tube concentration, and solvent viscosity. At fixed volume fraction, the anisotropy increases with increasing shear stress due to enhanced flow alignment. At fixed shear stress, the anisotropy increases with volume fraction due to rotational excluded-volume interactions. By considering the rotational diffusivity as a function of nanotube length, diameter, concentration, and solvent viscosity, we demonstrate a leading-order scaling relation for the optical anisotropy in terms of rotary Peclet number Pe. At low Pe, our results are in qualitative agreement with the theoretical predictions of Doi and Edwards. At high Pe, our data suggest that the degree of nanotube alignment scales as Pe16.