(232j) Orientational Ordering in Colloidal Rods by Sedimentation and Centrifugation

Sedimentation and the effective gravitational force of centrifugation were applied to produce 3D ordered assemblies of monodisperse colloidal rods that vary from 2 to 5 microns in the length and are of aspect ratio from 3.6 to 8.0. By means of confocal laser scanning microscopy and image processing, we quantified the extent of ordering, identified variables that controlled the assembly, and evaluated possible mechanisms to explain the orientational ordering. The quality of orientational order was quantified by direct calculation of the nematic order parameter (S). The dimensionless magnitude of the applied field was assessed by means of the Peclet number, a parameter that compares the time scale for rotational diffusion to the time scale for sedimentation. Experimental data showed that the effect of field strength on order parameter is a function that displays a complex dependence on aspect ratio. The scaling of the results for all aspect ratio with the equilibrium isotropic-nematic volume fraction suggested that the achievement of orientational order in the sediment is principally determined by its local volume fraction. Non- equilibrium effects, however, perturbed quantitative details of the dependence of S on the local sediment volume fraction from the known behavior for equilibrium rod suspensions.