Nematic order in suspensions of colloidal rods by application of a centrifugal field

An applied field generated by centrifugation induced nematic order in colloidal rods of aspect ratio, r, varying from 3.6 to 8. The dependence of the order parameter, S, on dimensionless applied field strength was quantified by identifying the directors of the micron-sized poly(methyl methacrylate) rods in the suspension with confocal microscopy. The influence of the centrifugal field on orientational order was highly aspect ratio dependent. Order increased with field strength for r = 3.6, while it decreased with field strength for r = 8.0. All the measurements, however, were well correlated by phi/phi(nem), where phi(nem) is the volume fraction of the nematic phase boundary. The most significant ordering of the rods, corresponding to S approximate to 0.55 +/- 0.02, was measured at the highest achievable volume fractions of phi/phi(nem) approximate to 1.35 +/- 0.09. At the largest centrifugal field strengths, the volume fraction for all three aspect ratios studied converged to phi/phi(nem) approximate to 1.25 +/- 0.07. Comparison to previous theory and simulation of the isotropic-nematic transition of rod particles indicates that the field strengths required to generate nematic order were larger than would have been predicted under the assumption of local equilibrium in the sediments.