Shear Induced Alignment of Low Aspect Ratio Gold Nanorods in Newtonian Fluids

The flow-induced alignment of small gold nanorods ranging in aspect ratio from 2.4 to 4.2 in aqueous sucrose solutions is reported. Optical absorption spectra have been measured over a range of shear rates using polarized incident light in an optically transparent quartz Couette cell. The measured spectral changes are directly attributed to the shear-induced anisotropy in the suspension due to particle alignment that saturates at Péclet number of around 200. The measured optical changes are reversible, indicating that the nanorods do not undergo aggregation during measurement. Numerical simulations show that the spectral shifts are consistent with the rods flipping between extreme orientations of the Jeffery's orbits and that the effect of the Brownian motion on the gold nanorods cannot be ignored even at large Péclet number.