Small angle X-ray scattering (SAXS) with in-situ shear was employed to study the ordering of gold nanorod dispersions in wormlike micelle solutions (WLM) formed by the surfactant cetylpyridinium chloride (CPyCl) and sodium salicylate salt (NaSal). Above a threshold CPyCl concentration, but below the isotropic-to-nematic transition of the micelles, the nanorods self-assembled under quiescent conditions into isotropically oriented hexagonal domains. Under gentle shear, these assemblies acquired orientational order where the hexagonal layers stacked coincident with the flow-vorticity (v, ω) plane. At higher CPyCl concentrations, in the nematic micellar phase, the sheared rods formed smectic-like layers in the gradient-vorticity (Δ, ω) plane. At even higher surfactant concentrations, where the micelles are deep within the hexagonal phase, the nanorods showed no translational but nematic-like order due to flow alignment. Depletion forces mediated by the WLMs are identified as the driving mechanism for these ordering behaviors, suggesting a novel mechanism for controlled, reconfigurable assembly of nanoparticles in solutions.
Study performed in the Department of Physics and Astronomy, Johns Hopkins University with Robert Leheny, Henry A. Rowland, and in collaboration with Brookhaven national laboratory.
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