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Direct observation of oscillatory-shear-induced order in colloidal suspensions

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Original languageEnglish
Pages (from-to)6859-6864
Number of pages6
JournalPhysical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics
Issue number6
Publication statusPublished - Jun 1998


We use optical microscopy to directly study ordering in concentrated colloidal suspensions of model hard spheres subjected to oscillatory shear in a parallel. plate geometry. At high strain amplitude the particles order quickly into hexagonal planes in the plane of shear, oriented so that a close-packed direction is parallel to the velocity axis, corresponding to previous scattering observations in other geometries. At low strain a polycrystalline structure forms, consisting of hexagonally ordered regions now with a distribution of different orientations, peaked around an orientation where a close-packed direction is now perpendicular to the velocity axis. Such polycrystallinity explains the evidence of disorder previously noted in scattering patterns. The ordered regions grow until "grains" fill the sample. Various examples of local disorder are observed,in the shear-ordered structures, including vacancies, dislocations, and stacking faults. The observed.motion of the planes during the cycle is complicated, but there is some evidence of a "zigzag" motion of planes at high strain, as has been proposed in previous studies.

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