Shear Induced Particle Migration in Locally Driven Brownian Suspensions

Complex fluids include elements of intermediate size; elements which are larger compared to the molecules forming the embedding liquid. Among many, colloidal dispersions are probably the simplest example. When these fluids are driven out of their thermal equilibrium, hydrodynamic interactions and the deformed microstructure of the particle phase result in a wide range of phenomenology. While complex fluids are traditionally studied by imposing bulk forces on a macroscopic samples, here we explore the response of a colloidal dispersion to a local, microscopic driving; a microscale version of rheometry. In particular, we examine the response of a two dimensional layer of Brownian particles to a flow field induced by a circular motion of a probe particle driven by optical tweezers. We observe that particles migrate from high to low strain rate regions and form strong gradients in the density profiles. This non-homogeneity in the density profile is localized and the emerging length scale is set by the competition between the Brownian and shear forces. We further demonstrate that our measurements are quantitatively described, over a large range of strain rates and particle densities, by a phenomenological two phase fluid constitutive model previously discussed in the literature.