Structural Anisotropy in Periodically Sheared Colloidal Suspensions

Periodically sheared dilute, non-Brownian suspensions explore new configurations through collisions in an otherwise reversible flow. Below a critical strain, the particles remain active until they find a configuration with no collisions and reach an absorbing state. Recent simulations by Hexner and Levine have shown that the configuration of particles in the critically absorbing state is hyperuniform. The structure factor ( S(q) ) of a hyperuniform system goes to zero at q=0 (as opposed to a constant positive value for the same suspension away from the critical state). We built a compact, uni-axial shear cell in order to shear colloidal suspensions while using small-angle light scattering to measure S(q) from angles of 0.03\degree to 1.5\degree. We observe a strong anisotropy in S(q) due to the anisotropy of the imposed strain. For a suspension of volume fraction 0.25, the critical strain is near 2; the structure factor along the shear does not decrease while S(q) along the vorticity direction decreases by almost an order of magnitude from the structure at thermal equilibrium.