Experiments on periodically sheared colloidal suspensions with diffusion

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. However, in a colloidal system at finite temperature, Brownian motion ensures that no state is ever truly absorbed and simulations by Hexner et. al. have shown that a small amount of diffusion enhances the order of the structures near the critical strain. We built a compact rotational shear cell to drive Brownian colloidal suspensions to explore the effect of diffusion on structures (S(q)) and dynamics (mean squared displacement) simultaneously using a confocal microscope. From dynamical measurements, we see that the effective diffusion constant is equal to the self-diffusion of the particles below the transition and increases linearly with strain amplitude above. For a 20% volume fraction suspension, we see S(q) at small q in the strain direction drop by a factor of 3 from thermal equilibrium in the strain direction with hyperuniform scaling S(q) ~ q^0.5 while in the vorticity direction, there is no hyperuniform scaling and S(q) at small q only decreases by 50%.