  We examine microstructural and mechanical changes which occur during
oscillatory shear flow and reformation after flow cessation of an intermediate
volume fraction colloidal gel using rheometry and Brownian Dynamics (BD)
simulations. A model depletion colloid-polymer mixture is used, comprising of a
hard sphere colloidal suspension with the addition of non-adsorbing linear
polymer chains. Results reveal three distinct regimes depending on the strain
amplitude of oscillatory shear. Large shear strain amplitudes fully break the
structure which results into a more homogenous and stronger gel after flow
cessation. Intermediate strain amplitudes densify the clusters and lead to
highly heterogeneous and weak gels. Shearing the gel to even lower strain
amplitudes creates a less heterogonous stronger solid. These three regimes of
shearing are connected to the microscopic shear-induced structural
heterogeneity. A comparison with steady shear flow reveals that the latter does
not produce structural heterogeneities as large as oscillatory shear. Therefore
oscillatory shear is a much more efficient way of tuning the mechanical
properties of colloidal gels. Moreover, colloidal gels presheared at large
strain amplitudes exhibit a distinct nonlinear response characterized largely
by a single yielding process while in those presheared at lower rates a two
step yield process is promoted due to the creation of highly heterogeneous
structures.
