(505g) Real-Space Characterization of the Evolution of Colloidal Gel Microstructure Under Steady Shear

Understanding the non-linear rheological response of colloidal gels to shear flow is critical in engineering complex fluids and relevant in a wide host of technologies including the food processing and ceramic industries. However, the complex interplay between the deformation of the microstructure due to shear and the subsequent effect on the rheological properties is poorly defined. In this study, the effect of steady shear flow on a density- and refractive index matched, depletion-induced colloidal gel system is directly visualized using confocal microscopy; the three dimensional image slice volumes thus obtained are analyzed using quantitative computational geometry based algorithms. Our results capture, for the first time in real space, local structural information during the evolution of the microstructure in response to applied shear. Three distinct flow regimes are identified from investigations in the flow-gradient plane. Initially, the spatial configuration of the gel changes to a predominant orientation along the extensional axis of flow - for large enough shear rates, this is followed by an unraveling of the gel backbone, forming extended linear domains. The continued application of shear flow ruptures the gel structure, producing disconnected clusters and chains that are translated primarily as a uniform plug along the flow direction. Finally, in the third regime, the steady shear flow induces cluster reaggregation resulting in the separation of the colloids into regularly alternating regions of high and low particle concentrations. Complementary studies in the flow-vorticity plane prior to gel rupture indicate the presence of heterogeneous flow profiles due to local rearrangement of the gel to accommodate the applied strain. Thus, temporal and spatial fluctuations in the velocity are observed in this plane contrary to the uniform velocity profile present in stable suspensions under shear. The results are compared with the response of gels formed by screening surface electrostatic charges and the key differences are highlighted.