(138a) Microstructural Evolution of Dilute Colloidal Gels during Shear Startup

Understanding the microstructural response of colloidal gels to transient shear flow is important for engineering their applications in the food, ceramic, pharmaceutical, and oil industries, to name a few. From a scientific standpoint, gel flow involves a complex interplay between the tenuous, nonequilibrium microstructure of these systems with the macro- and micro-scale forces at play, making predictions of the expected behavior extremely challenging. In this talk, our experimental and computational efforts in trying to understand this problem will be presented. Our experiments are performed on a density- and refractive index matched depletion-induced dilute colloidal gel, whose microstructural response to shear startup is directly visualized using fast-scanning confocal microscopy and quantified using algorithms of computational geometry. 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 stages of microstructural evolution are identified from investigations in the flow-gradient plane, including unraveling of the tenuous gel backbone and orientation of extended linear chains along the extensional axis of flow; gel rupture producing disconnected clusters and chains that are translated primarily as a uniform plug along the flow direction; and cluster reaggregation resulting in the separation of the colloids into regularly alternating regions of high and low particle concentrations, a phenomenon that is consistent with log-rolling or vorticity-alignment observed by other researchers. To examine whether hydrodynamic interactions play a critical role in the observed behavior, results of Multiple Particle Collision (MPC) dynamics simulations of dilute colloidal gels in shear startup will be presented. Our simulations are able to capture the gross features of the observed experimental behavior without the inclusion of hydrodynamic interactions. Finally, the implications of this behavior for the nonlinear rheology of dense colloidal gels will be discussed.