(317c) Yield in Colloidal Gels Under the Start-up Shear Flow: Role of Hydrodynamic Interactions

When a colloidal gel is subjected to external stress, it shows a solid-like to liquid-like transition, which is called yield behavior. Yield in colloidal gels was traditionally viewed as a one-step process. Applied stress ruptures the bonds between the particles and tears them apart from the particle network structure. However, recent studies revealed that gels can undergo yielding in two steps, depending on the particle volume fraction, bond strength, and flow strength. Our research aims to understand the physical origins that result in two-step yield in colloidal gels. We leverage a novel method, which we call Massively Parallelized Accelerated Stokesian Dynamics (MPASD), to describe large-scale colloidal gels where the particle dynamics are coupled by many-body hydrodynamic interactions and lubrication interactions. This approach enables us to identify the underlying mechanism of the two-step yield in colloidal gels subjected to the shear. We reveal that the primary yield occurs with a little loss of particle bonds, while the particle network structure remains intact during yielding. Moreover, we reproduce the pronounced shear stress peak during the secondary yield, which is not captured in the prior simulation studies neglecting hydrodynamics. In addition to the stress peak, a detailed interrogation of the bond dynamics and structural change guides us to demonstrate the role of lubrication interactions during secondary yield: Shear flow brings many particles close together, after which strong lubrication interactions play a role to make the particles hard to separate and result in the pronounced stress peak. Our work can not only advance the understanding of the underlying mechanism of two-step yield in colloidal gels, but also provide an avenue to develop an optimized formulation in cosmetics, inks, and food products, where two-step yielding can be useful.