2025 AIChE Annual Meeting

(588cg) Effect of Mechanical History on Microstructure and Dynamics of Jammed Suspensions of Soft Particles in Start up Flow

Mechanical aging in jammed suspensions of soft particles is governed by internal stresses and microstructural interactions and is essential for designing products with the goal of long-term stability in packaging, product efficiency and product functionality. This phenomenon is linked to the evolution of microstructure, and hence, large internal stress correlates with the dynamics of clusters of particles without any applied external flow underscoring the connection between mechanical aging and suspension stability. Slowly relaxing internal stresses affect rheological measurements, including viscoelastic moduli and yield stress determinations. The emergence of internal stress in these materials is a result of particle jamming and deformation, with interactions occurring through elastic contacts. In this work, we use a unique protocol to study the effect of pre-shear directionality and relaxation time on start up flow. In this protocol, the suspension is pre-sheared in the forward or backward direction, and the system is subsequently subjected to zero shear rate to relax the internal stress for a duration of tw before the final start up flow phase. Start up flow is then performed in backward-forward (-+) (pre-shear in the backward direction and start up in the forward direction) or forward-forward (++) directions (pre-shear and start up in the forward direction). Our start-up flow simulations show that suspensions retain their memory in start up flow with preshear directionality and different waiting times influencing the stress response and the microstructure of the suspensions. The transient stress response after flow cessation shows that the location and magnitude of the stress overshoot is a function of memory of the suspensions from the preshear stage. The microstructures of the suspensions for the two different pre-shear protocols are uniquely different since the preshearing protocols create microstructures with opposite extension and compression axes but converge to an identical structure at a steady state. Furthermore, the results will elucidate the effect of the directionality of the pre-shear condition and waiting time during the stress relaxation on the dynamics of suspensions in the final start up stage.