(178p) Sedimentation of a Rigid Particle in Yield Stress Fluids in Shear Flow

Yield stress fluids, composed of deformable particles jammed at volume fractions exceeding the random close packing of hard sphere suspensions, are widely used across consumer and industrial applications, including paints, pastes, and cosmetics, due to their distinctive flow behavior. In these formulations, rigid particles are often incorporated as functional additives. Nevertheless, the integration of rigid particles within yield stress fluids raises questions about the ability of the soft matrix to keep the rigid particle suspended when stresses are imposed on the system. When a rigid particle sediments, it can lead to phase separation and altered material properties. The dynamics of the rigid particles in these soft suspensions is a function of shear rates, volume fractions, and the pulling force on the rigid particle, which can be considered as particle weight. In this study, we use particle dynamics simulations to investigate the stability of suspensions under shear flow. Furthermore, we characterize the local flow behavior near the rigid particle and examine its sensitivity to shear rate and pulling force. The velocity streamlines show a typical shear flow pattern when the shear rate is high and pulling force is low and then transition to a channel-like pattern for the rigid particle to sediment through when the shear rate is low and the pulling force is high. Additionally, stability diagrams, which provide sedimentation conditions, are presented in terms of pulling force and shear rate for suspensions with varied volume fractions. By appropriately scaling the pulling force and shear rate, these volume fraction-dependent stability diagrams are collapsed onto a universal stability map which reveals the critical force threshold necessary to initiate sedimentation of the rigid particle in shear flow over a wide range of shear rates and volume fractions.¹ These results offer a novel approach to controlling and enhancing the stability of suspensions subjected to external forces.

References

1. ¹. Alrashdan, Rakan, and Fardin Khabaz. "Shear-driven stability of a rigid particle in yield stress fluids." Physics of Fluids10 (2024).