Impact of Hydrodynamic Interactions on Structural Relaxations in Dense Colloidal Suspensions: Helping the Cage Breakage

Increasing the concentration of colloidal suspensions reduces the influence of the long-range parts of hydrodynamic interactions (HIs) among colloidal particles. Due to this reduction, Brownian dynamics simulations without HIs are often assumed to approximate the dynamical behavior of highly dense colloidal suspensions. However, the validity of this assumption remains unestablished. This study numerically investigates the impact of HIs on the relaxation dynamics of dense colloidal suspensions. Our findings reveal that the structural relaxation dynamics without HIs are significantly slower than with HIs. Additionally, this study explores the qualitative differences underlying the observed quantitative distinctions in the relaxation dynamics with and without HIs. Without HIs, the motion of a tagged particle is accompanied by small backflow motions in the surrounding particles ahead of it. These backflows reflect a self-blocking effect, resulting in a hindrance of cage breakage. In contrast, with HIs, such vortex formation is notably reduced; instead, the surrounding solvent flows along with the motion of the tagged particle, pushing adjacent particles further. The cooperative motions of these particles, along with the solvent flow, facilitate the escape of the tagged particle from its cage.