Colloidal suspensions, in which particles ranging from tens of nanometers to a few microns are dispersed in a liquid, are the basis of many industrial applications, including paints, foods, cosmetics, and pharmaceuticals. Their macroscopic dynamic behavior, especially under external flows or stresses, is strongly influenced by the microscopic motion of the suspended particles and the interactions among them. Hydrodynamic interactions (HIs), which are mediated by solvent flows, often play a crucial role.

In dilute suspensions, HIs are well described by the Oseen tensor, which captures long-range interactions that decay as 1/r,where r is the interparticle distance. However, as the concentration increases, particle crowding progressively screens these long-range interactions. Therefore, based on the longstanding assumption that HIs can be effectively neglected in sufficiently dense systems, Brownian dynamic simulations that ignore solvent-mediated hydrodynamic effects have been used to model the dynamics of concentrated suspensions. However, the assumption has not been sufficiently validated.

In this study, comparative numerical simulations were conducted for a densely packed colloidal suspension, almost in the supercooled regime, with and without HIs. The inclusion of HIs significantly accelerated relaxation dynamics, representing not only a quantitative shift in the timescale but also a qualitative change in the underlying kinetic pathways.

In highly concentrated systems, particles are confined in “cages” formed by neighboring particles, undergoing extended localized rattling. Without HIs, a probe particle attempting to move forward directly pushes against its nearest neighbors (ahead of the probe), which then pushes the next layer of particles. In turn, the outer particles exert pushback forces on the inner layers, and subsequently on the probe particle. These interactions induce an elastic response that restores the particle to its original position, reinforcing the cage effect.

In contrast, with HIs, the motion of the probe particle first pushes the surrounding solvent, generating solvent flows that induce motion in neighboring particles via lubrication forces. These neighbors, in turn, further drive the solvent flows, producing a consecutive propagation of flow-induced motion. This solvent-mediated pathway suppresses the elastic restoration response and weakens the cage confinement, thereby facilitating more cooperative particle rearrangements and accelerating structural relaxation. These results demonstrate that near-field HIs can propagate their influence over substantial distances through dynamic coupling between the particles and solvent. Thus, HIs can play a crucial role even in highly dense systems.

This study re-examines the longstanding assumption that HIs are effectively negligible in dense suspensions and highlights their substantial impact on dynamic behavior. This emphasizes the need to reconsider solvent-mediated HIs, particularly when designing materials and technologies based on dense colloidal suspensions.

(written by Akira Furukawa on behalf of all authors)

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