Emergence of cooperative particle dynamics in quasi-two-dimensional glass-forming colloidal suspensions

As the concentration of particles in a glass-forming liquid increases, their dynamics slow down significantly, displaying solid-like behavior. This behavior is primarily attributed to cage formation, wherein particles are trapped by their neighbors. However, the particle dynamics near the onset of cage formation remain limitedly understood. In this study, we systematically investigated the dynamics of particles in a quasi-two-dimensional glass-forming colloidal suspension using a particle-level simulation. We utilized the "raspberry" model with a hybrid simulation approach. This approach combined lattice Boltzmann and molecular dynamics schemes for elucidating hydrodynamically interacting densely packed colloidal suspensions, with an area packing fraction of 0.45 <= phi <= 0.85. At a quiescent condition where particles underwent thermal motion, the string-like movements of particles became pronounced as phi increased. The hydrodynamic interactions between these particles were effective up to phi =0.6, wherein the string-like motion first appeared, but were mostly screened at higher phi values. Furthermore, we extended our analysis by imposing a small probing force, locally applied to the suspensions. The most significant response occurred at phi =0.6, where particles moved cooperatively during the cage formation process, similar to the experimental results of Li et al. [Nature 587, 225-229 (2020)]. By linking particle behaviors in two different scenarios, our study enhances our understanding of the emergence of highly cooperative particle movement and sheds light on the role of hydrodynamic interactions in glass-forming colloidal suspensions.