Atypical Violation of the Stokes–Einstein Relation in a Dense Binary Lennard-Jones Mixture

We study the dynamics of particles in binary mixtures near the freezing transition using molecular dynamics simulations. The particles are considered to interact via a Lennard–Jones potential, and the impact of varying their size-ratio on their dynamics is examined. By calculating the mean-squared displacements and the self-intermediate scattering function of the particles, we find that introducing size disparity in an equimolar mixture at a constant packing fraction hinders particle movement, leading to a decrease in the self-diffusion coefficient. Additionally, as the size disparity increases, the local cage relaxation time becomes longer. Interestingly, the increase in the system’s viscosity does not correspond to an expected decrease in self-diffusion, resulting in an unusual violation of the Stokes–Einstein relation. Unlike typical glass-forming mixtures, where this violation parameter increases as temperature decreases, we observe the opposite behaviour.