Relation between concentration fluctuations and dynamical heterogeneities in binary glass-forming liquids: A molecular dynamics simulation study

We perform molecular dynamics simulations of binary glass-forming liquids with high dynamical contrast. In detail, we mix water-like molecules with various polarities, which exhibit strongly different dynamics but very similar structures as neat liquids. It is found that these mixtures show concentration fluctuations, which strongly grow upon cooling towards an expected mixing-demixing phase transition. Because of the developing microphase segregation, the composition and temperature ranges accessible to our approach are limited, but the studied mixtures are ideal models to ascertain the relation between concentration fluctuations and molecular dynamics, in particular, the glassy slowdown of binary liquids, including aqueous solutions. We find that the dynamics of the components decouple upon cooling. While the dynamics of the slow component resembles that of neat glass formers, that of the fast component shows different features, including sub-linear diffusion on length and time scales beyond that of local particle cages and quasi-logarithmic decays of correlation functions. We show that this "anomalous dynamics" of the fast component receives a simple explanation based on a strong dependence of the particle mobility on the local concentration in combination with microphase segregation and dynamical contrast. Explicitly, spatially heterogeneous dynamics, which are more extended and tenacious than in neat glass formers, result from growing concentration fluctuations upon cooling. Due to a high dynamical asymmetry of the components at low temperatures, the slow species sustains the microphase segregation in the mixture, while the fast component moves along the concentration and mobility gradients, leading to remarkable dynamical features.