Evolution of the structure in a soft binary colloidal mixture during thermodynamic processes of cooling and heating.

Abstract

The study of the structural evolution of a material under equilibrium or nonequilibrium thermodynamic conditions is fundamental for understanding its stability and predicting its phase behavior. To the best of our knowledge, the structural transformations induced by different temperature protocols have not been fully understood. This study provides a detailed molecular resolution of the structural evolution occurring in a bidisperse colloidal mixture of soft spheres, as it is subjected to a sequence of controlled thermodynamic processes of heating and cooling. The structural transformations are studied between two equilibrium configurations at different temperatures through extensive molecular dynamics simulations. By exploring the interplay of multiple length and time scales, we uncover how these protocols influence the progression of the colloidal suspension toward thermodynamic equilibrium. Our results show that under fast temperature changes, heating and cooling processes follow distinct thermodynamic pathways toward the corresponding equilibrium configuration because of the emergence of different structural mechanisms, which are discussed here in detail; these distinct pathways are defined as thermodynamic asymmetries that depend strongly on the temperature protocol and the composition of the dispersion. In contrast, for sufficiently slow temperature changes, we identify the condition under which both protocols follow symmetric and reversible pathways.