Heterogeneous dynamics, ageing, and rejuvenating in van der Waals liquids
It has been shown over the past ten years that the dynamics close to the glass transition is strongly heterogeneous: fast domains coexist with domains three or four decades slower, the size of these regions being about 3 nm at Tg. The authors extend here a model that has been proposed recently for the glass transition in van der Waals liquids. The authors describe in more details the mechanisms of the α relaxation in such liquids. It allows then to interpret physical ageing in van der Waals liquids as the evolution of the density fluctuation distribution towards the equilibrium one. The authors derive the expression of macroscopic quantities (volume, compliance, etc.). Numerical results are compared with experimental data (shape, times to reach equilibrium) for simple thermal histories (quenches, annealings). The authors explain the existence of a “Kovacs memory effect” and the temporal asymmetry between down jump and up jump temperatures experiments, even for systems for which there is no energy barriers. Their model allows also for calculating the evolution of small probe diffusion coefficients during ageing.