Evidence of dynamic crossover phenomena in water and other glass-forming liquids:experiments, MD simulations and theory

Abstract

In a recent quasi-elastic neutron scattering experiment on water confined in a Portland cementpaste, we find that this 3D confined water shows a dynamic crossover phenomenon atTL = 227 ± 5 K. The DSC heat-flow scan upon cooling and an independent measurement of specific heatat constant pressure of confined water in silica gel show a prominent peak at the sametemperature. We show in this paper that this type of behavior is common to many otherglassy liquids, which also show the crossover temperature in coincidence with thetemperature of a small specific heat peak. We also demonstrate with MD simulations thatthe dynamic crossover phenomenon in confined water is an intrinsic propertyof bulk water, and is not due to the confinement effect. Recently, an extendedversion of the mode coupling theory (MCT) including the hopping effect wasdeveloped. This theory shows that, instead of a structural arrest transition atTC predicted by the idealized MCT, a fragile-to-strong dynamic crossover phenomenon takes place insteadat TC, confirming both the experimental and the numerical results. The coherent and incoherentα relaxation times can be scaled with the calculated viscosity, showing thesame crossover phenomenon. We thus demonstrated with experiments,simulations and theory that a genuine change of dynamical behavior ofboth water and many glassy liquids happens at the crossover temperatureTL, which is 10–30% higher than the calorimetric glass transition temperatureTg.