title: Transport properties of glass-forming liquids suggest that dynamic crossover temperature is as important as the glass transition temperature.
abstract: It is becoming common practice to partition glass-forming liquids into two classes based on the dependence of the shear viscosity η on temperature T. In an Arrhenius plot, ln η vs 1/T, a strong liquid shows linear behavior whereas a fragile liquid exhibits an upward curvature [super-Arrhenius (SA) behavior], a situation customarily described by using the Vogel-Fulcher-Tammann law. Here we analyze existing data of the transport coefficients of 84 glass-forming liquids. We show the data are consistent, on decreasing temperature, with the onset of a well-defined dynamical crossover η(×), where η(×) has the same value, η(×) ≈ 10(3) Poise, for all 84 liquids. The crossover temperature, T(×), located well above the calorimetric glass transition temperature T(g), marks significant variations in the system thermodynamics, evidenced by the change of the SA-like T dependence above T(×) to Arrhenius behavior below T(×). We also show that below T(×) the familiar Stokes-Einstein relation D/T ∼ η(-1) breaks down and is replaced by a fractional form D/T ∼ η(-ζ), with ζ ≈ 0.85.
authors.affiliation: Dipartimento di Fisica and Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Università di Messina, I-98166 Messina, Italy. francesco.mallamace@unime.it
authors.forename: Francesco
authors.initials: F
authors.lastname: Mallamace
authors.forename: Caterina
authors.initials: C
authors.lastname: Branca
authors.forename: Carmelo
authors.initials: C
authors.lastname: Corsaro
authors.forename: Nancy
authors.initials: N
authors.lastname: Leone
authors.forename: Jeroen
authors.initials: J
authors.lastname: Spooren
authors.forename: Sow-Hsin
authors.initials: SH
authors.lastname: Chen
authors.forename: H Eugene
authors.initials: HE
authors.lastname: Stanley
mesh_terms: D000465:Algorithms; D002151:Calorimetry; D005898:Glass / Q000737:chemistry*; D044367:Phase Transition; D012997:Solvents / Q000737:chemistry*; D013816:Thermodynamics; D044366:Transition Temperature; D014783:Viscosity
chemical_list: D012997:Solvents