Transport properties of glass-forming liquids suggestthat dynamic crossover temperature is as importantas the glass transition temperature

It is becoming common practice to partition glass-forming liquids into two classes based on the dependence of the shear viscosity eta on temperature T. In an Arrhenius plot, ln eta 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 eta(x), where eta(x) has the same value, eta(x) approximate to 10(3) Poise, for all 84 liquids. The crossover temperature, T-x, 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-x to Arrhenius behavior below T-x. We also show that below T-x the familiar Stokes-Einstein relation D/T similar to eta(-1) breaks down and is replaced by a fractional form D/T similar to eta(-zeta), with zeta approximate to 0.85.