An experimental critique on the existence of fragile-to-strong transition in glass-forming liquids

A number of recent studies in the literature have reported the existence of unusual temperature dependence of the shear viscosity for a variety of glass-forming liquids that cannot be described with a single fragility index. Rather, these liquids display an apparent fragile-to-strong transition (FST) with lowering of temperature that was suggested to be indicative of a structural transition. In the present work we critique the accuracy of the high temperature oscillation viscometry technique that was used almost exclusively for viscosity measurements of these liquids above their liquidus temperature. Viscosity measurements are carried out on binary Ge-Se chalcogenide liquids in their supercooled and stable states using conventional capillary and parallel-plate rheometry techniques and are compared with the oscillation viscometry data in the literature. Such comparisons conclusively demonstrate that the latter measurement technique often underestimates the viscosity by nearly an order of magnitude, especially at high temperatures. These results, when taken together, indicate that the observation of FST in glass-forming liquids is likely an artifact of the inaccuracy of the oscillation viscometry technique. Moreover, these results cast serious doubt on the reliability of the high-temperature oscillation viscometry data for metals and alloys that are prevalent in the literature.