Investigating correlation between fragility and intermolecular potential in glass-forming liquids

You have probably held an ice cream cone in hot temperatures, which made it melt and get on your fingers. But you probably have not wondered how and why it flows the way it does in varying temperatures. Having a technical understanding of the viscous flow of glassy materials is essential for many industries, especially glassblowing. But many of the physical parameters are still little understood.

Using the Krausser-Samwer-Zaccone (KSZ) model, which describes the viscosity of liquids in terms of physical parameters, Lunkenheimer et al. investigated nearly 30 different glass-forming liquids, including polymers, plastic crystals and oxide network liquids. In particular, the team used the model to determine the correlation between the fragility parameter, m, and the steepness of the interparticle repulsion of glass-forming liquids.

“We prove the concept that so-called strong liquids, where the viscosity varies only weakly with temperature and the fragility parameter m is small, have rather soft repulsive potentials, while for fragile liquids with high m, a hard-sphere like potential seems to apply,” said author Konrad Samwer. “We find that this is true for very different classes of liquids with almost all types of bonding characters.

The model showed a linear dependence of fragility on the steepness of the interatomic repulsive potential.

“For various classes of liquids, we needed sets of data over many decades in molecular mobility or viscosity and temperature ranges from the glass-transition temperature up to almost boiling temperature,” said Samwer. “The big surprise was how universal the linear correlation came across.”

While the research is focused on repulsive intermolecular interactions, the team is turning their attention to attractive, anharmonic interactions and their influence on other glass physics parameters.

Source: “Universal correlations between the fragility and interparticle repulsion of glass-forming liquids,” by Peter Lunkenheimer, Felix Humann, Alois Loidl, and Konrad Samwer, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0014457 .