Glassy dynamics in a liquid of anisotropic molecules: Bifurcation of relaxation spectrum

S Kumar and S Sarkar and B Bagchi, JOURNAL OF CHEMICAL PHYSICS, 160, 224501 (2024).

DOI: 10.1063/5.0210699

In experimental and theoretical studies of glass transition phenomena, one often finds a sharp crossover in dynamical properties at a temperature T-cr. A bifurcation of a relaxation spectrum is also observed at a temperature T-B approximate to T-cr; both lie significantly above the glass transition temperature. In order to better understand these phenomena, we introduce a new model of glass-forming liquids, a binary mixture of prolate and oblate ellipsoids. This model system exhibits sharp thermodynamic and dynamic anomalies, such as the specific heat jump during heating and a sharp variation in the thermal expansion coefficient around a temperature identified as the glass transition temperature, T-g. The same temperature is obtained from the fit of the calculated relaxation times to the Vogel-Fulcher-Tammann (VFT) form. As the temperature is lowered, the calculated single peak rotational relaxation spectrum splits into two peaks at T-B above the estimated T-g. Similar bifurcation is also observed in the distribution of short-to-intermediate time translational diffusion. Interrogation of the two peaks reveals a lower extent of dynamic heterogeneity in the population of the faster mode. We observe an unexpected appearance of a sharp peak in the product of rotational relaxation time tau(2) and diffusion constant D at a temperature T-cr, close to T-B, but above the glass transition temperature. Additionally, we coarse-grain the system into cubic boxes, each containing, on average, similar to 62 particles, to study the average dynamical properties. Clear evidence of large-scale sudden changes in the diffusion coefficient and rotational correlation time signals first-order transitions between low and high-mobility domains.

Return to Publications page