Communication: High pressure specific heat spectroscopy reveals simple relaxation behavior of glass forming molecular liquid

Density scaling and isodynes in glycerol-water mixtures

This paper presents dielectric and neutron spectroscopy data on two different glycerol-water mixtures at elevated pressures. Glycerol-water liquid mixtures have a high concentration of hydrogen bonds which usually is expected to lead to complex dynamics. However, with regard to the pressure dependence of the dynamics we reveal a surprisingly simple picture. Different aspects of the dynamics have the same pressure dependence, in other words the phase diagram of the liquids have so-called isodynes, density scaling is also observed to hold reasonably well and there is even some reminiscence of isochronal superposition. This suggests that these aspect of liquid dynamics are very general and hold for different types of intermolecular interactions.

Time-scale ordering in hydrogen- and van der Waals-bonded liquids

The time scales of structural relaxation are investigated on the basis of five different response functions for 1,2, 6-hexanetriol, a hydrogen-bonded liquid with a minor secondary contribution, and 2,6,10,15,19,23-hexamethyl-tetracosane (squalane), a van der Waals-bonded liquid with a prominent secondary relaxation process. Time scales of structural relaxation are derived as inverse peak frequencies for each investigated response function. For 1,2,6-hexanetriol, the ratios of the time scales are temperature-independent, while a decoupling of time scales is observed for squalane in accordance with the literature. An alternative evaluation approach is made on the squalane data, extracting time scales from the terminal relaxation mode instead of the peak position, and in this case, temperature-independent time-scale ratios are also found for squalane, despite its strong secondary relaxation contribution. Interestingly, the very same ordering of response-function-specific time scales is observed for these two liquids, which is also consistent with the observation made for simple van der Waals-bonded liquids reported previously [Jakobsen et al., J. Chem. Phys. 136, 081102 (2012)]. This time-scale ordering is based on the following response functions, from fast to slow dynamics: shear modulus, bulk modulus, dielectric permittivity, longitudinal thermal expansivity coefficient, and longitudinal specific heat. These findings indicate a general relation between the time scales of different response functions and, as inter-molecular interactions apparently play a subordinate role, suggest a rather generic nature of the process of structural relaxation.

Perspective: Searching for simplicity rather than universality in glass-forming liquids

This article gives an overview of experimental results on dynamics in bulk glass-forming molecular liquids. Rather than looking for phenomenology that is universal, in the sense that it is seen in all liquids, the focus is on identifying the basic characteristics, or "stylized facts," of the glass transition problem, i.e., the central observations that a theory of the physics of glass formation should aim to explain in a unified manner.

Slow rheological mode in glycerol and glycerol–water mixtures

Glycerol–water mixtures were studied at molar concentrations ranging from x_gly = 1 (neat glycerol) to x_gly = 0.3 using shear mechanical spectroscopy.