Glasses are non-equilibrium amorphous solids formed by rapidly cooling a liquid below the melting point. Physical aging is the study of how the properties of a system change over time as it relaxes toward equilibrium. Physical aging can be observed in glasses due to their non-equilibrium character. Aging is best studied just below the glass-transition, where relaxation is slow, but still fast enough to be observed. Physical aging, the glass transition and the puzzles of supercooled liquids are closely linked phenomena. In a series of papers [1-5], we have shown how physical aging in organic liquids close to their glass transition temperature can be described by a single-parameter scenario through very precise measurements and careful temperature protocols. We have moreover demonstrated that the non-linear aging can be predicted from the equilibrium relaxation (or fluctuations) in the molecular liquids VEC and NMEC as well as in computer simulations of a binary Lennard-Jones liquid [5]. We conjecture that these findings may be general for supercooled liquids.
The close connection between physical aging of glasses and equilibrium relaxation in liquids calls for a joint understanding of the aging rate and the equilibrium relaxation rate.
The aging rate of glasses has traditionally been modeled as a function of temperature, T, and fictive temperature, while density, ρ, is not explicitly included as a parameter. However, this description does not naturally connect to the modern understanding of what governs the relaxation rate in equilibrium. In equilibrium, it is well known that the relaxation rate, γeq, depends on temperature and density. In addition, a large class of systems obeys density scaling, which means the rate specifically depends on the scaling parameter, Γ= e (ρ)/T, where e(ρ) is a system specific function.
Here we present a conjecture on how density scaling may be generalized to describe aging [1]. Moreover, we discuss how this connects to single parameter aging [1-5] and other recent related results in literature [4,5].
References
[1] T. Hecksher, N. B Olsen, K. Niss & J. C. Dyre, J. Chem. Phys., 133, 174514 (2010).
[2] T. Hecksher, N. B. Olsen & J. C. Dyre, PNAS, 116, 16736-16741 (2019)
[3] L. A. Roed, T. Hecksher, J. C. Dyre & K. Niss, J. Chem. Phys., 150, 044501 (2019)
[4] K. Niss, J. C. Dyre & T. Hecksher, J. Chem. Phys., 152, 041103 (2020)
[5] B. Riechers, L. A. Roed, S. Mehri, T. S. Ingebrigtsen, T. Hecksher, J. C. Dyre & K. Niss, Sci. Adv., 8, 9809 (2022)
[6] K. Niss, J. Chem. Phys., 157, 054503 (2022).
[7] T. Hecksher and K. Niss, arXiv:2408.12401 (2024).
[8] V. Di Lisio, and V.-M. Stavropoulou, and D. Cangialosi, , J. Chem. Phys., 064505 159 (2023).
[9] M. Hénot, X. A. Nguyen, and F. Ladieu, J. Phys. Chem. Lett., 15, 11 (2024).