Abstract
Impulsive stimulated thermal scattering (ISTS) allows one to access the structural relaxation dynamics in supercooled molecular liquids on a time scale ranging from nanoseconds to milliseconds. Till now, a heuristic semi-empirical model has been commonly adopted to account for the ISTS signals. This model implicitly assumes that the relaxation of specific heat, C, and thermal expansion coefficient, γ, occur on the same time scale and accounts for them via a single stretched exponential. This work proposes two models that assume disentangled relaxations, respectively, based on the Debye and Havriliak-Negami assumptions for the relaxation spectrum and explicitly accounting for the relaxation of C and γ separately in the ISTS response. A theoretical analysis was conducted to test and compare the disentangled relaxation models against the stretched exponential. The former models were applied to rationalize the experimental ISTS signals acquired on supercooled glycerol. This allows us to simultaneously retrieve the frequency-dependent specific heat and thermal expansion up to the sub-100MHz frequency range and further to compare the fragility and time scale probed by thermal, mechanical, and dielectric susceptibilities.
Highlights
The intriguing behavior of glass-forming liquids is the object of a continuous research effort.[1–9] By virtue of its ability to simultaneously probe multiple relaxation processes, such as thermal expansion, acoustic, and orientational response,[10,11] the use of impulsive stimulated thermal scattering (ISTS) in a periodical grating geometry allowed gaining new insights from the thermoelastic response to impulsive photothermal excitation in several glass formers.[11–17] Standard thermo-mechanical modeling,[18] based on the assumption of frequency-independent specific heat and thermal expansion coefficient, has been shown to be inadequate to characterize the dynamics triggered in an Impulsive stimulated thermal scattering (ISTS) experiment, this being especially true for viscous systems
Satisfactory fitting quality was achieved for all ISTS waveforms, supporting the validity of the models developed in this work
We were able to investigate the relaxation of C and γ, up to several tens of MHz, greatly extending the upper frequency bound so far achieved by thermal susceptibility spectroscopy, by nearly 3 and 7 decades for C(ω) and γ(ω), respectively
Summary
The intriguing behavior of glass-forming liquids is the object of a continuous research effort.[1–9] By virtue of its ability to simultaneously probe multiple relaxation processes, such as thermal expansion, acoustic, and orientational response,[10,11] the use of impulsive stimulated thermal scattering (ISTS) in a periodical grating geometry allowed gaining new insights from the thermoelastic response to impulsive photothermal excitation in several glass formers.[11–17] Standard thermo-mechanical modeling,[18] based on the assumption of frequency-independent (non-relaxing) specific heat and thermal expansion coefficient, has been shown to be inadequate to characterize the dynamics triggered in an ISTS experiment, this being especially true for viscous systems. Scitation.org/journal/jcp expansion rise of the ISTS signal, proved effective in describing the ISTS response of glycerol, salol, and DC704 oil.[12,14,18] The latter model accounts for the concomitant volume and temperature change upon system’s heating. The procedure allows retrieving C(ω) and γ(ω) all the way to the sub-100 MHz range This largely extends the upper limit of the previously accessible bandwidth, 100 kHz20 and 1 Hz22,24 for C and γ, respectively, enabling a comparison of relaxation times and fragility values from the obtained heat capacity and thermal expansion with those of mechanical, and dielectric susceptibilities in an extended frequency and temperature range. III, a continuum mechanics thermoelastic model is used to calculate the response of the material strain to photothermal excitation This is achieved by considering the temperature change as a source term in the equation of motion. We compare in detail the thermal, mechanical, and dielectric relaxation for glycerol
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