Density fluctuations in the intermediate glass-former glycerol: A Brillouin light scattering study

Abstract

Brillouin scattering has been used to measure the dynamic structure factor of glycerol as a function of temperature from the high temperature liquid to the glassy state. Our investigation aims at understanding the number and the nature of the relaxation processes active in this prototype glass forming system in the high frequency region. The associated character of glycerol is reflected by a rather simple relaxations pattern, while the contributions coming from intra-molecular channels are negligible in the GHz frequency region. The temperature behavior of the characteristic frequency and lifetime of the longitudinal acoustic modes is analyzed, suggesting that a phenomenological model which only includes the structural (α) process and the unrelaxed viscosity is able to catch the leading contributions to the dynamics of the density fluctuations. This ansatz is also supported by a combined analysis of light and inelastic x-ray scattering spectra. The temperature dependence of the characteristic time of the α-process, τα, obtained by a full-spectrum analysis conforms to the α-scale universality, i.e. the values τα revealed by different experimental techniques are proportional the ones to the others. The non-erogodicity parameter smoothly decreases on increasing the temperature, and no signature of the cusp-like behavior predicted by the idealized mode coupling theory and observed in other glass-formers is found in glycerol.

Breakdown of the Debye approximation for the acoustic modes with nanometric wavelengths in glasses

“…2A. There, the data derived from the present experiment are compared with those obtained by using lower-frequency techniques (21)(22)(23). We observe that the sound velocity measured in the present experiment does not reach the corresponding macroscopic value, not even at the lowest probed q value of ∼1 nm −1 .…”

“…where-for both longitudinal and transverse branches-A, w, E * , and Q have to be considered simply as fitting parameters, and v ≡ v(E = 0) is the macroscopic sound velocity (21,27). This empirical model is found to describe well our data, as shown in Fig.…”

Time-domain Brillouin scattering for the determination of laser-induced temperature gradients in liquids

“…Note that the twofold change in Brillouin frequency with temperature in Fig. 2(c) is mainly due to the significant change in speed of sound over the examined temperature range (200 K -400 K) [22], rather than the change of the index of refraction which varies only a few percent over the same temperature range [18]. A slight Brillouin frequency shift of about 0.8 GHz in between our experimental data and the calculated calibration curve can be observed in Fig.…”

“…At such relatively low fluence, the overheating caused by the multiple laser pump pulses is moderate. (c) Temperature dependent Brillouin frequency in glycerol, from our measurements, and calculated from Comez et al [22] and Klieber et al [18]. The temperature calibration curves displayed in (c) can be used to estimate the absolute liquid temperature from the measured Brillouin frequency.…”

“…The experimental temperature calibration of the Brillouin frequency has been further fitted by a smooth polynomial function in order to extract an even more reliable temperature behavior of the Brillouin scattering frequency of glycerol under our experimental conditions at 395 nm probe wavelength and a normal incidence scattering angle. Since we did not have the possibility to record the Brillouin scattering frequency at higher temperatures than 320 K, we have used a calculated calibration curve from literature data obtained from Comez et al [22] and Klieber et al [18] as a reference function to link the Brillouin frequency to the absolute glycerol temperature. Note that the twofold change in Brillouin frequency with temperature in Fig.…”