| 1. |
- Elamin, Khalid, 1977, et al.
(author)
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Long-range diffusion in xylitol-water mixtures
- 2013
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 117:24, s. 7363-7369
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Journal article (peer-reviewed)abstract
- Dynamic light scattering (DLS) and small-angle neutron scattering (SANS) were employed to study mixtures of xylitol and water. The results were also related to a previous dielectric relaxation study on the same system. In the temperature range of the DLS measurements the viscosity related structural (α) relaxation is too fast to be observed on the experimental time scale, but a considerably slower exponential and hydrodynamic relaxation process is clearly observable in the polarized light scattering data. A similar ultraslow process has been observed in many other types of binary liquids and commonly assigned to long-range concentration or density fluctuations. In some studies this interpretation has been supported by observations of substantial structural inhomogeneities in static light scattering or SANS experiments. However, in this study we observe such an ultraslow process without any indication of structural inhomogeneities on length-scales above 2 nm. Hence, we suggest that our observed ultraslow process is due to long-range diffusion of single xylitol molecules or small clusters of a few xylitol molecules (and perhaps some associated water molecules) which are randomly dispersed and sufficiently small to not be structurally detected in our SANS study. In the q-range of the DLS measurements this ultraslow relaxation process is around room temperature several orders of magnitude slower than the structural α-relaxation. However, if its 1/q2-dependent relaxation time is extrapolated to q-values where relaxation times from dielectric spectroscopy and quasielastic neutron scattering are compatible (about 10 nm-1), a relaxation time similar to that of the dielectric α-relaxation is obtained. Thus, the large difference in time scale between the two relaxation processes in the q-range of a DLS study is due to the fact that the α-relaxation is cooperative in nature, rather than caused by long-range single particle diffusion, and thus q-independent at low q-values.
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| 2. |
- Izzo, M. G., et al.
(author)
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Rayleigh scattering and disorder-induced mixing of polarizations in amorphous solids at the nanoscale: 1-octyl-3-methylimidazolium chloride glass
- 2020
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In: Physical Review B. - 2469-9969 .- 2469-9950. ; 102:21
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Journal article (peer-reviewed)abstract
- Acousticlike excitations in topologically disordered media at mesocale/nanoscale present anomalous features with respect to the Debye's theory. The so-called Rayleigh scattering manifests in a strong increase of the attenuation of the acousticlike excitations and a softening of the phase velocity with respect to its continuum limit value. Mean field models developed in the random media theory framework can successfully predict the occurrence, at the proper length scale, of Rayleigh scattering. The overall attenuation in the Rayleigh region is, however, underestimated. In the framework of random media theory we developed an analytical model, which permits a quantitative description of the acousticlike excitations in topological glasses in the whole first pseudo-Brillouin zone. The underestimation of the Rayleigh scattering is avoided and, importantly, the model allows to account also for the polarization properties of the acousticlike excitations. In a three-dimensional medium an acoustic wave is characterized by its phase velocity, intensity, and polarization. Rayleigh scattering emphasizes how the topological disorder affects the first two properties. The topological disorder is, however, expected to influence also the third one. In common with the Rayleigh scattering, hallmarks possibly related to the mixing of polarizations have been traced in different classes of amorphous solids at nanoscale. The quantitative theoretical approach developed permits to demonstrate how the mixing of polarizations generates a distinctive feature in the dynamic structure factor of amorphous solids. The modeling capability of the proposed mean field theory is tested on glassy 1-octyl-3-methylimidazolium chloride, whose spatial distribution of the elastic moduli is well assessed and can be experimentally characterized. Contrast between theoretical and experimental features for the selected glass reveals an excellent agreement. The mean field approach we present retains a certain degree of generality and can be possibly extended to different stochastic media or different wave fields.
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| 3. |
- Izzo, Maria G., et al.
(author)
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The Mixing of Polarizations in the Acoustic Excitations of Disordered Media With Local Isotropy
- 2018
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In: Frontiers in Physics. - : Frontiers Media SA. - 2296-424X. ; 6
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Journal article (peer-reviewed)abstract
- An approximate solution of the Dyson equation related to a stochastic Helmholtz equation, which describes the acoustic dynamics of a three-dimensional isotropic random medium with elastic tensor fluctuating in space, is obtained in the framework of the Random Media Theory. The wavevector-dependence of the self-energy is preserved, thus allowing a description of the acoustic dynamics at wavelengths comparable with the size of heterogeneity domains. This in turn permits to quantitatively describe the mixing of longitudinal and transverse dynamics induced by the medium's elastic heterogeneity and occurring at such wavelengths. A functional analysis aimed to attest the mathematical coherence and to define the region of validity in the frequency-wavector plane of the proposed approximate solution is presented, with particular emphasis dedicated to the case of disorder characterized by an exponential decay of the covariance function.
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