Models Of Supercooled Liquids Research Articles

Assessing the structural heterogeneity of supercooled liquids through community inference.

We present an information-theoretic approach inspired by distributional clustering to assess the structural heterogeneity of particulate systems. Our method identifies communities of particles that share a similar local structure by harvesting the information hidden in the spatial variation of two- or three-body static correlations. This corresponds to an unsupervised machine learning approach that infers communities solely from the particle positions and their species. We apply this method to three models of supercooled liquids and find that it detects subtle forms of local order, as demonstrated by a comparison with the statistics of Voronoi cells. Finally, we analyze the time-dependent correlation between structural communities and particle mobility and show that our method captures relevant information about glassy dynamics.

The shoving model for the glass-former LiCl·6H2O: A molecular dynamics simulation study

Abstract Molecular dynamics (MD) simulations of LiCl·6H2O showed that the diffusion coefficient D, and also the structural relaxation time , follow a power law at high temperatures, D−1 ∝ (T − To)−μ, with the same experimental parameters for viscosity (To = 207 K, μ = 2.08). Decoupling between D and occurs at Tx ∼ 1.1To. High frequency acoustic excitations for the LiCl·6H2O model were obtained by the calculation of time correlation functions of mass current fluctuations. The temperature dependence of the instantaneous shear modulus, G∞(T), was considered in the shoving model for supercooled liquids [J.C. Dyre, T. Christensen, N.B. Olsen, J. Non-Cryst. Solids 352 (2006) 4635] resulting in a linear relationship log (D−1) vs. G∞/T.

Thermal and dielectric studies of 2,2’-dihydroxybenzophenone

Thermal and dielectric properties of 2,2’-dihydroxybenzophenone were studied in relation with the potential progress of crystal nucleation and growth below the ordinary glass transition temperature, Tgα. Differential scanning calorimetry was carried out in a range 100-350 K. The α glass transition was found to occur at Tgα=239 K. Crystallization and fusion were observed to take place when the sample was cooled down to 103 K, but not observed when cooled to 203 K. Crystal nucleation was interpreted as having happened during annealing for a short time at 103 K which is much below the Tgα. Heat capacities were measured in a range 7-350 K by an intermittent heating method with an adiabatic calorimeter. The temperature, enthalpy and entropy of fusion were determined to be 334.46 K, 20.07 kJ mol-1 and 60.01 J K-1mol-1, respectively. Crystal growth was found to proceed even at 220 K below the Tgα, but no glass transition was detected below 220 K. Dielectric losses were measured in a temperature range of 100-250 K and a frequency range of 30Hz-10 kHz. β-Relaxation process was found dielectrically with the activation energy of 22.6 kJ mol-1, and the corresponding glass transition was expected to occur at 76.9 K. It is discussed, based on the “structurally ordered clusters aggregation” model for supercooled liquids and glasses, that the β process is potentially attributed to the crystal nucleation progressing at 103 K.

Glass and polycrystal states in a lattice spin model

We numerically study a nondisordered lattice spin system with a first order liquid–crystal transition, as a model for supercooled liquids and glasses. Below the melting temperature the system can be kept in the metastable liquid phase, and it displays a dynamic phenomenology analogous to fragile supercooled liquids, with stretched exponential relaxation, power law increase of the relaxation time, and high fragility index. At an effective spinodal temperature Tsp the relaxation time exceeds the crystal nucleation time, and the supercooled liquid loses stability. Below Tsp liquid properties cannot be extrapolated, in line with Kauzmann’s scenario of a lower metastability limit of supercooled liquids as a solution of Kauzmann’s paradox. The off-equilibrium dynamics below Tsp corresponds to fast nucleation of small, but stable, crystal droplets, followed by extremely slow growth, due to the presence of pinning energy barriers. In the early time region, which is longer the lower the temperature, this crystal-growth phase is indistinguishable from an off-equilibrium glass, both from a structural and a dynamical point of view: crystal growth has not advanced enough to be structurally detectable, and a violation of the fluctuation–dissipation theorem (FDT) typical of structural glasses is observed. On the other hand, for longer times crystallization reaches a threshold beyond which crystal domains are easily identified, and FDT violation becomes compatible with ordinary domain growth.

Cationic environment in silicate glasses studied by neutron diffraction with isotopic substitution

The method of neutron diffraction coupled with isotopic substitution is presented and recent investigations on the environment around cations in silicate (Ti in K 2O·TiO 2·2SiO 2, Ca and Ni in 2CaO·NiO·3SiO 2) and aluminosilicate (Li in Li 2O·Al 2O 3·2SiO 2) glasses are reviewed. The examination of the cation-centered pairs obtained from the first difference function presents striking similarities for all investigated cations. These functions indicate a well-defined short- and medium-range environment around cations. The local site generally presents a lower coordination number than that found in the crystals of similar composition. The environment around Ti in vitreous K 2O·TiO 2·2SiO 2 corresponds to a square-based pyramid and direct TiO 5–TiO 5 linkages were observed experimentally in the second difference function, contrary to crystals. A detailed description of the cation site distortion for Li and Ca may be given by this method. The distribution of cations at medium range, which can be extracted by the double difference method, reveals the presence of cation-rich regions in silicate glasses. The cation–cation distances often indicate a two-dimensional character in the cationic organization. On the contrary, Li-aluminosilicate glass shows a more homogeneous cation distribution, in relation with the charge-compensating role of Li in this glass. This non-homogeneous distribution of cations may be related to the nano-inhomogeneities proposed in the models of supercooled liquids.

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Reply to "Comment on 'Fitting of viscosity: Distinguishing the temperature dependences predicted by various models of supercooled liquids' "

Comment on "Fitting of viscosity: Distinguishing the temperature dependences predicted by various models of supercooled liquids"

In a recent paper, Kivelson et al. [Phys. Rev. E 53, 751 (1996)] presented fits to viscosity and relaxation time data for a variety of glass-forming liquids and concluded that the success of their model in fitting the data provides support for the existence of a high-temperature avoided critical point ${\mathit{T}}^{\mathrm{*}}$. We compare fits obtained with the theory of Kivelson et al. with those of the free-volume theory and conclude that the analysis of Kivelson et al. does not provide convincing evidence for the applicability of their theory to supercooled liquids. \textcopyright{} 1996 The American Physical Society.

Fitting of viscosity: Distinguishing the temperature dependences predicted by various models of supercooled liquids.

In an effort to assess the applicability of various competing theoretical models, we have examined the temperature dependence of the viscosity \ensuremath{\eta}(T) and of the \ensuremath{\alpha}-relaxation times of a wide variety of supercooled liquids. The overall best fits over the entire temperature range above the glass-transition temperature and for all the diverse supercooled liquids investigated are given by the expression ln[\ensuremath{\eta}(T)/${\mathrm{\ensuremath{\eta}}}_{\mathrm{\ensuremath{\infty}}}$] =${\mathit{E}}_{\mathrm{\ensuremath{\infty}}}$+${\mathit{BT}}^{\mathrm{*}}$[(${\mathit{T}}^{\mathrm{*}}$-T)/${\mathit{T}}^{\mathrm{*}}$${]}^{8/3}$FTHETA (${\mathit{T}}^{\mathrm{*}}$-T), where FTHETA(${\mathit{T}}^{\mathrm{*}}$-T) is a step function that is 1 for T${\mathit{T}}^{\mathrm{*}}$ and 0 for T>${\mathit{T}}^{\mathrm{*}}$, and ${\mathit{T}}^{\mathrm{*}}$ is usually greater than the melting point temperature. Our analysis supports the notion that there is a single dominant species-independent, nonmolecular mechanism underlying \ensuremath{\alpha} relaxation for all supercooled liquids throughout the entire T-range characteristic of supercooled liquids. \textcopyright{} 1996 The American Physical Society.