Compressed-exponential relaxations in supercooled liquid trehalose

We investigated the α-relaxations in supercooled liquid trehalose by using photon correlation spectroscopy (PCS) and found an interesting compressed-exponential relaxation at temperatures above 140 °C. The q^?1 dependence of its relaxation time corresponds to an ultraslow ballistic motion due to the local structure rearrangements. In the same temperature range, we found the glycosidic bond structure changes in trehalose molecule from the Raman scattering and the X-ray direction measurements. We concluded that the compressed-exponential relaxation in supercooled liquid trehalose might originate from the intra-molecular (glycosidic bond) structure change.     

Density and bond-orientational relaxations in supercooled water

ABSTRACT

Recent computational studies have reported evidence of a metastable liquid–liquid phase transition (LLPT) in molecular models of water under deeply supercooled conditions. A competing hypothesis suggests, however, that non-equilibrium artefacts associated with coarsening of the stable crystal phase have been mistaken for an LLPT in these models. Such artefacts are posited to arise due to a separation of time scales in which density fluctuations in the supercooled liquid relax orders of magnitude faster than those associated with bond-orientational order. Here, we use molecular simulation to investigate the relaxation of density and bond-orientational fluctuations in three molecular models of water (ST2, TIP5P and TIP4P/2005) in the vicinity of their reported LLPT. For each model, we find that density is the slowly relaxing variable under such conditions. We also observe similar behaviour in the coarse-grained mW model of water. Our findings, therefore, challenge the key physical assumption underlying the competing hypothesis.     

Dynamics of a supercooled liquid

The theory of collective motion in liquids suggested by the authors has been found to explain successfully the recent experimental results of temperature dependent disorder in supercooled liquid gallium. Metastability limit is exhibited through a singularity of S(k, ω = 0) and corresponds to a critical value of correlation between different particles beyond which the supercooled liquid goes to the thermodynamically stable solid state.     

Dynamics of nanoparticles in a supercooled liquid

The dynamic properties of nanoparticles suspended in a supercooled glass forming liquid are studied by x-ray photon correlation spectroscopy. While at high temperatures the particles undergo Brownian motion the measurements closer to the glass transition indicate hyperdiffusive behavior. In this state the dynamics is independent of the local structural arrangement of nanoparticles, suggesting a cooperative behavior governed by the near-vitreous solvent.     

Microscopic theory of heterogeneity and nonexponential relaxations in supercooled liquids.

Recent experiments show that supercooled liquids around the glass transition temperature are "dynamically heterogeneous" [H. Sillescu, J. Non-Cryst. Solids 243, 81 (1999)]. Such heterogeneity is expected from the random first order transition theory of the glass transition. Using a microscopic approach based on this theory, we derive a relation between the departure from Debye relaxation as characterized by the beta value of a stretched exponential response function, phi(t) = e(-(t/tau(KWW))beta), and the fragility of the liquid. The beta value is also predicted to depend on temperature and to vanish as the ideal glass transition is approached at the Kauzmann temperature.     

The primary-relaxation process in a supercooled liquid

Summary Dielectric and ultrasonic measurements have been performed in the temperature and in the frequency domain to investigate the α (or primary) relaxation in the supercooled state of a ?fragile? molecular liquid. This relaxation is strictly connected with the phenomenon of the glass transition and therefore an understanding of the former is a prerequisite for the explanation of the latter. The dynamic susceptibilities relative to these probes have been analysed with the Cole-Davidson function employing (where required) a Vogel-Fulcher-Tamman law to relate the main relaxation time to temperature. The results obtained are consistent with the dynamic behaviour of liquids of high fragility. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Solidification of a supercooled liquid in a narrow channel

We simulate solidification in a narrow channel through the use of a phase-field model with an adaptive grid. In different regimes, we find that the solid can grow in fingerlike steady-state shapes, or become unstable, exhibiting unsteady growth. At low melt undercoolings, we find good agreement between our results, theoretical predictions, and experiment. For high undercoolings, we report evidence for a new stable steady-state finger shape which exists in experimentally accessible ranges for typical materials.     

Dielectric heating and relaxations in smectic a liquid crystals

The dependence of the temperature changes ΔT on frequency occurring in the smectic A, nematic and isotropic phase of a positive dielectric liquid crystal material initiated by dielectric heating is investigated. The same relaxation mechanism governing the strong increase of ΔT in nematics is shown to be responsible for inducing comparably strong dielectric heating effects in smectic A phases.     

Anisotropic local stress and particle hopping in a deeply supercooled liquid

The origin of the microscopic motions that lead to stress relaxation in deeply supercooled liquid remains unclear. We show that in such a liquid the stress relaxation is locally anisotropic which can serve as the driving force for the hopping of the system on its free energy surface. However, not all hoppings are equally effective in relaxing the local stress, suggesting that diffusion can decouple from viscosity even at the local level. On the other hand, orientational relaxation is found to be always coupled to stress relaxation.     

Metal-to-semimetal transition in supercooled liquid silicon

Computer simulations, using the Stillinger-Weber potential, have previously been employed to demonstrate a liquid-liquid transition in supercooled silicon near 1060 K. From calculations of electronic structure using an empirical psuedopotential, we show that silicon undergoes an associated metal to semimetal transition with a resistivity jump of roughly 1 order of magnitude. We show that the electronic states near the Fermi energy become localized in the low temperature phase, and that changes in electronic structure between the two phases arise from a change in atomic structure, and not from a change in density.     

Correlation between supercooled liquid region and crystallization behavior with alloy composition of La-Al-Cu metallic glasses

Bulk metallic glasses (BMGs) with large supercooled liquid region are promising materials for superplastic forming. In this paper, we demonstrate a microstructure-based strategy to pinpoint the composition with the largest supercooled liquid region in La 86 x Al 14 Cu x (x=16 at%-20 at%) metallic glass system. By monitoring the changes in crystallization behavior of the glassy alloys with composition to search for the alloys exhibiting eutectic crystallization, the glassy alloys with the largest supercooled...     

VH spectra from liquid and supercooled salol

VH spectra of light scattered by salol have been measured over a wide temperature range. A hydrodynamic model using two relaxing processes fits accurately the line shapes in the full viscosity range.     

Structural study of supercooled liquid silicon

For many years, theoretical studies using model and ab initio potentials have predicated the existence of a liquid/liquid phase transition in silicon, based on a continuous change of the liquid A5 structure to A4. In contrast, we report here a quantitative analysis of data from high-energy X-ray diffraction measurements of containerlessly-processed supercooled liquid silicon that demonstrates that the fractions of regions with A5 and A4 order instead remain essentially constant with supercooling, but that the coherence length of the A5 order increases.     

Mechanical responses and stress fluctuations of a supercooled liquid in a sheared non-equilibrium state

A steady shear flow can drive supercooled liquids into a non-equilibrium state. Using molecular dynamics simulations under steady shear flow superimposed with oscillatory shear strain for a probe, non-equilibrium mechanical responses are studied for a model supercooled liquid composed of binary soft spheres. We found that even in the strongly sheared situation, the supercooled liquid exhibits surprisingly isotropic responses to oscillating shear strains applied in three different components of the strain tensor. Based on this isotropic feature, we successfully constructed a simple two-mode Maxwell model that can capture the key features of the storage and loss moduli, even for highly non-equilibrium state. Furthermore, we examined the correlation functions of the shear stress fluctuations, which also exhibit isotropic relaxation behaviors in the sheared non-equilibrium situation. In contrast to the isotropic features, the supercooled liquid additionally demonstrates anisotropies in both its responses and its correlations to the shear stress fluctuations. Using the constitutive equation (a two-mode Maxwell model), we demonstrated that the anisotropic responses are caused by the coupling between the oscillating strain and the driving shear flow. Due to these anisotropic responses and fluctuations, the violation of the fluctuation-dissipation theorem (FDT) is distinct for different components. We measured the magnitude of this violation in terms of the effective temperature. It was demonstrated that the effective temperature is notably different between different components, which indicates that a simple scalar mapping, such as the concept of an effective temperature, oversimplifies the true nature of supercooled liquids under shear flow. An understanding of the mechanism of isotropies and anisotropies in the responses and fluctuations will lead to a better appreciation of these violations of the FDT, as well as certain consequent modifications to the concept of an effective temperature.     

An Al-rich metallic glass with a large supercooled liquid region

The addition of small amounts (microalloying) of Ti was previously shown to improve glass formation and stability in rapidly quenched Al–Y–Fe alloys. Here, microalloying with V is demonstrated to have an even more dramatic effect on glass stability. Al_85.35Y₈Fe₆V_0.65 exhibits a crystallization-onset temperature (T_x ) of 365°C and a supercooled liquid region (ΔT_x ) of 80°C, the largest currently known for Al-rich metallic glasses. The rapidly quenched oxygenated alloys Al₈₅Y₈Fe₆V_0.65O_0.35 and Al_84.35Y₈Fe₆V_0.65O₁ are also glasses, establishing that the glasses with V have a higher tolerance to oxygen contamination than the alloys prepared with Ti. The partial devitrification of Al_85.35Y₈Fe₆V_0.65 produces an unusual, non-uniform microstructure that suggests an autocatalytic nucleation mechanism.     

Local influence of boundary conditions on a confined supercooled colloidal liquid

We study confined colloidal suspensions as a model system which approximates the behavior of confined small molecule glass-formers. Dense colloidal suspensions become glassier when confined between parallel glass plates. We use confocal microscopy to study the motion of confined colloidal particles. In particular, we examine the influence particles stuck to the glass plates have on nearby free particles. Confinement appears to be the primary influence slowing free particle motion, and proximity to stuck particles causes a secondary reduction in the mobility of free particles. Overall, particle mobility is fairly constant across the width of the sample chamber, but a strong asymmetry in boundary conditions results in a slight gradient of particle mobility.