Orientational dynamics in supercooled liquids near T(c) and comparison with ideal mode-coupling theory

Orientational dynamics in supercooled salol and ortho-terphenyl were measured near their critical temperatures, T(c), with optical Kerr effect experiments spanning a very broad range of times. Above T(c), the decays are shown to be in excellent agreement with the master curve predicted by ideal mode-coupling theory when higher order terms are included. Between the critical decay and the von Schweidler power laws, the intermediate time range of the data can be modeled by a power law. This intermediate power law, located at 2相似文献   

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.     

Diffusion in ordinary and supercooled liquids

Summary We discuss a simplified approach for the recollision part of the memory function for self-motion in ordinary and supercooled liquids. This part accounts for the substantial decrease of the diffusion coefficient with respect to the binary prediction, an effect which becomes larger and larger on cooling the system. The quantitative results compare satisfactorily with previous and new simulation data obtained for a liquid rubidium model both near the triple point and in moderately supercooled states. Paper presented at the workshop ?Highlights on Simple Liquids”, held in Turin at ISI on 1–3 May, 1989.     

Viscoelasticity and metastability limit in supercooled liquids

A supercooled liquid is said to have a kinetic spinodal if a temperature Tsp exists below which the liquid relaxation time exceeds the crystal nucleation time. We revisit classical nucleation theory taking into account the viscoelastic response of the liquid to the formation of crystal nuclei and find that the kinetic spinodal is strongly influenced by elastic effects. We introduce a dimensionless parameter lambda, which is essentially the ratio between the infinite frequency shear modulus and the enthalpy of fusion of the crystal. In systems where lambda is larger than a critical value lambda(c) the metastability limit is totally suppressed, independently of the surface tension. On the other hand, if lambda相似文献   

Inhomogeneous fluctuations in liquids and gases

The coupled inhomogeneous density and temperature oscillations in liquids and gases are described with the help of dynamical equations of motion for fluctuations. It is proved that stationary points are asymptotically stable when the preset temperature distribution is inhomogeneous. It is shown that the temperature setting in a gas is a result of the competition between viscosity and heat conduction mechanisms. The phase trajectories in the T-ρ plane are constructed in the inhomogeneous case by numerically solving the derived equations.     

Long wavelength density fluctuations in supercooled liquids

Long wavelength density fluctuations in a classical liquid are discussed in terms of a model which has the correct small frequency behaviour and the correct high frequency limit, neglecting thermal relaxation effects. Moreover, the first three frequency moments are correct, so that the model should be well suited for the interpretation of inelastic neutron scattering experiments on supercooled liquids.     

Heterodyne detected transient gratings in supercooled molecular liquids

We present an analysis, based on a phenomenological set of Generalised Navier-Stokes equations, of Heterodyne Detected Transient Gratings on supercooled molecular liquids of anisotropic molecules. This set of equations generalises equations proven in Franosch, Latz and Pick [24] for the same type of liquids. It also takes into account the three different sources generated by the laser pumping process pertinent for these experiments. We give analytical expressions for the response functions that can be measured using the different polarisation of the experimental set-up. Specialising to the case of parallel polarisation (where longitudinal phonons are launched), we show that each response function is a sum of the same seven elementary response functions (ERFs) whose time and temperature evolutions are individually analysed. We also show that the response functions corresponding to two of the sources can be directly connected to the Laplace Transform of a light scattering signal. The ERFs generated by the heat-absorption process, which is the third source, are of a different nature. They do not have the same time and temperature behaviours and they can provide, inter alia, unique information on the rotation-translation coupling function characteristic of these liquids.Received: 5 January 2004, Published online: 29 June 2004PACS: 64.70.Pf Glass transitions - 78.47. + p Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter - 61.25.Em Molecular liquidsR. Gupta: Present address: Dept of Physics,1110 W Green St, Urbana, Ill 61801, USA.     

Heterogeneities in supercooled liquids: a density-functional study

A metastable state, characterized by a low degree of mass localization, is identified using density-functional theory (DFT). This free energy minimum, located through the proper evaluation of competing terms in the free energy functional, is independent of the specific form of the DFT used. Computer simulation results on particle motion indicate that this heterogeneous state corresponds to the deeply supercooled state.     

Upper critical dimension, dynamic exponent, and scaling functions in the mode-coupling theory for the Kardar-Parisi-Zhang equation

We study the mode-coupling approximation for the Kardar-Parisi-Zhang equation in the strong-coupling regime. By constructing an ansatz consistent with the asymptotic forms of the correlation and response functions we determine the upper critical dimension d(c) = 4 and the expansion z = 2-(d-4)/4+O((4-d)2) around dc. We find the exact z = 3/2 value in d = 1, and estimate the values z approximately 1.62, z approximately 1.78 in d = 2, 3. The result dc = 4 and the expansion around dc are very robust and can be derived just from a mild assumption on the relative scale on which the response and correlation functions vary as z approaches 2.     

Pressure dependence of diffusion in simple glasses and supercooled liquids

Using molecular dynamics simulation, we have calculated the pressure dependence of the diffusion coefficient in a binary Lennard-Jones glass. We observe four temperature regimes. The apparent activation volume drops from high values in the hot liquid to a plateau value. Near the critical temperature of the mode coupling theory it rises steeply, but in the glassy state we find again small values, similar to the ones in the liquid. The peak of the activation volume at the critical temperature is in agreement with the prediction of mode coupling theory.     

On the behaviour of supercooled liquids and polymers in nano-confinement

ABSTRACT

Size effects play an important role in structural phase transitions, melting transitions, in martensitic materials, glass transitions, etc. Very often the question arises, whether a measured size effect originates from the geometrical confinement itself, or if it appears due to the interaction with the limiting surface. Using dynamic mechanical analysis (DMA) technique we have studied various microphase segregated polymers, molecular glass forming liquids and supercooled water confined in nanoporous silica as well as in biological tissues. Here we show on some selected examples that DMA measurements can be used to study relaxation processes in detail and to disentangle in favourable cases pure pore size effects from effects that are induced by the confining surface.     

Saddles in the energy landscape probed by supercooled liquids

We numerically investigate the supercooled dynamics of two simple model liquids exploiting the partition of the multidimensional configuration space in basins of attraction of the stationary points (inherent saddles) of the potential energy surface. We find that the inherent saddle order and potential energy are well-defined functions of the temperature T. Moreover, by decreasing T, the saddle order vanishes at the same temperature (T(MCT)) where the inverse diffusivity appears to diverge as a power law. This allows a topological interpretation of T(MCT): it marks the transition from a dynamics between basins of saddles (T > T(MCT)) to a dynamics between basins of minima (T < T(MCT)).     

Apparent finite-size effects in the dynamics of supercooled liquids

Molecular dynamics simulations are performed for a supercooled simple liquid with changing the system size from N=108 to 10(4) to examine possible finite-size effects. Although almost no systematic deviation is detected in the static pair correlation functions, it is demonstrated that the structural alpha relaxation in a small system becomes considerably slower than that in larger systems for temperatures below T(c) at which the size of the cooperative particle motions becomes comparable to the unit cell length of the small system. The discrepancy increases with decreasing temperature.     

Dynamics of supercooled liquids and understanding the glass transition

The self-consistent mode coupling theory of glass transition is briefly reviewed. The existance of a temperature T_c, higher than the usual calorimetric glass transition temperature, across which the dynamics of the fluid becomes quite different are indicated through different experimental results. Above T_c the viscosity tends to diverge with a power law while for lower temperature this sharp transition is cutoff. Such changes in the transport properties can be understood from the self-consistent mode coupling theory. The relaxation functions predicted by the mode-coupling theory over different time scales are indicated. The models with proper wave-vector dependence are also discussed.     

Atomic mechanism of glass formation in supercooled monatomic liquids

Atomic mechanism of glass formation in supercooled monatomic liquids is monitored via analyzing the spatial arrangement of solid-like atoms. The supercooled states are obtained by cooling from the melt using molecular dynamics (MD) simulation. Solid-like atoms, detected via Lindemann-like freezing criterion, are found throughout the liquid. Their number increases with decreasing temperature and they form clusters. In the deeply supercooled region, all solid-like atoms form a single percolation cluster which spans throughout the system. The number of atoms in this cluster increases steeply with further cooling. Glass formation in supercooled liquids occurs when a single percolation cluster of solid-like atoms involves the majority of atoms in the system to form a relatively rigid glassy solid. By analyzing the temperature dependence of static and dynamic properties, we identify three characteristic temperatures of glass formation in supercooled liquids including the Vogel–Fulcher temperature.