Theory of aging in structural glasses

The random first-order transition theory of the dynamics of supercooled liquids is extended to treat aging phenomena in nonequilibrium structural glasses. A reformulation of the idea of "entropic droplets" in terms of libraries of local energy landscapes is introduced which treats in a uniform way the supercooled liquid (reproducing earlier results) and glassy regimes. The resulting microscopic theory of aging makes contact with the Nayaranaswamy-Moynihan-Tool nonlinear relaxation formalism and the Hodge-Scherer extrapolation of the Adam-Gibbs formula, but deviations from both approaches are predicted and shown to be consistent with experiment. The nonlinearity of glassy relaxation is shown to quantitatively correlate with liquid fragility. The residual non-Arrhenius temperature dependence of relaxation observed in quenched glasses is explained. The broadening of relaxation spectra in the nonequilibrium glass with decreasing temperature is quantitatively predicted. The theory leads to the prediction of spatially fluctuating fictive temperatures in the long-aged glassy state, which have non-Gaussian statistics. This can give rise to "ultraslow" relaxations in systems after deep quenches.     

Strain-induced yielding in bubble clusters

We study how shearing clusters of two or four bubbles induces bubble separation or topological rearrangement. The critical deformation at which this yielding occurs is measured as a function of shear rate, liquid composition, and liquid content in the cluster. We establish a geometrical yield criterion in the quasistatic case on the basis of these experimental data as well as simulations. In the dynamic regime, the deformation where the cluster yields increases with the strain rate, and we derive a scaling law describing this phenomenon based on the dynamical inertial rupture of the liquid meniscus linking the two bubbles. Our experiments show that the same scaling law applies to two- and four-bubble clusters.     

LiCl熔盐急冷形成非晶固体的分子动力学计算机模拟研究

卤化物玻璃目前已成为引人注目的光纤新材料,用分子动力学方法研究液态急冷形成非晶态的过程,对于卤化物玻璃的形成过程研究也应是有用的。鉴于碱金属卤化物是最简单的熔盐,其动态结构亦很清楚。用分子动力学方法研究其急冷以形成玻     

Comparative study of temperature dependent orientational relaxation in a model thermotropic liquid crystal and in a model supercooled liquid

Recent optical Kerr effect experiments have revealed a power law decay of the measured signal with a temperature independent exponent at short-to-intermediate times for a number of liquid crystals in the isotropic phase near the isotropic-nematic transition and supercooled molecular liquids above the mode coupling theory critical temperature. In this work, the authors investigate the temperature dependence of short-to-intermediate time orientational relaxation in a model thermotropic liquid crystal across the isotropic-nematic transition and in a binary mixture across the supercooled liquid regime in molecular dynamics simulations. The measure of the experimentally observable optical Kerr effect signal is found to follow a power law decay at short-to-intermediate times for both systems in agreement with recent experiments. In addition, the temperature dependence of the power law exponent is found to be rather weak. As the model liquid crystalline system settles into the nematic phase upon cooling, the decay of the single-particle second-rank orientational time correlation function exhibits a pattern that is similar to what has been observed for supercooled liquids.     

Nonlinear behavior of polyethylene/layered double hydroxide nanocomposites under shear flow

The structural evolution of filler clusters in polyethylene/layered double hydroxide-based nanocomposites is investigated under application of a simple shear flow and is described in the framework of a modified Wagner model. Overall, the structural behavior of these polymer-clay nanocomposites is found to be similar to the behavior of filled elastomers for which breakdown of filler clusters at increasing strain and their reaggregation at decreasing strain were observed under oscillatory shear (Payne effect). Similar to the filled elastomers and other jammed systems, the polymer-clay nanocomposites demonstrate an asymmetric behavior upon approaching the steady state depending on whether the system was initially at higher or lower shear strain. In particular, the reaggregation time of filler structure in the quiescent state is found to be about one order of magnitude larger than the characteristic breakage time in the nonlinear shear regime. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A. 2008, Vol. 50, No. 5, pp. 868–881. This article was submitted by the authors in English.     

Deformation of inherent structures to detect long-range correlations in supercooled liquids

We propose deformations of inherent structures as a suitable tool for detecting structural changes underlying the onset of cooperativity in supercooled liquids. The non-affine displacement (NAD) field resulting from the applied deformation shows characteristic differences between the high temperature liquid and supercooled state, which are typically observed in dynamic quantities. The average magnitude of the NAD is very sensitive to temperature changes in the supercooled regime and is found to be strongly correlated with the inherent structure energy. In addition, the NAD field is characterized by a correlation length that increases upon lowering the temperature towards the supercooled regime.     

Molecular dynamics simulations of nucleation from vapor to solid composed of Lennard-Jones molecules

We performed molecular dynamics (MD) simulations of nucleation from vapor at temperatures below the triple point for systems consisting of 10(4)-10(5) Lennard-Jones (L-J) type molecules in order to test nucleation theories at relatively low temperatures. Simulations are performed for a wide range of initial supersaturation ratio (S(0) ? 10-10(8)) and temperature (kT = 0.2-0.6ε), where ε and k are the depth of the L-J potential and the Boltzmann constant, respectively. Clusters are nucleated as supercooled liquid droplets because of their small size. Crystallization of the supercooled liquid nuclei is observed after their growth slows. The classical nucleation theory (CNT) significantly underestimates the nucleation rates (or the number density of critical clusters) in the low-T region. The semi-phenomenological (SP) model, which corrects the CNT prediction of the formation energy of clusters using the second virial coefficient of a vapor, reproduces the nucleation rate and the cluster size distributions with good accuracy in the low-T region, as well as in the higher-T cases considered in our previous study. The sticking probability of vapor molecules onto the clusters is also obtained in the present MD simulations. Using the obtained values of sticking probability in the SP model, we can further refine the accuracy of the SP model.     

Enhanced small-angle scattering connected to the Widom line in simulations of supercooled water

We present extensive simulations on the TIP4P∕2005 water model showing significantly enhanced small-angle scattering (SAS) in the supercooled regime. The SAS is related to the presence of a Widom line (T(W)) characterized by maxima in thermodynamic response functions and Ornstein-Zernike correlation length. Recent experimental small-angle x-ray scattering data [Huang et al., J. Chem. Phys. 133, 134504 (2010)] are excellently reproduced, albeit with an increasing temperature offset at lower temperatures. Assuming the same origin of the SAS in experiment and model this suggests the existence of a Widom line also in real supercooled water. Simulations performed at 1000 bar show an increased abruptness of a crossover from dominating high-density (HDL) to dominating low-density (LDL) liquid and strongly enhanced SAS associated with crossing T(W), consistent with a recent determination of the critical pressure of TIP4P∕2005 at 1350 bar. Furthermore, good agreement with experimental isothermal compressibilities at 1000, 1500, and 2000 bar shows that the high pressure supercooled thermodynamic behavior of water is well described by TIP4P∕2005. Analysis of the tetrahedrality parameter Q reveals that the HDL-LDL structural transition is very sharp at 1000 bar, and that structural fluctuations become strongly coupled to density fluctuations upon approaching T(W). Furthermore, the tetrahedrality distribution becomes bimodal at ambient temperatures, an observation that possibly provides a link between HDL-LDL fluctuations and the structural bimodality in liquid water indicated by x-ray spectroscopic techniques. Computed x-ray absorption spectra are indeed found to show sensitivity to the tetrahedrality parameter.     

Structural study of supercooled liquid transition metals

Local structural models for supercooled liquid transition metals (Ti, Ni, and Zr) are proposed based on a reverse Monte Carlo analysis of high-energy x-ray diffraction data taken from the electrostatically levitated liquids. The resulting structures are characterized by their bond angle distributions, Honeycutt and Andersen indices [J. Phys. Chem. 91, 4950 (1987)], and bond orientational order parameters. All analyses suggest that an icosahedral short-range order is present in these supercooled liquids, but it is distorted in liquid Ti. These results are in agreement with the observed evolution of the high-q shoulder on the second peak in the structure factor S(q) and with an earlier analysis based on a local cluster model.     

A small-angle scattering study on equilibrium clusters in lysozyme solutions

We use small-angle scattering experiments to investigate the structural properties of aqueous lysozyme solutions under conditions where the existence of equilibrium clusters has recently been demonstrated (Nature 2004, 432, 492). We also discuss the possible emergence of a low angle scattering contribution, which recently attracted interest due to its appearance in solutions of various proteins. We demonstrate that in lysozyme solutions under our experimental conditions such rising low q intensities can only be observed under special circumstances and can thus not be attributed to the existence of a universal long-range attraction. We then focus on the structural properties of the equilibrium clusters as a function of protein concentration, temperature, and ionic strength. We show that the experimental structure factors obtained from the scattering measurements exhibit the typical cluster-cluster peak q(c) reflecting the mean distance between charged clusters as well as a monomer-monomer peak q(m), which represents the nearest neighbor shell of monomers within a single cluster. The underlying principle for the formation of these structures is the coexistence of two opposing forces, a short-range attraction and a long-range repulsion due to residual charges. We can quantitatively analyze our scattering data by applying a simple equilibrium cluster model and calculate an average cluster aggregation number, N(c). The thus obtained cluster aggregation number increases linearly with volume fraction. We also observe an increasing N(c) as temperature decreases and as the screening of residual charges increases. We point out the importance of the existence of equilibrium clusters and the universality of this phenomenon for self-assembling processes observed in nature. Finally, we discuss the limitations of our simple globular cluster model in view of recent findings from computer simulations.     

Surface clusters of colloid particles produced by deposition on sites

The possibility of producing surface clusters of well-defined structure formed by colloid particles was analyzed theoretically and experimentally. Theoretical results were derived by performing Monte Carlo-type simulations according to the generalized random sequential adsorption (RSA) mechanism. In these simulations, the jamming coverage of particles adsorbing irreversibly on spherical sites was determined as a function of the particle-to-site size ratio lambda. It was revealed that, by properly choosing lambda, a targeted site coordination can be achieved; for example, there can be one, two, three, and so forth particles attached to one site. The structure of the heterogeneous clusters produced in this way was described in terms of the pair correlation function. It was predicted that the extent of ordering within surface clusters was diminished as the concentration of sites increased. These theoretical predictions were checked by performing deposition experiments of negatively charged polystyrene latex particles (average diameter 0.9 mum) under the diffusion-controlled transport regime. Mica sheets precovered by positively charged polystyrene latex (average diameters 0.45 and 0.95 microm) were used as the substrate surface in these experiments. Positive latex (site) deposition was also carried out under diffusion-controlled transport conditions. The concentration of the sites and the adsorbed particles was determined by direct particle counting using optical microscopy. It was found, in quantitative agreement with theoretical simulations, that the structure of surface clusters produced in this way exhibits a significant degree of short-range ordering. It also was proven experimentally that clusters containing a targeted number of colloid particles (e.g., 2 and 4) could be produced by the deposition procedure.     

How short-range attractions impact the structural order, self-diffusivity, and viscosity of a fluid

We present molecular simulation data for viscosity, self-diffusivity, and the local structural ordering of (i) a hard-sphere fluid and (ii) a square-well fluid with short-range attractions. The latter fluid exhibits a region of dynamic anomalies in its phase diagram, where its mobility increases upon isochoric cooling, which is found to be a subset of a larger region of structural anomalies, in which its pair correlations strengthen upon isochoric heating. This "cascade of anomalies" qualitatively resembles that found in recent simulations of liquid water. The results for the hard-sphere and square-well systems also show that the breakdown of the Stokes-Einstein relation upon supercooling occurs for conditions where viscosity and self-diffusivity develop different couplings to the degree of pairwise structural ordering of the liquid. We discuss how these couplings reflect dynamic heterogeneities. Finally, we note that the simulation data suggest how repulsive and attractive glasses may generally be characterized by two distinct levels of short-range structural order.     

Measuring diffusivity in supercooled liquid nanoscale films using inert gas permeation. II. Diffusion of Ar, Kr, Xe, and CH4 through methanol

We present an experimental technique to measure the diffusivity of supercooled liquids at temperatures near their T(g). The approach uses the permeation of inert gases through supercooled liquid overlayers as a measure of the diffusivity of the supercooled liquid itself. The desorption spectra of the probe gas are used to extract the low temperature supercooled liquid diffusivities. In the preceding companion paper, we derived equations using ideal model simulations from which the diffusivity could be extracted using the desorption peak times for isothermal or peak temperatures for temperature programmed desorption experiments. Here, we discuss the experimental conditions for which these equations are valid and demonstrate their utility using amorphous methanol with Ar, Kr, Xe, and CH(4) as probe gases. The approach offers a new method by which the diffusivities of supercooled liquids can be measured in the experimentally challenging temperature regime near the glass transition temperature.     

A non-polarizable model of water that yields the dielectric constant and the density anomalies of the liquid: TIP4Q

A four-site rigid water model is presented, whose parameters are fitted to reproduce the experimental static dielectric constant at 298 K, the maximum density of liquid water and the equation of state at low pressures. The model has a positive charge on each of the three atomic nuclei and a negative charge located at the bisector of the HOH bending angle. This charge distribution allows increasing the molecular dipole moment relative to four-site models with only three charges and improves the liquid dielectric constant at different temperatures. Several other properties of the liquid and of ice Ih resulting from numerical simulations with the model are in good agreement with experimental values over a wide range of temperatures and pressures. Moreover, the model yields the minimum density of supercooled water at 190 K and the minimum thermal compressibility at 310 K, close to the experimental values. A discussion is presented on the structural changes of liquid water in the supercooled region where the derivative of density with respect to temperature is a maximum.     

Athermal simulation of plastic deformation in amorphous solids at constant pressure

An algorithm is introduced for the molecular simulation of constant-pressure plastic deformation in amorphous solids at zero temperature. This allows to directly study the volume changes associated with plastic deformation (dilatancy) in glassy solids. In particular, the dilatancy of polymer glasses is an important aspect of their mechanical behavior. The new method is closely related to Berendsen's barostat, which is widely used for molecular dynamics simulations at constant pressure. The new algorithm is applied to plane strain compression of a binary Lennard-Jones glass. Conditions of constant volume lead to an increase of pressure with strain, and to a concommitant increase in shear stress. At constant (zero) pressure, by contrast, the shear stress remains constant up to the largest strains investigated (ε = 1), while the system density decreases linearly with strain. The linearity of this decrease suggests that each elementary shear relaxation event brings about an increase in volume which is proportional to the amount of shear. In contrast to the stress–strain behavior, the strain-induced structural relaxation, as measured by the self-part of the intermediate structure factor, was found to be the same in both cases. This suggests that the energy barriers that must be overcome for their nucleation continually grow in the case of constant-volume deformation, but remain the same if the deformation is carried out at constant pressure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2057–2065, 2004     

Molecular packing in highly stable glasses of vapor-deposited tris-naphthylbenzene isomers

Physical vapor deposition of organic molecules can produce glasses with high kinetic stability and low enthalpy. Previous experiments utilizing wide-angle x-ray scattering (WAXS) have shown that, relative to the ordinary glasses prepared by cooling the supercooled liquid, such glasses exhibit excess scattering characteristic of anisotropic packing. We have used vapor deposition to prepare glasses of four isomers of tris-naphthylbenzene (TNB), and measured both the WAXS patterns and the kinetic stability. While vapor-deposited glasses of all four TNB isomers exhibit high and nearly uniform kinetic stability, the level of excess scattering varies significantly. In addition, for α,α,β-TNB, glasses of essentially identical kinetic stability can have excess scattering levels that vary by a factor of two. These results indicate that anisotropic packing is not the source of kinetic stability in vapor-deposited glasses but rather a secondary feature that depends upon the chemical structure of the glass-forming molecules. We also show that the time required for these stable vapor-deposited glasses to transform into the supercooled liquid greatly exceeds the structural relaxation time τ(α) of the liquid and scales approximately as τ(α) (0.6). The kinetic stability of the vapor-deposited TNB glasses matches that expected for ordinary glasses that have been aged for 10(2) to 10(7) years.     

Crystallization at the glass transition in supercooled thin films of methanol

The stability of an amorphous material depends on how fast and by what mechanism crystallization occurs. Based on crystallization rate measurements through optical reflectivity changes in supercooled methanol thin films, it is observed for the first time that there is a definitive and detectable change of the crystallization mechanism at the glass transition temperature T(g). For methanol glasses below T(g)=103.4 K, crystallization occurs as an interface controlled, one-dimension process at frozen-in embryo sites, while in the deep supercooled liquid phase above T(g) crystallization is diffusion controlled in two dimensions with a constant nucleation rate and an activation energy of 107.8(+/-4.7) kJ/mol.     

On the reduced moment in the transient regime of homogeneous nucleation

One of the parameters characterizing the evolution of nucleation in the transient regime is the so-called reduced moment, a dimensionless quantity. This parameter describes the steepness with which the nucleation rate approaches its steady state. Until recently, very little had been known about this parameter in real systems, although a widely quoted 1969 theory [D. Kashchiev, Surf. Sci. 14, 209 (1969)] existed that formally described nucleation in the transient regime. This theory has been shown to be incorrect in its implication about the reduced moment. Molecular dynamics simulations have recently greatly clarified what happens in the transient regime. It turns out that the reduced moment depends strongly on the size of the nucleus under consideration, and, for a rapidly quenched liquid, it substantially exceeds unity for small nuclei but approaches unity as nuclei increases in size. The objective of this paper is to illustrate the behavior of the reduced moment and to show how this behavior is a natural consequence of the kinetics of the nucleation process.