Aging without disorder on long time scales

We study the Metropolis dynamics of a simple spin system without disorder, which exhibits glassy dynamics at low temperatures. We use an implementation of the algorithm of Bortz, Kalos and Lebowitz [1]. This method turns out to be very efficient for the study of glassy systems, which get trapped in local minima on many different time scales. We find strong evidence of aging effects at low temperatures. We relate these effects to the distribution function of the trapping times of single configurations.     

Fragile to strong crossover and Widom line in supercooled water: A comparative study

The aim of this paper is to discuss the relationship between the dynamics and thermodynamics of water in the supercooled region. Reviewed case studies comprehend bulk water simulated with the SPC/E, TIP4P and TIP4P/2005 potentials, water at protein interfaces, and water in solution with electrolytes. Upon supercooling, the fragile to strong crossover in the α-relaxation of water is found to occur when the Widom line emanating from the liquid-liquid critical point is crossed. This appears to be a general characteristic of supercooled water, not depending on the applied interaction potential and/or different local environments.     

非晶态合金表面的水润湿动力学

采用分子动力学模拟方法研究了改进的Simple pointcharge模型SPC/E水滴在Cu_(50)Zr_(50)非晶薄膜上的润湿行为和铺展过程.通过与CuZr(110)和(100)晶面对比研究发现,水滴在Cu_(50)Zr_(50)非晶薄膜表面上表现出较高的铺展速度.水滴在非晶合金表面的铺展过程中形成了明显的吸附层;而在晶态表面,水滴铺展前沿呈脚状形态.分析结果表明非晶表面的水分子在吸附层内呈现完全无序的单层排列方式,而在晶态表面,特别是(100)晶面,吸附层水分子呈双层有序排列.这种吸附层结构的差异导致了吸附层内水分子方向的差异:非晶表面吸附层内水分子方向倾向平行于表面,而晶态基底上吸附层内的水分子倾向于指向液滴内部.由此造成了非晶表面吸附层中的水分子与液滴内部以相对较弱的氢键相互作用,这使得上层水分子能够较容易扩散至吸附层前沿,促进液滴铺展.     

Dynamical and structural properties of adsorbed water molecules at the TiO2 rutile-(110) surface: interfacial hydrogen bonding probed by ab-initio molecular dynamics

ABSTRACT

Ab-initio molecular dynamics (AIMD) simulations have been carried out to study a range of different and energetically-accessible adsorbed-water configurations and motifs for their vibrational and structural characteristics, in contact with rutile-(110) interfaces at 100?K. The radial pair distribution function between the titanium atoms at the interface and the hydrogen and oxygen atoms in the water monolayer show an orientation of the water molecules parallel to the surface of titania, and with hydrogen atoms pointed in the opposite direction to the surface. In some cases, a distinctive vibrational frequency region between 2500 and 3000?cm^?1 has also been observed, due to a strong dispersion interaction between water molecules. This behaviour is also seen in experimental studies of thin-film water coverage on TiO₂ surfaces.     

Glassy dynamics of water at interface with biomolecules: A Mode Coupling Theory test

We study the slow dynamics of hydration water upon cooling in two different biological aqueous solutions, one containing a molecule of lysozyme and another with trehalose molecules. In particular we test if the glassy behaviour of these solutions fulfils the predictions of the popular Mode Coupling Theory of glassy dynamics. In particular we test the Time Temperature Superposition Principle and the matching of the exponents of the theory. Our results confirm that this theory is able to describe the dynamical behaviour of supercooled water also in non ideal cases as the ones under investigation in the region of mild supercooling.     

Kovacs effect in facilitated spin models of strong and fragile glasses

We investigate the Kovacs (or crossover) effect in facilitated f-spin models of glassy dynamics. Although the Kovacs hump shows a behavior qualitatively similar for all cases we have examined (irrespective of the facilitation parameter f and the spatial dimension d), we find that the dependence of the Kovacs peak time on the temperature of the second quench allows to distinguish among different microscopic mechanisms responsible for the glassy relaxation (e.g. cooperative vs defect diffusion). We also analyze the inherent structure dynamics underlying the Kovacs protocol, and find that the class of facilitated spin models with d>1 and f>1 shows features resembling those obtained recently in a realistic model of fragile glass forming liquid.     

Tagged-particle motion in glassy systems under shear: Comparison of mode coupling theory and Brownian dynamics simulations

We study the dynamics of a tagged particle in a glassy system under shear. The recently developed integration through transients approach, based on mode coupling theory, is continued to arrive at the equations for the tagged-particle correlators and the mean squared displacements. The equations are solved numerically for a two-dimensional system, including a nonlinear stability analysis of the glass solution, the so called β-analysis. We perform Brownian Dynamics simulations in 2D and compare with theory. After switch on, transient glassy correlation functions show strong fingerprints of the stress overshoot scenario, including, additionally to previously studied superexponential decay, a shoulder-like slowing down after the overshoot. We also find a new type of Taylor dispersion in glassy states which has intriguing similarity to the known low-density case. The theory qualitatively captures most features of the simulations with quantitative deviations concerning the shear-induced time scales. We attribute these deviations to an underestimation of the overshoot scenario in the theory.     

Relaxation dynamics near the sol–gel transition: From cluster approach to mode-coupling theory

A long standing problem in glassy dynamics is the geometrical interpretation of clusters and the role they play in the observed scaling laws. In this context, the mode-coupling theory (MCT) of type-A transition and the sol–gel transition are both characterized by a structural arrest to a disordered state in which the long-time limit of the correlator continuously approaches zero at the transition point. In this paper, we describe a cluster approach to the sol-gel transition and explore its predictions, including universal scaling laws and a new stretched relaxation regime close to criticality. We show that while MCT consistently describes gelation at mean-field level, the percolation approach elucidates the geometrical character underlying MCT scaling laws.     

Crossover from Rouse dynamics to the α-relaxation in poly (vinyl ethylene)

By means of neutron spin echo (NSE) we have explored the dynamics of poly(vinyl ethylene) on length scales covering Rouse dynamics and below. The results establish the simultaneous existence of a generic sublinear diffusion regime which underlies the α-process in addition to the Rouse process. Both regimes are separated by a well-defined dynamic crossover. From that the size of the Gaussian blobs making up the Rouse model is determined directly. The glassy dynamics may thus be identified with subdiffusive motions occurring within these Gaussian blobs.     

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)).     

Study of tetrabutylammonium bromide in aqueous solution by neutron scattering

The study of electrolyte solutions by neutron scattering is an example of the large range of possibilities of the technique. Structure and dynamics at different time and length scales, discrimination of global from local motions, separation of coherent from incoherent contributions are necessary to embrace the complexity of a subject where charge and hydrophobicity play important and competitive roles. The behaviour of aqueous solutions of tetrabutylammonium bromide is studied here by several neutron scattering techniques: Small Angle Neutron Scattering, Neutron Diffraction, Time-of-Flight and Neutron Spin Echo. We concentrate on the conformation and dynamics of the hydrophobic cations. In particular, the center-of-mass (CoM) motion of the cation at the microscopic scale is best described via the low Q coherent signal, as measured by Neutron Spin Echo. Due to a possible cage formation effect in the TBABr solution, at the scale of the distance between cations, the cationic CoM relaxation time is larger than that predicted by a simple extrapolation of results issued from the hydrodynamic regime and those obtained from the incoherent signal analysis.     

Peptide friction in water nanofilm on mica surface

Peptide frictions in water nanofilms of various thicknesses on a mica surface are studied via molecular dynamics simulations. We find that the forced lateral motion of the peptide exhibits stick-slip behaviour at low water coverage; in contrast, the smooth gliding motion is observed at higher water coverage. The adsorbed peptide can form direct peptide-surface hydrogen bonds as well as indirect peptide-water-surface hydrogen bonds with the substrate. We propose that the stick-slip phenomenon is attributed to the overall effects of direct and indirect hydrogen bonds formed between the surface and the peptide.     

Effective spin-glass Hamiltonian for the anomalous dynamics of the HMF model

We discuss an effective spin-glass Hamiltonian which can be used to study the glassy-like dynamics observed in the metastable states of the Hamiltonian mean field (HMF) model. By means of the Replica formalism, we were able to find a self-consistent equation for the glassy order parameter which reproduces, in a restricted energy region below the phase transition, the microcanonical simulations for the polarization order parameter recently introduced in the HMF model.     

Anomalous scattering in supercooled ST2 water

ABSTRACT

Large-scale molecular dynamics (MD) simulations of systems containing up to 256,000 molecules were performed to investigate the scattering behaviour of the ST2 water model at deeply supercooled conditions. The simulations reveal that ST2 exhibits anomalous scattering, reminiscent of that observed in experiment, which is characterised by an increase in the static structure factor at low wavenumbers. This unusual behaviour in ST2 is linked with coupled fluctuations in density and local tetrahedral order in the liquid. The Ornstein–Zernike correlation length estimated from the anomalous scattering component exhibits power-law growth upon cooling, consistent with the existence of a liquid–liquid critical point (LLCP) in the ST2 model at ca. 245 K. Further, spontaneous liquid–liquid phase separation is observed upon thermally quenching a large system with 256,000 water molecules below the predicted critical temperature into the two-phase region. The large-scale MD simulations therefore confirm the existence of a metastable liquid–liquid phase transition in ST2 and support findings from previous computational studies performed using smaller systems containing only a few hundred molecules. We anticipate that our analysis may prove useful in interpreting recent scattering experiments that have been performed to search for an LLCP in deeply supercooled water.     

Cooperative Behavior of Kinetically Constrained Lattice Gas Models of Glassy Dynamics

Kinetically constrained lattice models of glasses introduced by Kob and Andersen (KA) are analyzed. It is proved that only two behaviors are possible on hypercubic lattices: either ergodicity at all densities or trivial non-ergodicity, depending on the constraint parameter and the dimensionality. But in the ergodic cases, the dynamics is shown to be intrinsically cooperative at high densities giving rise to glassy dynamics as observed in simulations. The cooperativity is characterized by two length scales whose behavior controls finite-size effects: these are essential for interpreting simulations. In contrast to hypercubic lattices, on Bethe lattices KA models undergo a dynamical (jamming) phase transition at a critical density: this is characterized by diverging time and length scales and a discontinuous jump in the long-time limit of the density autocorrelation function. By analyzing generalized Bethe lattices (with loops) that interpolate between hypercubic lattices and standard Bethe lattices, the crossover between the dynamical transition that exists on these lattices and its absence in the hypercubic lattice limit is explored. Contact with earlier results are made via analysis of the related Fredrickson--Andersen models, followed by brief discussions of universality, of other approaches to glass transitions, and of some issues relevant for experiments.     

Formation of argon cluster with proton seeding

ABSTRACT

We employ force-field molecular dynamics simulations to investigate the kinetics of nucleation to new liquid or solid phases in a dense gas of particles, seeded with ions. We use precise atomic pair interactions, with physically correct long-range behaviour, between argon atoms and protons. Time dependence of molecular cluster formation is analysed at different proton concentration, temperature and argon gas density. The modified phase transitions with proton seeding of the argon gas are identified and analysed. The seeding of the gas enhances the formation of nano-size atomic clusters and their aggregation. The strong attraction between protons and bath gas atoms stabilises large nano-clusters and the critical temperature for evaporation. An analytical model is proposed to describe the stability of argon-proton droplets and is compared with the molecular dynamics simulations.     

Subdiffusion and cage effect in a sheared granular material

We investigate experimentally the diffusion properties of a bidimensional bidisperse dry granular material under quasistatic cyclic shear. The comparison of these properties with results obtained both in computer simulations of hard spheres systems and Lennard-Jones liquids and experiments on colloidal systems near the glass transition demonstrates a strong analogy between the statistical behavior of granular matter and these systems, despite their intrinsic microscopic differences (thermal vs athermal). More specifically, we study in detail the cage dynamics responsible for the subdiffusion in the slow relaxation regime, and obtain the values of relevant time and length scales.     

Solvent density mode instability in non-polar solutions

We analyse the origin of the multiple long time scales associated with the long time decay observed in non-polar solvation dynamics by linear stability analysis of solvent density modes where the effects of compressibility and solvent structure are systematically incorporated. The coupling of the solute-solvent interactions at both ground and excited states of the solute with the compressibility and solvent structure is found to have important effects on the time scales. The present theory suggests that the relatively longer time constant is controlled by the solvent compressibility, while the solvent structure at the nearest-neighbour length scale dominates the shorter time constant.      

Fluctuation-dissipation relations and energy landscape in an out-of-equilibrium strong-glass-forming liquid

We study the out-of-equilibrium dynamics following a temperature jump in a model for silica, a strong liquid, and compare it with the well known case of fragile liquids. We calculate the fluctuation-dissipation relation, from which it is possible to estimate an effective temperature T(eff) associated with the slow out-of-equilibrium structural degrees of freedom. We find the striking and unexplained result that, different from the fragile liquid cases, T(eff) is smaller than the bath temperature.     

Non-ergodicity transition and multiple glasses in binary mixtures: on the accuracy of the input static structure in the mode coupling theory

We examine the question of the accuracy of the static correlation functions used as input in the mode coupling theory (MCT) of non-ergodic states in binary mixtures. We first consider hard-sphere mixtures and compute the static pair structure from the Ornstein-Zernike equations with the Percus-Yevick closure and more accurate ones that use bridge functions deduced from Rosenfeld's fundamental measures functional. The corresponding MCT predictions for the non-ergodicity lines and the transitions between multiple glassy states are determined from the long-time limit of the density autocorrelation functions. We find that while the non-ergodicity transition line is not very sensitive to the input static structure, up to diameter ratios D(2)/D(1)?=?10, quantitative differences exist for the transitions between different glasses. The discrepancies with the more accurate closures become even qualitative for sufficiently asymmetric mixtures. They are correlated with the incorrect behavior of the PY structure at high size asymmetry. From the example of ultra-soft potential it is argued that this issue is of general relevance beyond the hard-sphere model.