Isotropic tensor fields in amorphous solids: Correlation functions of displacement and strain tensor fields.

Generalizing recent work on isotropic tensor fields in isotropic and achiral condensed matter systems from two to arbitrary dimensions we address both mathematical aspects assuming perfectly isotropic systems and applications focusing on correlation functions of displacement and strain field components in amorphous solids where isotropy may not hold. Various general points are exemplified using simulated polydisperse Lennard-Jones particles. It is shown that the strain components in reciprocal space have essentially a complex circularly symmetric Gaussian distribution albeit weak non-Gaussianity effects become visible for large wave numbers q where also anisotropy effects become relevant.

General Relations between Stress Fluctuations and Viscoelasticity in Amorphous Polymer and Glass-Forming Systems.

Mechanical stress governs the dynamics of viscoelastic polymer systems and supercooled glass-forming fluids. It was recently established that liquids with long terminal relaxation times are characterized by transiently frozen stress fields, which, moreover, exhibit long-range correlations contributing to the dynamically heterogeneous nature of such systems. Recent studies show that stress correlations and relaxation elastic moduli are intimately related in isotropic viscoelastic systems.

Correlations of tensor field components in isotropic systems with an application to stress correlations in elastic bodies.

Correlation functions of components of second-order tensor fields in isotropic systems can be reduced to an isotropic fourth-order tensor field characterized by a few invariant correlation functions (ICFs). It is emphasized that components of this field depend in general on the coordinates of the field vector variable and thus on the orientation of the coordinate system. These angular dependencies are distinct from those of ordinary anisotropic systems.

Strain correlation functions in isotropic elastic bodies: large wavelength limit for two-dimensional systems.

Strain correlation functions in two-dimensional isotropic elastic bodies are shown both theoretically (using the general structure of isotropic tensor fields) and numerically (using a glass-forming model system) to depend on the coordinates of the field variable (position vector in real space or wavevector in reciprocal space) and thus on the direction of the field vector and the orientation of the coordinate system. Since the fluctuations of the longitudinal and transverse components of the strain field in reciprocal space are known in the long-wavelength limit from the equipartition theorem, all components of the correlation function tensor field are imposed and no additional physical assumptions are needed. An observed dependence on the field vector direction thus cannot be used as an indication for anisotropy or for a plastic rearrangement.

Alcohol use during the COVID-19 pandemic: gender, parenthood, intimate partner violence, and stress.

Some preliminary work during the COVID-19 pandemic indicates that adult alcohol use increased, particularly for parents. This cross-sectional study examined the quantity and frequency of adults' alcohol use during the early stages of the pandemic. Additionally, the influences of gender, parenthood, COVID-19-related stressors and intimate partner violence (IPV) on alcohol consumption were examined.

Molecular Simulations of Controlled Polymer Crystallization in Polyethylene.

Multilamella polymer crystals are grown from the melt for the first time, in molecular dynamics simulations of a united-monomer model, with in excess of 1500000 united-monomers. Two-component systems comprised of equal weight fractions of 2000 united-monomer long chains and 200 united-monomer short chains are considered, with varying numbers of short butyl branches placed along the long chains. Utilizing two different cooling protocols, continuous-cooling and self-seeding, drastically different multilamella structures are revealed, which depend heavily on the branch content and crystallization protocol used.

Drinking to cope and coping strategies in Deaf/Hard of hearing college students.

Introduction: Deaf and hard of hearing (DHH) students experience unique stressors as a minority linguistic and cultural group that may contribute to problematic substance and alcohol use behavior. Proper coping strategies may be one protective factor to avoid reliance on alcohol as a means to reduce stress.

Methods: The current study compared the endorsement of coping strategies by DHH students and hearing individuals and their relationship to drinking to cope behavior.

Different types of spatial correlation functions for non-ergodic stochastic processes of macroscopic systems.

Focusing on non-ergodic macroscopic systems, we reconsider the variances [Formula: see text] of time averages [Formula: see text] of time-series [Formula: see text]. The total variance [Formula: see text] (direct average over all time series) is known to be the sum of an internal variance [Formula: see text] (fluctuations within the meta-basins) and an external variance [Formula: see text] (fluctuations between meta-basins). It is shown that whenever [Formula: see text] can be expressed as a volume average of a local field [Formula: see text] the three variances can be written as volume averages of correlation functions [Formula: see text], [Formula: see text] and [Formula: see text] with [Formula: see text].

Role of torsional potential in chain conformation, thermodynamics, and glass formation of simulated polybutadiene melts.

For polymer chains, the torsional potential is an important intramolecular energy influencing chain flexibility and segmental dynamics. Through molecular dynamics simulations of an atomistic model for melts of cis-trans-1,4-polybutadiene (PBD), we explore the effect of the torsions on conformational properties (bond vector correlations and mean-square internal distances), fundamental thermodynamic quantities (density, compressibility, internal energy, and specific heat), and glass transition temperature T. This is achieved by systematically reducing the strength of the torsional potential, starting from the chemically realistic chain (CRC) model with the full potential toward the freely rotating chain (FRC) model without the torsional potential.

The memory of thin polymer films generated by spin coating.

We present results from isothermal and temperature-sweep creep experiments adapted to filaments which were derived from spin coated and subsequently crumpled thin polystyrene films. Due to the existence of residual stresses induced by preparation, the filaments showed significant shrinkage which we followed as a function of time at various temperatures. In addition, the influence of preparation conditions and subsequent annealing of supported thin polymer films on shrinkage and relaxation behavior was investigated.

Theory of length-scale dependent relaxation moduli and stress fluctuations in glass-forming and viscoelastic liquids.

The spatiotemporal correlations of the local stress tensor in supercooled liquids are studied both theoretically and by molecular dynamics simulations of a two-dimensional (2D) polydisperse Lennard-Jones system. Asymptotically exact theoretical equations defining the dynamical structure factor and all components of the stress correlation tensor for low wave-vector q are presented in terms of the generalized (q-dependent) shear and longitudinal relaxation moduli, G(q, t) and K(q, t). We developed a rigorous approach (valid for low q) to calculate K(q, t) in terms of certain bulk correlation functions (for q = 0), the static structure factor S(q), and thermal conductivity κ.

Simple models for strictly non-ergodic stochastic processes of macroscopic systems.

We investigate simple models for strictly non-ergodic stochastic processes [Formula: see text] (t being the discrete time step) focusing on the expectation value v and the standard deviation [Formula: see text] of the empirical variance [Formula: see text] of finite time series [Formula: see text]. [Formula: see text] is averaged over a fluctuating field [Formula: see text] ([Formula: see text] being the microcell position) characterized by a quenched spatially correlated Gaussian field [Formula: see text]. Due to the quenched [Formula: see text]-field [Formula: see text] becomes a finite constant, [Formula: see text], for large sampling times [Formula: see text].

Relaxation moduli of glass-forming systems: temperature effects and fluctuations.

Equilibrium and dynamical properties of a two-dimensional polydisperse colloidal model system are characterized by means of molecular dynamics (MD) and Monte Carlo (MC) simulations. We employed several methods to prepare quasi-equilibrated systems: in particular, by slow cooling and tempering with MD (method SC-MD), and by tempering with MC dynamics involving swaps of particle diameters (methods Sw-MD, Sw-MC). It is revealed that the Sw-methods are much more efficient for equilibration below the glass transition temperature T leading to denser and more rigid systems which show much slower self-diffusion and shear-stress relaxation than their counterparts prepared with the SC-MD method.

The role of affective urgency and emotion regulation in vaping susceptibility.

Introduction: The prevalence of E-cigarette use is increasing along with concerns about the negative health effects of their use. Understanding the psychological constructs associated with susceptibility to beginning regular e-cigarette use may be helpful for prevention efforts. Factors such as emotion regulation (ER) and impulsivity, specifically urgency, have been significantly correlated with patterns of drug addiction in the past.

General relations to obtain the time-dependent heat capacity from isothermal simulations.

It is well-known that time-dependent correlation functions related to temperature and energy can crucially depend on the thermostatting mechanism used in computer simulations of molecular systems. We argue, however, that linear response functions must be considered as universal properties of physical systems. This implies that the classical fluctuation equation for the transient heat capacity, c(t), is not applicable to the thermostatted molecular dynamics (apart from long enough times).

Fluctuations of non-ergodic stochastic processes.

We investigate the standard deviation [Formula: see text] of the variance [Formula: see text] of time series [Formula: see text] measured over a finite sampling time [Formula: see text] focusing on non-ergodic systems where independent "configurations" c get trapped in meta-basins of a generalized phase space. It is thus relevant in which order averages over the configurations c and over time series k of a configuration c are performed. Three variances of [Formula: see text] must be distinguished: the total variance [Formula: see text] and its contributions [Formula: see text], the typical internal variance within the meta-basins, and [Formula: see text], characterizing the dispersion between the different basins.

Ensemble fluctuations matter for variances of macroscopic variables.

Extending recent work on stress fluctuations in complex fluids and amorphous solids we describe in general terms the ensemble average [Formula: see text] and the standard deviation [Formula: see text] of the variance [Formula: see text] of time series [Formula: see text] of a stochastic process x(t) measured over a finite sampling time [Formula: see text]. Assuming a stationary, Gaussian and ergodic process, [Formula: see text] is given by a functional [Formula: see text] of the autocorrelation function h(t). [Formula: see text] is shown to become large and similar to [Formula: see text] if [Formula: see text] corresponds to a fast relaxation process.

Composition fluctuations in polydisperse liquids: Glasslike effects well above the glass transition.

We study a two-dimensional glass-forming system of slightly polydisperse (LJ) particles using molecular dynamics simulations and demonstrate that in the liquid regime (well above the vitrification temperature) this model shows a number of features typical of the glass transition: (i) the relation between compressibility and structure factor S(q) is strongly violated; (ii) the dynamical structure factor S(q,t) at low q shows a two-step relaxation; (iii) the time-dependent heat capacity c_{v}(t) shows a long-time power-law tail. We show that these phenomena can be rationalized with the idea of composition fluctuations and provide a quantitative theory for the effects (i) and (ii). It implies that such effects must be inherent in all polydisperse colloidal models, including binary LJ mixtures.

Glassy dynamics of a binary Voronoi fluid: a mode-coupling analysis.

The binary Voronoi mixture is a fluid model whose interactions are derived from the Voronoi-Laguerre tessellation of the configurations of the system. The resulting interactions are local and many-body. Here we perform molecular-dynamics (MD) simulations of an equimolar mixture that is weakly polydisperse and additive.

Microscopic Structure of Compacted Polyelectrolyte Complexes: Insights from Molecular Dynamics Simulations.

We utilize atomistic molecular dynamics (MD) simulations to study local structural changes inside a polyelectrolyte complex consisting of poly(styrenesulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA) upon densification, in analogy to ultracentrifugation in experiments. In particular, we focus on the water content and on the reinforcement of the PSS-PDADMA network for various external accelerations. We demonstrate that apart from the formation of mesoscopic pores observed experimentally also the microscopic structure and the local relaxation processes likely affect the unique rheological properties of compacted polyelectrolyte complexes, as densification increases both the number of PSS-PDADMA coordinations and the intermixing of PSS and PDADMA.

Long-range stress correlations in viscoelastic and glass-forming fluids.

A simple and rigorous approach to obtain stress correlations in viscoelastic liquids (including supercooled liquid and equilibrium amorphous systems) is proposed. The long-range dynamical correlations of local shear stress are calculated and analyzed in 2-dimensional space. It is established how the long-range character of the stress correlations gradually emerges as the relevant dynamical correlation length l grows in time.

Continuous-time random-walk approach to supercooled liquids: Self-part of the van Hove function and related quantities.

From equilibrium molecular dynamics (MD) simulations of a bead-spring model for short-chain glass-forming polymer melts we calculate several quantities characterizing the single-monomer dynamics near the (extrapolated) critical temperature [Formula: see text] of mode-coupling theory: the mean-square displacement g(t), the non-Gaussian parameter [Formula: see text] and the self-part of the van Hove function [Formula: see text] which measures the distribution of monomer displacements r in time t. We also determine these quantities from a continuous-time random walk (CTRW) approach. The CTRW is defined in terms of various probability distributions which we know from previous analysis.

Voronoi glass-forming liquids: A structural study.

We introduce a theoretical model of simple fluid, whose interactions, defined in terms of the Voronoi cells of the configurations, are local and many-body. The resulting system is studied both theoretically and numerically. We show that the fluid, though sharing the global features of other models of fluids with soft interactions, has several unusual characteristics, which are investigated and discussed.

Shear-stress fluctuations and relaxation in polymer glasses.

We investigate by means of molecular dynamics simulation a coarse-grained polymer glass model focusing on (quasistatic and dynamical) shear-stress fluctuations as a function of temperature T and sampling time Δt. The linear response is characterized using (ensemble-averaged) expectation values of the contributions (time averaged for each shear plane) to the stress-fluctuation relation μ_{sf} for the shear modulus and the shear-stress relaxation modulus G(t). Using 100 independent configurations, we pay attention to the respective standard deviations.

Shear Modulus and Shear-Stress Fluctuations in Polymer Glasses.

Using molecular dynamics simulation of a standard coarse-grained polymer glass model, we investigate by means of the stress-fluctuation formalism the shear modulus μ as a function of temperature T and sampling time Δt. While the ensemble-averaged modulus μ(T) is found to decrease continuously for all Δt sampled, its standard deviation δμ(T) is nonmonotonic, with a striking peak at the glass transition. Confirming the effective time-translational invariance of our systems, μ(Δt) can be understood using a weighted integral over the shear-stress relaxation modulus G(t).