Microscopic Dynamic Properties Research Articles

Multi-scale modeling of the concrete SHPB test based on DEM-FDM coupling method

The split Hopkinson pressure bar (SHPB) test is the most commonly used method for testing the dynamic mechanical behavior of materials at high strain rates. In this paper, for the SHPB test of concrete, an efficient multi-scale mechanical calculation method based on coupling of discrete element method (DEM) and finite difference method (FDM) is proposed, and SHPB test system for analysis of the macroscopic and microscopic dynamic mechanical properties and damage evolution of concrete under impact loading is established by this coupling method. A three-phase mesoscopic scale model of concrete is constructed by DEM, and the crushability, real geometry-shape, and particle size distribution of aggregates are considered by combining 3D scanning technology and the “ball-clump-cluster” method. Meanwhile, FDM is adopted to simulate the SHPB model, and the “coupling wall” mechanism is applied to realize the signal transmission between the models. On this basis, the stress waves obtained by the direct monitoring method through numerical simulation are basically consistent with those obtained by the theoretical calculation method, which verifies the dynamic characteristics of the SHPB model. Finally, the different types of microcrack propagation are analyzed in the premise of examining the static and dynamic mechanical response of the concrete model. It was found that the distribution of mortar and aggregate microcracks on the outside of the concrete was significantly more than that on the inside due to transverse inertia force, and the superiority of the method in simulating the damage evolution behavior of concrete meso-structural is demonstrated.

Study on the Effect of Sandstone Microscopic Damage and Dynamic Compressive Properties After Heat Treatment

Study on the Effect of Sandstone Microscopic Damage and Dynamic Compressive Properties After Heat Treatment

Structure and Dynamics of Highly Attractive Polymer Nanocomposites in the Semi-Dilute Regime: The Role of Interfacial Domains and Bridging Chains.

Detailed molecular dynamics (MD) simulations are employed to study how the presence of adsorbed domains and nanoparticle bridging chains affect the structural, conformational, thermodynamic, and dynamic properties of attractive polymer nanocomposite melts in the semi-dilute regime. As a model system we have chosen an unentangled poly(ethylene glycol) (PEG) matrix containing amorphous spherical silica nanoparticles with different diameters and at different concentrations. Emphasis is placed on properties such as the polymer mass density profile around nanoparticles, the compressibility of the system, the mean squared end-to-end distance of PEG chains, their orientational and diffusive dynamics, the single chain form factor, and the scattering functions. Our analysis reveals a significant impact of the adsorbed, interfacial polymer on the microscopic dynamic and conformational properties of the nanocomposite, especially under conditions favoring higher surface-to-volume ratios (e.g., for small nanoparticle sizes at fixed nanoparticle loading, or for higher silica concentrations). Simultaneously, adsorbed polymer chains adopt graft-like conformations, a feature that allows them to considerably extend away from the nanoparticle surface to form bridges with other nanoparticles. These bridges drive the formation of a nanoparticle network whose strength (number of tie chains per nanoparticle) increases substantially with increasing concentration of the polymer matrix in nanoparticles, or with decreasing nanoparticle size at fixed nanoparticle concentration. The presence of hydroxyl groups at the ends of PEG chains plays a key role in the formation of the network. If hydroxyl groups are substituted by methoxy ones, the simulations reveal that the number of bridging chains per nanoparticle decreases dramatically, thus the network formed is less dense and less strong mechanically, and has a smaller impact on the properties of the nanocomposite. Our simulations predict further that the isothermal compressibility and thermal expansion coefficient of PEG-silica nanocomposites are significantly lower than those of pure PEG, with their values decreasing practically linear with increasing concentration of the nanocomposite in nanoparticles.

Study on the microscopic damage evolution and dynamic fracture properties of sandstone under freeze-thaw cycles

Freeze-thaw has great deterioration effect on rock mechanical performance, which strongly affect the stability of rock masses engineering. Meanwhile, natural fractures in real rocks are ubiquitous with considerable variations in size, number, and orientation, which further accelerate the risk of geological disasters. In this study, the microstructure changes of specimens induced by cyclic freeze-thaw were measured using nuclear magnetic resonance (NMR) technique. Dynamic fracture tests were performed using single cleavage triangle (SCT) red sandstone specimens. Crack propagation gauges (CPGs) were applied to determine the crack velocity. Meanwhile, the microstructure of the fracture surface was obtained with the aid of scanning electron microscope (SEM). The results show that the cyclic freeze-thaw plays an important part in rock dynamic fracture behavior. The porosity and crack velocity increase with freeze-thaw cycles, whereas the crack initiation time and dynamic fracture toughness decrease with freeze-thaw cycles. Sandstone has three pore types and is susceptible to the freeze-thaw weathering. The fracture surface of specimen without freeze-thaw is smooth and the fracture mode is transgranular. With the rise of freeze-thaw cycles, the fracture surface becomes rougher, indicating that intergranular fracture plays a dominant role in sandstone failure.

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. For the first time we study the structural relaxation of this mixture in the supercooled-liquid regime. From the simulations we determine the time- and temperature-dependent coherent and incoherent scattering functions for a large range of wave vectors, as well as the mean-square displacements of both particle species. We perform a detailed analysis of the dynamics by comparing the MD results with the first-principles-based idealized mode-coupling theory (MCT). To this end, we employ two approaches: fits to the asymptotic predictions of the theory, and fit-parameter-free binary MCT calculations based on static-structure-factor input from the simulations. We find that many-body interactions of the Voronoi mixture do not lead to strong qualitative differences relative to similar analyses carried out for simple liquids with pair-wise interactions. For instance, the fits give an exponent parameter λ ≈ 0.746 comparable to typical values found for simple liquids, the wavevector dependence of the Kohlrausch relaxation time is in good qualitative agreement with literature results for polydisperse hard spheres, and the MCT calculations based on static input overestimate the critical temperature, albeit only by a factor of about 1.2. This overestimation appears to be weak relative to other well-studied supercooled-liquid models such as the binary Kob–Andersen Lennard-Jones mixture. Overall, the agreement between MCT and simulation suggests that it is possible to predict several microscopic dynamic properties with qualitative, and in some cases near-quantitative, accuracy based solely on static two-point structural correlations, even though the system itself is inherently governed by many-body interactions.

Effect of water cooling on microscopic damage and dynamic properties of high-temperature granite

The microscopic damage and dynamic mechanical properties of high-temperature granite after water cooling were studied through wave velocity test, nuclear magnetic resonance (NMR) test, split Hopkinson pressure bar (SHPB) impact test and scanning electron microscope（SEM）test. The variation of porosity and dynamic mechanical parameters of granite were analyzed. The results show that the wave velocity of high-temperature granite decreases nonlinearly after water cooling and the components of porosity with large pore increase with the increase of temperature. Moreover, water cooling leads to the more cracks and greater crack sizes than that of natural cooling. The dynamic parameters of high-temperature granite after water cooling show that the increase in temperature results in a decrease in the peak stress, an increase in the peak strain, and an increase at first then decrease in the elastic modulus. Additional thermal stress, resulted from sharply decrease in surface temperature of the high-temperature granite, leads to increased internal damage and decreased wave velocity and peak stress. Compared with natural cooling, the plasticity of high-temperature granite is reduced, because the cold hardening effect improves the hardness of surface granite. After water cooling, the granite specimens exhibit the brittle failure characteristics and their peak strain decreases but their elastic modulus increases. Cooling way has a minor effect on the cracks induced by shock before 400 ℃. As the temperature up to 800 ℃, the impact fracture surface of granite after natural cooling is characterized by honeycomb and irregular shape, in contrast, the impact fracture surface of granite after the water cooling is relatively flat.

Study on the effect of samplers on the disturbance of dredger-filled soil structure by numerical simulation layering method

Through the use of the particle flow software simulation, the influences on the microscopic dynamic properties of high viscosity soft soil caused by the inner diameter, cut angle and wall thickness of the samplers were studied with the example of the dredged mud. The motion behavior of the inflection point of particle characteristics was compared in seven soil samples by dividing the soil layers. The results show that a convective displacement field of the soil particles is formed at the bottom, and the particles above the convection are mainly subject to tensile expansion while the particles below the convection are mainly compressed and contracted, which result in the regular changes in porosity and bending deformation of the soil layers. There is a synergistic relationship between the intersection of the porosity curve and the initial porosity curve and the convective position of the particle displacement field. There is a characteristic inflection point in the interlayer particle displacement with the soil layers behave as a bending deformation. The particle disturbance between the inflection points is not obvious and the particle disturbance outside the inflection points is larger. The deformation feature can be fitted to a rotating paraboloid with the lower opening and the disturbance of the seven groups soil samples can be assessed initially by the a value of the surface equation. The line connecting the inflection points of the interlayer features can be refitted to a paraboloid of rotation. The ratio of the volume of the paraboloid to the volume of the soil sample can be used to evaluate the originality of the soil sample.

Molecular Dynamics Analysis of Charge Transport in Ionic-Liquid Electrolytes Containing Added Salt with Mono, Di, and Trivalent Metal Cations.

Among many other applications, room-temperature ionic liquids (ILs) are used as electrolytes for storage and energy-conversion devices. In this work, we investigate, at the microscopic level, the structural and dynamical properties of 1-methyl-1-butyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide [C4 PYR]+ [Tf2 N]- IL-based electrolytes for metal-ion batteries. We carried out molecular dynamics simulations of electrolytes mainly composed of [C4 PYR]+ [Tf2 N]- IL with the addition of Mn+ -[Tf2 N]- metal salts (M=Li+ , Na+ , Ni2+ , Zn2+ , Co2+ , Cd2+ , and Al3+ , n=1, 2, and 3) dissolved in the IL. The addition of low salt concentrations lowers the charge transport and conductivity of the electrolytes. This effect is due to the strong interaction of the metal cations with the [Tf2 N]- anions, which allows for molecular aggregation between them. We analyze how the conformation of the [Tf2 N]- anions surrounding the metal cations determine the charge-transport properties of the electrolyte. We found two main conformations based on the size and charge of the metal cation: monodentate and bidentate (number of oxygen atoms of the anion pointing to the metal atoms). The microscopic local structure of the Mn+ -[Tf2 N]- aggregates influences the microscopic charge transport as well as the macroscopic conductivity of the total electrolyte.

Microstructures and dynamics of tetraalkylphosphonium chloride ionic liquids.

Atomistic simulations have been performed to investigate the effect of aliphatic chain length in tetraalkylphosphonium cations on liquid morphologies, microscopic ionic structures, and dynamical quantities of tetraalkylphosphonium chloride ionic liquids. The liquid morphologies are characterized by sponge-like interpenetrating polar and apolar networks in ionic liquids consisting of tetraalkylphosphonium cations with short aliphatic chains. The lengthening aliphatic chains in tetraalkylphosphonium cations lead to polar domains consisting of chloride anions and central polar groups in cations being partially or totally segregated in ionic liquid matrices due to a progressive expansion of apolar domains in between. Prominent polarity alternation peaks and adjacency correlation peaks are observed at low and high q range in total X-ray scattering structural functions, respectively, and their peak positions gradually shift to lower q values with lengthening aliphatic chains in tetraalkylphosphonium cations. The charge alternation peaks registered in the intermediate q range exhibit complicated tendencies due to a cancellation of peaks and anti-peaks in partial structural functions for ionic subcomponents. The particular microstructures and liquid morphologies in tetraalkylphosphonium chloride ionic liquids intrinsically contribute to distinct dynamics characterized by mean square displacements, van Hove correlation functions, and non-Gaussian parameters for ionic species in the heterogeneous ionic environment. Most tetraalkylphosphonium cations have higher translational mobilities than their partner anions due to strong coordination of chloride anions with central polar groups in tetraalkylphosphonium cations through strong Coulombic and hydrogen bonding interactions. The increase of aliphatic chain length in tetraalkylphosphonium cations leads to a concomitant shift of van Hove correlation functions and non-Gaussian parameters to larger radial distances and longer time scales, respectively, indicating the enhanced translational dynamical heterogeneities of tetraalkylphosphonium cations and the corresponding chloride anions.

On the composition dependence of the microscopic structure, thermodynamic, dynamic and dielectric properties of water-dimethyl formamide model mixtures. Molecular dynamics simulation results

Isothermal-isobaric molecular dynamics simulations have been performed to examine an ample set of properties of the model water-N,N-dimethylformamide (DMF) mixture as a function of composition. The SPC-E and TIP4P-Ew water models together with two united atom models for DMF [Chalaris M., Samios J., J. Chem. Phys., 2000, 112, 8581; Cordeiro J., Int. J. Quantum Chem., 1997, 65, 709] were used. Our principal analyses concern the behaviour of structural properties in terms of radial distribution functions, and the number of hydrogen bonds between molecules of different species as well as thermodynamic properties. Namely, we explore the density, excess mixing molar volume and enthalpy, the heat capacity and excess mixing heat capacity. Finally, the self-diffusion coefficients of species and the dielectric constant of the system are discussed. In addition, surface tension of water-DMF mixtures has been calculated and analyzed.

Molecular modeling of polycarbonate materials: Glass transition and mechanical properties

Linking the experimentally accessible macroscopic properties of thermoplastic polymers to their microscopic static and dynamic properties is a key requirement for targeted material design. Classical molecular dynamics simulations enable us to study the structural and dynamic behavior of molecules on microscopic scales, and statistical physics provides a framework for relating these properties to the macroscopic properties. We take a first step toward creating an automated workflow for the theoretical prediction of thermoplastic material properties by developing an expeditious method for parameterizing a simple yet surprisingly powerful coarse-grained bisphenol-A polycarbonate model which goes beyond previous coarse-grained models and successfully reproduces the thermal expansion behavior, the glass transition temperature as a function of the molecular weight, and several elastic properties.

Co-evolution of cooperation and limited resources on interdependent networks

In this article, based on interdependent networks, cooperation in spatial prisoner's dilemma game (PDG) with coevolving resources is studied. By means of a strategy-independent rule, limited resources can be continually re-allocated among different players in the same network or across different networks. The coevolution of dynamics is discussed respectively for two cases: game circumstances on two coupled networks are identical or asymmetry. In the first case, we obtain that the involvement of resources can be significantly beneficial for cooperative behaviors, and the heterogeneous distribution of resources can positively enhance interdependent network reciprocity. Furthermore, an optimal value (ρ ≈ 0.8) of the interdependent strength exists for cooperation, which is obviously larger than the previous results (ρ ≈ 0.5) without coevolving resources. Besides, we also find resources follow the power law distribution, where cooperators and interconnected players tend to obtain more resources. In the second case, we mainly focus on the flow of resources between networks as well as the intertwined effect of the interdependent strength and resources distribution on cooperation. Microscopic dynamical properties of the flow of resources jointly caused by various factors have been discussed. In certain conditions, instead of promoting the cooperation, the influx of resources could even be positive for defection.

膜の動的特性を利用した分子認識 / 分子変換

Biomembranes have attracted much attention for the design of functional membranes because biomembranes possesses the variety of potential functions and induce the necessary functions in response to the environmental changes. We have previously revealed the dynamic property of a closed bilayer membranes composed of phospholipids (liposome) that is usually used as a model biomembrane, with a dielectric dispersion analysis. A lateral diffusion, a rotational Brownian motion of phospholipids within a lipid membranes and lipid–lipid (or other molecules) interactions comprised a dynamics of lipid membranes. These modes strongly correlated with each other to hierarchically constitute the hydration structure, phase separation behavior, membrane fluidity, or permeability to solute, and membrane undulation. Based on the hierarchical dynamics of membranes, chemical conversion on membranes was demonstrated: (i) oxidation reaction of hydroxyl methyl furfural; (ii) aldorl condensation reaction of benzaldehyde; polymerization reaction of (iii) lactide and (iv) aniline. From these case studies, the microscopic and macroscopic dynamic properties could well control the reaction process on lipid membranes.

Reparametrized E3B (Explicit Three-Body) Water Model Using the TIP4P/2005 Model as a Reference.

In this study, we present the third version of a water model that explicitly includes three-body interactions. The major difference between this version and the previous two is in the two-body water model we use as a reference potential; here we use the TIP4P/2005 model (previous versions used the TIP4P water model). We alter four parameters from our previous version of the model by fitting to the diffusion coefficient of the ambient liquid, the liquid and ice densities, and the melting point. We evaluate the performance of this version by calculating many other microscopic and thermodynamic static and dynamic properties as a function of temperature and near the critical point and comparing to experiment, the TIP4P/2005 model and the previous version of our three-body model.

Microscopic damage and dynamic mechanical properties of rock under freeze–thaw environment

For understanding the rock microscopic damage and dynamic mechanical properties subjected to recurrent freeze–thaw cycles, experiments for five groups of homogeneous sandstone under different freeze–thaw cycles were conducted. After freeze– thaw, nuclear magnetic resonance (NMR) tests and impact loading tests were carried out, from which microscopic damage characteristics of sandstone and dynamic mechanical parameters were obtained. The results indicate that the porosity increases with the increase of cycle number, the rate of porosity growth descends at the beginning of freeze–thaw, yet accelerates after a certain number of cycles. The proportion of pores with different sizes changes dynamically and the multi-scale distribution of pores tends to develop on pore structure with the continuing impact of freeze–thaw and thawing. Dynamic compressive stress–strain curve of sandstone undergoing freeze–thaw can be divided into four phases, and the phase of compaction is inconspicuous compared with the static curve. Elastic modulus and dynamic peak intensity of sandstone gradually decrease with freeze–thaw cycles, while peak strain increases. The higher the porosity is, the more serious the degradation of dynamic intensity is. The porosity is of a polynomial relationship with the dynamic peak intensity.

Microscopic dynamics of water around unfolded structures of barstar at room temperature.

The breaking of the native structure of a protein and its influences on the dynamic response of the surrounding solvent is an important issue in protein folding. In this work, we have carried out atomistic molecular dynamics simulations to unfold the protein barstar at two different temperatures (400 K and 450 K). The two unfolded forms obtained at such high temperatures are further studied at room temperature to explore the effects of nonuniform unfolding of the protein secondary structures along two different pathways on the microscopic dynamical properties of the surface water molecules. It is demonstrated that though the structural transition of the protein in general results in less restricted water motions around its segments, but there are evidences of formation of new conformational motifs upon unfolding with increasingly confined environment around them, thereby resulting in further restricted water mobility in their hydration layers. Moreover, it is noticed that the effects of nonuniform unfolding of the protein segments on the relaxation times of the protein-water (PW) and the water-water (WW) hydrogen bonds are correlated with hindered hydration water motions. However, the kinetics of breaking and reformation of such hydrogen bonds are found to be influenced differently at the interface. It is observed that while the effects of unfolding on the PW hydrogen bond kinetics seem to be minimum, but the kinetics involving the WW hydrogen bonds around the protein segments exhibit noticeably heterogeneous characteristics. We believe that this is an important observation, which can provide valuable insights on the origin of heterogeneous influence of unfolding of a protein on the microscopic properties of its hydration water.

Relating the Poisson's ratio to molecular dynamics of glass-formers

Despite the importance of Poisson's ratio in materials science and engineering, its connection to molecular dynamics and relaxation processes in glass-forming systems remains unclear. The existence of such connection made the family of metallic glasses in our previous study [K.L. Ngai et al. J. Chem. Phys. 140 (2014) 044511], where it was also justified by molecular dynamics simulation, physics of glass-forming materials, and theoretical considerations. Here we extend the consideration to molecular glass-formers and amorphous polymers, and demonstrate that the Poisson's ratio correlates with several microscopic dynamic properties, and explained theoretically in exactly the same way as given in our previous publication.

Microscopic dynamics perspective on the relationship between Poisson's ratio and ductility of metallic glasses.

In metallic glasses a clear correlation had been established between plasticity or ductility with the Poisson's ratio νPoisson and alternatively the ratio of the elastic bulk modulus to the shear modulus, K/G. Such a correlation between these two macroscopic mechanical properties is intriguing and is challenging to explain from the dynamics on a microscopic level. A recent experimental study has found a connection of ductility to the secondary β-relaxation in metallic glasses. The strain rate and temperature dependencies of the ductile-brittle transition are similar to the reciprocal of the secondary β-relaxation time, τβ. Moreover, metallic glass is more ductile if the relaxation strength of the β-relaxation is larger and τβ is shorter. The findings indicate the β-relaxation is related to and instrumental for ductility. On the other hand, K/G or νPoisson is related to the effective Debye-Waller factor (i.e., the non-ergodicity parameter), f0, characterizing the dynamics of a structural unit inside a cage formed by other units, and manifested as the nearly constant loss shown in the frequency dependent susceptibility. We make the connection of f0 to the non-exponentiality parameter n in the Kohlrausch stretched exponential correlation function of the structural α-relaxation function, ϕ(t)=exp[-(t/τα)(1-n)]. This connection follows from the fact that both f0 and n are determined by the inter-particle potential, and 1/f0 or (1 - f0) and n both increase with anharmonicity of the potential. A well tested result from the Coupling Model is used to show that τβ is completely determined by τα and n. From the string of relations, (i) K/G or νPoisson with 1/f0 or (1 - f0), (ii) 1/f0 or (1 - f0) with n, and (iii) τα and n with τβ, we arrive at the desired relation between K/G or νPoisson and τβ. On combining this relation with that between ductility and τβ, we have finally an explanation of the empirical correlation between ductility and the Poisson's ratio νPoisson or K/G based on microscopic dynamical properties.

Surface and Interface Analyses of Polymer Brushes by Synchrotron Radiation

In the present review, we focus on the characterization of polymer brushes by quantum beam, which is regarded as a promising probe of surface and interface analysis. The polymer brushes were prepared on various shapes of surface, and proved to be benefit to various applications. Among them, the polymer brushes grafted on sphere nanoparticles and flat substrates are investigated as representative cases. The static structure of polymer brushes, especially the chain dimension of polymer brushes grafted on nanoparticles and a flat substrate, have been studied by small angle X-ray scattering (SAXS) and neutron reflectivity (NR), respectively. The microscopic dynamical properties of polymer brushes are also expected to be revealed by quantum beam. X-ray photon correlation spectroscopy (XPCS), a technique using a coherent X-ray, is one of the promising methods for microscopically understanding of dynamical properties of polymer brushes. Some of our recent studies about dynamical behavior of polymer brush immobilized nanoparticle by XPCS are also presented.

Importance of Protein Conformational Motions and Electrostatic Anchoring Sites on the Dynamics and Hydrogen Bond Properties of Hydration Water

The microscopic dynamic properties of water molecules present in the vicinity of a protein are expected to be sensitive to its local conformational motions and the presence of polar and charged groups at the surface capable of anchoring water molecules through hydrogen bonds. In this work, we attempt to understand such sensitivity by performing detailed molecular dynamics simulations of the globular protein barstar solvated in aqueous medium. Our calculations demonstrate that enhanced confinement at the protein surface on freezing its local motions leads to increasingly restricted water mobility with long residence times around the secondary structures. It is found that the inability of the surface water molecules to bind with the protein residues by hydrogen bonds in the absence of protein-water (PW) electrostatic interactions is compensated by enhanced water-water hydrogen bonds around the protein with uniform bulklike behaviors. Importantly, it is further noticed that in contrast to the PW hydrogen bond relaxation time scale, the kinetics of the breaking and formation of such bonds are not affected on freezing the protein's conformational motions.