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We study the shock-induced hot spot formation mechanism of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine-based explosives by molecular dynamics, compare different kinds of desensitizers and different shock velocities. A set of programs is written to calculate the physical picture of shock loading. Based on the simulations and analyses, the hot spots are found at the interface and are heated by plastic work in three ways: the interface intrinsic dissipation, the pore collapse, and the coating layer deformation. The work/heat transition rate is proved to be increasing with a loading speed. [ABSTRACT FROM AUTHOR]

This paper demonstrates a method for calculating the tension of a system with a curved interface from a molecular dynamics simulation. To do so, the pressure of a subset of the system is determined by applying a local (virtual) mechanical deformation, fitting the response to that of a bulk fluid, and then using the Young-Laplace equation to infer the tension of the interface. The accuracy of the method is tested by calculating the local pressure of a series of water simulations at various external pressures. The tension of a simulated curved octane-water interface is computed with the method and compares well with the planar tension (≈ 46.7 dyn/cm). Finally, an ambiguity is resolved between the Harasima and Irving-Kirkwood methods of calculating the local pressure as a means for computing the tension. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations have been carried out to obtain the interfacial and coexistence properties of soft-sphere attractive Yukawa (SAY) fluids with short attraction range, κ = 10, 9, 8, 7, 6, and 5. All our simulation results are new. These data are also compared with the recently reported results in the literature of hard-core attractive Yukawa (HAY) fluids. We show that the interfacial and coexistence properties of both potentials are different. For the surveyed systems, here we show that all coexistence curves collapse into a master curve when we rescale with their respective critical points and the surface tension curves form a single master curve when we plot γ* vs. T/Tc. [ABSTRACT FROM AUTHOR]

Bulk and surface properties of the ionic liquids 1-alkyl-3-methyl-imidazolium iodides ([Cnmim]I) were simulated by classical molecular dynamics using all atom non-polarizable force field (n = 4, butyl; 6, hexyl; 8, octyl). The structure of ionic liquids were initially optimized by density functional theory and atomic charges obtained by CHELPG method. Reduction of partial atomic charges (by 20% for simulation of density and surface tension, and by 10% for viscosity) found to improve the accuracy, while a non-polarizable force field was applied. Additionally, the simulation ensembles approach the equilibrium faster when the charge reduction is applied. By these refined force field parameters, simulated surface tensions in the range of 323-393 k are quite in agreement with the experiments. Simulation of temperature dependent surface tension of [C4mim]I well beyond room temperature (up to 700 K) permits prediction of the critical temperature in agreement with that predicted from experimental surface tension data. Simulated densities in the range of 298-450 K for the three ionic liquids are within 0.8% of the experimental data. Structural properties for [C4mim]I were found to be in agreement with the results of Car-Parrinello molecular dynamics simulation we performed, which indicates a rather well-structured cation-anion interaction and occurs essentially through the imidazolium ring cation. Diffusion coefficient changes with alkyl chain length in the order of [C8mim]I > [C6mim]I > [C4mim]I for the cation and the anion. Formation of a dense domain in subsurface region is quite evident, and progressively becomes denser as the alkyl chain length increases. Bivariate orientational analysis was used to determine the average orientation of molecule in ionic liquids surface, subsurface, and bulk regions. Dynamic bisector-wise and side-wise movement of the imodazolium ring cation in the surface region can be deduced from the bivariate maps. Atom-atom density profile and bivariate analysis indicate that the imidazolium cation takes a spoon like configuration in the surface region and the tilt of alkyl group is a function length of alkyl chain exposing as linear as possible to the vapor phase. [ABSTRACT FROM AUTHOR]

Data for the flow rate of water in carbon nanopores is widely scattered, both in experiments and simulations. In this work, we aim at precisely quantifying the characteristic large slip length and flow rate of water flowing in a planar graphene nanochannel. First, we quantify the slip length using the intrinsic interfacial friction coefficient between water and graphene, which is found from equilibrium molecular dynamics (EMD) simulations. We then calculate the flow rate and the slip length from the streaming velocity profiles obtained using non-equilibrium molecular dynamics (NEMD) simulations and compare with the predictions from the EMD simulations. The slip length calculated from NEMD simulations is found to be extremely sensitive to the curvature of the velocity profile and it possesses large statistical errors. We therefore pose the question: Can a micrometer range slip length be reliably determined using velocity profiles obtained from NEMD simulations? Our answer is 'not practical, if not impossible' based on the analysis given as the results. In the case of high slip systems such as water in carbon nanochannels, the EMD method results are more reliable, accurate, and computationally more efficient compared to the direct NEMD method for predicting the nanofluidic flow rate and hydrodynamic boundary condition. [ABSTRACT FROM AUTHOR]

First-principles molecular dynamics (FPMD) simulations and static quantum chemical (QC) calculations are used to evaluate the tensile strengths, σc, of interfaces consisting of (0001) surfaces of α-Al2O3 separated by small organic species. The evaluation of σc with FPMD was achieved by performing simulations in which the simulation cell was extending in a direction normal to the fracture plane until rupture of the interface occurred. The static QC calculations employed an approach which treated fracture of the interface as a competition between uniform extension of the simulation cell and crack formation at the rupture site, which is analogous to that used in the construction of universal binding energy relationships. The results showed that the static QC calculations accurately reproduced the FPMD simulations with respect to tensile strength and the cell extension at which rupture occurred, provided that the rupture site employed in the static calculations matched the site at which rupture occurred during the FPMD simulations. A simple strategy for identifying the rupture site, even in complex systems containing many potential rupture sites, is proposed. Overall, the work extends the calculation of tensile strengths with static QC methods to highly heterogeneous interfaces, thus providing a computationally efficient alternative to demanding FPMD simulations for this purpose. [ABSTRACT FROM AUTHOR]

A new molecular dynamics simulation method, with coupling to external baths, is used to perform equilibrium simulations on polyamide-6,6 trimers nanoconfined between graphene surfaces, in equilibrium with the bulk polymer. The method is coupled with the reverse nonequilibrium molecular dynamics simulation technique to exchange heat in the direction normal to the surfaces. To be able to study the effect of confinement on the heat conductance in nanoconfined pores, in this work a number of simulations on systems with different pore sizes are done. It is concluded that the coefficient of heat conductivity depends on the degree of polymer layering between the surfaces and on the pore width. Our results further indicate a considerable temperature drop at the interface between the surfaces and polymer. The calculated Kapitza lengths depend on the intersurface distance and on the layering of the polymer nanoconfined between the surfaces. [ABSTRACT FROM AUTHOR]

While the interaction of colloidal particles (sizes in excess of 100 nm) with liquid interfaces may be understood in terms of continuum models, which are grounded in macroscopic properties such as surface and line tensions, the behaviour of nanoparticles at liquid interfaces may be more complex. Recent simulations [D. L. Cheung and S. A. F. Bon, Phys. Rev. Lett. 102, 066103 (2009)] of nanoparticles at an idealised liquid-liquid interface showed that the nanoparticle-interface interaction range was larger than expected due, in part, to the action of thermal capillary waves. In this paper, molecular dynamics simulations of a Lennard-Jones nanoparticle in a binary Lennard-Jones mixture are used to confirm that these previous results hold for more realistic models. Furthermore by including attractive interactions between the nanoparticle and the solvent, it is found that the detachment energy decreases as the nanoparticle-solvent attraction increases. Comparison between the simulation results and recent theoretical predictions [H. Lehle and M. Oettel, J. Phys. Condens. Matter 20, 404224 (2008)] shows that for small particles the incorporation of capillary waves into the predicted effective nanoparticle-interface interaction improves agreement between simulation and theory. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations have been used to investigate the kinetics of spontaneous cavitation and crystallization in a Lennard-Jones liquid at negative pressures in the temperature range where these processes compete with each other. The nucleation rate has been calculated in NVE and NpT ensembles by the method of mean lifetime and the transition interface sampling method with parallel path swapping. The data obtained have been used to determine in the framework of classical nucleation theory the value of the ratio of the solid-liquid and the liquid-void interfacial free energy for critical crystals and cavities and the values of their volumes at points where the cavitation rate of the liquid is equal to the rate of its crystallization. [ABSTRACT FROM AUTHOR]

Though the local dielectric constant at interfaces is an important phenomenological parameter in the analysis of surface spectroscopy, its microscopic definition has been uncertain. Here, we present a full molecular theory on the local field at interfaces with the help of molecular dynamics simulation, and thereby provide microscopic basis for the local dielectric constant so as to be consistent to the phenomenological three-layer model of interface systems. To demonstrate its performance, we applied the theory to the water/vapor interface, and obtained the local field properties near the interface where the simple dielectric model breaks down. Some computational issues pertinent to Ewald calculations of the dielectric properties are also discussed. [ABSTRACT FROM AUTHOR]

Commonly, the confinement effects are studied from the grand canonical Monte Carlo (GCMC) simulations from the computation of the density of liquid in the confined phase. The GCMC modeling and chemical potential (μ) calculations are based on the insertion/deletion of the real and ghost particle, respectively. At high density, i.e., at high pressure or low temperature, the insertions fail from the Widom insertions while the performing methods as expanded method or perturbation approach are not efficient to treat the large and complex molecules. To overcome this problem we use a simple and efficient method to compute the liquid's density in the confined medium. This method does not require the precalculation of μ and is an alternative to the GCMC simulations. From the isothermal-isosurface-isobaric statistical ensemble we consider the explicit framework/liquid external interface to model an explicit liquid's reservoir. In this procedure only the liquid molecules undergo the volume changes while the volume of the framework is kept constant. Therefore, this method is described in the NpnAVfT statistical ensemble, where N is the number of particles, pn is the normal pressure, Vf is the volume of framework, A is the surface of the solid/fluid interface, and T is the temperature. This approach is applied and validated from the computation of the density of the methanol and water confined in the mesoporous cylindrical silica nanopores and the MIL-53(Cr) metal organic framework type, respectively. [ABSTRACT FROM AUTHOR]

Density functional molecular dynamics simulations have been performed in the NVT ensemble (moles (N), volume (V) and temperature (T)) on a system formed by ten acetone molecules at a temperature of 2000 K and density ρ = 1.322 g cm-3. These conditions resemble closely those realized at the interface of an acetone vapor bubble in the early stages of supercompression experiments and result in an average pressure of 5 GPa. Two relevant reactive events occur during the simulation: the condensation of two acetone molecules to give hexane-2,5-dione and dihydrogen and the isomerization to the enolic propen-2-ol form. The mechanisms of these events are discussed in detail. [ABSTRACT FROM AUTHOR]

In this paper we calculate the interfacial resistances to heat and mass transfer through a liquid-vapor interface in a binary mixture. We use two methods, the direct calculation from the actual nonequilibrium solution and integral relations, derived earlier. We verify, that integral relations, being a relatively faster and cheaper method, indeed gives the same results as the direct processing of a nonequilibrium solution. Furthermore we compare the absolute values of the interfacial resistances with the ones obtained from kinetic theory. Matching the diagonal resistances for the binary mixture we find that kinetic theory underestimates the cross coefficients. The heat of transfer is, as a consequence, correspondingly larger. [ABSTRACT FROM AUTHOR]

We consider the computation of the interfacial properties of molecular chains from direct simulation of the vapor-liquid interface. The molecules are modeled as fully flexible chains formed from tangentially bonded monomers with truncated Lennard-Jones interactions. Four different model systems comprising of 4, 8, 12, and 16 monomers per molecule are considered. The simulations are performed in the canonical ensemble, and the vapor-liquid interfacial tension is evaluated using the test area and the wandering interface methods. In addition to the surface tension, we also obtain density profiles, coexistence densities, critical temperature and density, and interfacial thickness as functions of temperature, paying particular attention to the effect of the chain length on these properties. According to our results, the main effect of increasing the chain length (at fixed temperature) is to sharpen the vapor-liquid interface and to increase the width of the biphasic coexistence region. As a result, the interfacial thickness decreases and the surface tension increases as the molecular chains get longer. The interfacial thickness and surface tension appear to exhibit an asymptotic limiting behavior for long chains. A similar behavior is also observed for the coexistence densities and critical properties. Our simulation results indicate that the asymptotic regime is reached for Lennard-Jones chains formed from eight monomer segments. We also include a preliminary study on the effect of the cutoff distance on the interfacial properties. Our results indicate that all of the properties exhibit a dependence with the distance at which the interactions are truncated, though the relative effect varies from one property to the other. The interfacial thickness and, more particularly, the interfacial tension are found to be strongly dependent on the particular choice of cutoff, whereas the density profiles and coexistence densities are, in general, less sensitive to the truncation. [ABSTRACT FROM AUTHOR]

The Wolf method for the calculation of electrostatic interactions is applied in a liquid phase and at the liquid-vapor interface of water and its results are compared with those from the Ewald sums method. Molecular dynamics simulations are performed to calculate the radial distribution functions at room temperature. The interface simulations are used to obtain the coexisting densities and surface tension along the coexistence curve. The water model is a flexible version of the extended simple point charge model. The Wolf method gives good structural results, fair coexistence densities, and poor surface tensions as compared with those obtained using the Ewald sums method. [ABSTRACT FROM AUTHOR]

We report molecular dynamics simulations of three globular proteins: ubiquitin, apo-calbindin D9K, and the C-terminal SH2 domain of phospholipase C-γ1 in explicit water. The proteins differ in their overall charge and fold type and were chosen to represent to some degree the structural variability found in medium-sized proteins. The length of each simulation was at least 15 ns, and larger than usual solvent boxes were used. We computed radial distribution functions, as well as orientational correlation functions about the surface residues. Two solvent shells could be clearly discerned about charged and polar amino acids. Near apolar amino acids the water density near such residues was almost devoid of structure. The mean residence time of water molecules was determined for water shells about the full protein, as well as for water layers about individual amino acids. In the dynamic properties, two solvent shells could be characterized as well. However, by comparison to simulations of pure water it could be shown that the influence of the protein reaches beyond 6 Å, i.e., beyond the first two shells. In the first shell (r≤=3.5 Å), the structural and dynamical properties of solvent waters varied considerably and depended primarily on the physicochemical properties of the closest amino acid side chain, with which the waters interact. By contrast, the solvent properties seem not to depend on the specifics of the protein studied (such as the net charge) or on the secondary structure element in which an amino acid is located. While differing considerably from the neat liquid, the properties of waters in the second solvation shell (3.5≤r≤=6 Å) are rather uniform; a direct influence from surface amino acids are already mostly shielded. [ABSTRACT FROM AUTHOR]

We report molecular dynamics (MD) simulations of three protein-water systems (ubiquitin, apo-calbindin D9K, and the C-terminal SH2 domain of phospholipase C-γ1), from which we compute the dielectric properties of the solutions. Since two of the proteins studied have a net charge, we develop the necessary theory to account for the presence of charged species in a form suitable for computer simulations. In order to ensure convergence of the time correlation functions needed for the analysis, the minimum length of the MD simulations was 20 ns. The system sizes (box length, number of waters) were chosen so that the resulting protein concentrations are comparable to experimental conditions. A dielectric component analysis was carried out to analyze the contributions from protein and water to the frequency-dependent dielectric susceptibility χ(ω) of the solutions. Additionally, an even finer decomposition into protein, two solvation shells, and the remaining water (bulk water) was carried out. The results of these dielectric decompositions were used to study protein solvation at mesoscopic resolution, i.e., in terms of protein, first and second solvation layers, and bulk water. This study, therefore, complements the structural and dynamical analyses at molecular resolution that are presented in the companion paper. The dielectric component contributions from the second shell and bulk water are very similar in all three systems. We find that the proteins influence the dielectric properties of water even beyond the second solvation shell, in agreement with what was observed for the mean residence times of water molecules in protein solutions. By contrast, the protein contributions, as well as the contributions of the first solvation shell, are system specific. Most importantly, the protein and the first water shell around ubiquitin and apo-calbindin are anticorrelated, whereas the first water shell around the SH2 domain is positively correlated. [ABSTRACT FROM AUTHOR]

We describe a novel algorithm that includes the effect of host lattice flexibility into molecular dynamics simulations that use rigid lattices. It uses a Lowe-Andersen thermostat for interface-fluid collisions to take the most important aspects of flexibility into account. The same diffusivities and other properties of the flexible framework system are reproduced at a small fraction of the computational cost of an explicit simulation. We study the influence of flexibility on the self-diffusion of simple gases inside single walled carbon nanotubes. Results are shown for different guest molecules (methane, helium, and sulfur hexafluoride), temperatures, and types of carbon nanotubes. We show, surprisingly, that at low loadings flexibility is always relevant. Notably, it has a crucial influence on the diffusive dynamics of the guest molecules. [ABSTRACT FROM AUTHOR]

Using the molecular dynamics simulation technique, we study the effect of a highly ordered physisorbed alkyl monolayer on the surface induced liquid structure near a smooth solid–liquid interface. The system considered here is 1,12-dodecanediol adsorbed at the benzene–graphite interface. We compare the results on the graphite–1,12-dodecanediol–benzene interface to previous simulations on the bare graphite–benzene interface. We find that the surface induced liquid structure near the adsorbate covered interface differs from the structure induced by the bare interface only within the first solvation shell. Beyond the first solvation shell, there is no discernible difference, even though the surface induced structure is still pronounced. In addition, we study the structure of the hydrogen bond stabilized molecular adsorbate network. Here we find that the hydrogen bonds link the 1,12-dodecanediol molecules in a chain-like fashion across the boundaries of the herringbone structure formed by the adsorbate. [ABSTRACT FROM AUTHOR]

The atomic scale interfacial microstructure evolution and heat dissipation process in nanoscale friction are investigated by 3D non-equilibrium molecular dynamics (MD) simulations. Two Ni blocks of different orientations are built to simulate the self-mate friction. The embedded atom (EAM) potentials are employed in these simulations. The microstructure evolution is observed. The temperature and velocity profiles along the height direction, which is perpendicular to the direction of motion, are calculated under sliding velocity. The heat dissipation process is studied. The effect of sliding velocity is also obtained. The results show that extensive plastic deformation and temperature rise occur in the interface. Atomic scale mechanical mixing and generation of mixing layer are observed in the regions near the contact interface. The sliding velocity has great impact on temperature rise. The study of the growth dynamics of mixing layer also sheds light on the formation process of mixing layer. [ABSTRACT FROM AUTHOR]

We review our simulation results on properties of supercooled confined water. We consider two situations: water confined in a hydrophilic pore that mimics an MCM-41 environment and water at interface with a protein. The behavior upon cooling of the a relaxation of water in both environments is well interpreted in terms of the Mode Coupling Theory of glassy dynamics. Moreover, we find a crossover from a fragile to a strong regime. We relate this crossover to the crossing of theWidom line emanating from the liquid-liquid critical point, and in confinement we connect this crossover also to a crossover of the two body excess entropy of water upon cooling. Hydration water exhibits a second, distinctly slower relaxation caused by its dynamical coupling with the protein. The crossover upon cooling of this long relaxation is related to the protein dynamics. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are used to study the structure of the vapor–liquid interface of three-dimensional fluids. Particles interact via a truncated Lennard-Jones pair potential in the absence of external fields. The effect of the surface area on the surface tension is investigated. It is found that the surface tension increases with the decrease of the surface area. However, this finite-size effect is pronounced only in small surface areas. In addition, our simulation results show that the finite-size correction of the surface tension is directly proportional to the reciprocal of the surface area, in accord with the prediction of the Gaussian model of capillary waves. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Grain boundaries are the interfaces between differently oriented crystals of the same material. The underlying structures of grain boundary play a significant role in mechanical properties of polycrystalline materials. This influence becomes more significant when the grain size is reduced to ultrafine or nano size scale where the dislocation activities in the interior of grains lessen and mechanisms mediated by the grain boundary become dominant. This paper reviewed recent results in the atomistic simulation of the nanoscale behavior of grain boundary in face-centered cubic (fcc) metals. Three different simulation models were introduced to investigate the grain boundary behavior during plastic deformation, including three-dimensional (3D) nanocrystalline model, columnar nanocrystalline model and bicrystal model. The grain boundary was found to contribute to plastic deformation through the process of dislocation absorption, transmission or nucleation at boundary plane, as well as grain boundary accommodation mechanisms such as GB sliding and GB migration. These grain boundary mediated mechanisms were widely studied by the previous atomistic simulation works and were extensively reviewed here. Future challenges and directions in the computational study of grain boundary behaviors were also discussed. [ABSTRACT FROM AUTHOR]

The quantum mechanical (QM)/molecular mechanical (MM) interface between Chemistry at HARvard Molecular Mechanics (CHARMM) and TURBOMOLE is described. CHARMM provides an extensive set of simulation algorithms, like molecular dynamics (MD) and free energy perturbation, and support for mature nonpolarizable and Drude polarizable force fields. TURBOMOLE provides fast QM calculations using density functional theory or wave function methods and excited state properties. CHARMM-TURBOMOLE is well-suited for extended QM/MM MD simulations using first principles methods with large (triple- ζ) basis sets. We demonstrate these capabilities with a QM/MM simulation of Mg2+(aq), where the MM outer sphere water molecules are represented using the SWM4-NDP Drude polarizable force field and the ion and inner coordination sphere are represented using QM PBE, PBE0, and MP2 methods. The relative solvation free energies of Mg2+ and Zn2+ were calculated using thermodynamic integration. We also demonstrate the features for excited state properties. We calculate the time-averaged solution absorption spectrum of indole, the emission spectrum of the indole [ABSTRACT FROM AUTHOR]

Nonequilibrium molecular dynamics (NEMD) simulations on conceptual binary Lennard-Jones systems show that the thermal conductivity (κ) of a superlattice (SL) can be significantly reduced by randomizing the thicknesses of its layers, by which a SL becomes a random multilayer (RML). Such reduction in κ is a clear signature of coherent phonon that can be localized in RMLs. We build a two-phonon model that divides the overall heat conduction into coherent and incoherent phonon contributions. In SL both coherent and incoherent phonons contribute to heat conduction, while in RML coherent phonons are localized so only incoherent phonons contribute. This model can fit the length dependence of the thermal conductances predicted in our NEMD simulations very well. The ballistic-limit thermal conductance and the intrinsic mean free path (MFP) of both coherent and incoherent phonons, and the localization length of coherent phonons, are obtained by fitting our model to the NEMD simulation results. The significant increase in κ of SL with total length is due to the long MFP of coherent phonons, and the lower κ of RML than SL is caused by the localization of coherent phonons. [ABSTRACT FROM AUTHOR]

Abstract: The flow-induced translocation of polymer chains through a microfluidic channel is investigated using particle-based Dissipative Particle Dynamics and modified Langevin Dynamics approaches. Adaptive no-slip wall boundary conditions have been implemented to model fluid flow in the microfluidic channel, paying attention to controlling fluid density fluctuations. By varying the magnitude of the external body force driving the flow, an extensive simulation study of the dynamics of flow-induced translocation of polymers with and without considering hydrodynamic interactions (HIs) was performed. The results show that the HIs can increase the translocation probability and reduce the translocation time. In addition, the results also demonstrate that the solid wall interfacial property exerts a considerable influence on the dynamics of polymer translocation, i.e., an attractive interaction between the solid wall and the polymer increases the translocation time, whereas a repulsive interaction decreases it. [Copyright &y& Elsevier]

Abstract: Laser-induced thin film spallation experiments have been previously developed to characterize the intrinsic interfacial strength of thin films. In order to gain insights of atomic level thin film debonding processes and the interfacial strength dependence on film/substrate lattice structures, in this study, molecular dynamics simulations of thin film interfacial failure under laser-induced stress waves were performed. Various loading amplitudes and pulse durations were employed to identify the optimum simulation condition. Stress propagation as a function of time was revealed in conjunction with the interface structures. Parametric studies confirmed that while the interfacial strength between a thin film and a substrate does not depend on the film thickness and the duration of the laser pulse, a thicker film and a shorter duration do provide advantage to effectively load the interface to failure. With the optimized simulation condition, further studies were focused on bulk Au/Au bi-crystals with mismatched orientations, and Ni/Al, Cu/Al, Cu/Ag and Cu/Au bi-crystals with mismatched lattices. The interfacial strength was found to decrease with increasing orientation mismatch and lattice mismatch but more significantly dominated by the bonding elements' atomic structure and valence electron occupancy. [Copyright &y& Elsevier]

The fundamental processes of shear-induced chemical mixing in heterogeneous Cu-based alloy systems have been studied by molecular dynamics computer simulations. These simulations reveal that two very disparate mechanisms operate depending on whether or not the two phases are coherent. For the coherent systems, mixing occurs as dislocations transfer across phase boundaries. The mixing in these systems is 'superdiffusive,' and for spherical precipitates, the rate of mixing increases quadratically with precipitate radius. In systems that have incoherent phases, the mixing occurs by a local shuffling of atoms at the interface, and for them, the mixing is diffusive, with the mixing rates of spherical precipitates scaling linearly with particle radius. The morphologies of the interfaces for the two situations are also different. Coherent precipitates form rough interfaces that are relatively sharp, whereas the interfaces of incoherent precipitates are smooth but diffuse. These simulations also show that for incoherent precipitates, shear-induced mixing can be very different at different crystallographic interfacial planes as well as for different strain directions. [ABSTRACT FROM AUTHOR]

Abstract: The adhesion between two immiscible polymers stitched together via mobile chains is studied with large scale molecular simulations employing a coarse-grained bead-spring model. An adhesion model is presented that incorporates both connector molecular slipping out viscously and bulk dissipation in two dissimilar glassy polymers, in which one is dense melt and another is loose. The contributions to the separation work from thermodynamics and chain suction are studied in dependence of the connector length, at constant temperature, and at fixed basic molecular parameters. It is shown that connector length relative to its entanglement length can enhance the adhesion toughness and interfacial strength. Bulk dissipation is not considerable with low connector areal density in mushroom regime, while becomes more evident in the loose melt with increasing connector length when the coverage density is increased up to overlapping brush regime. The results provide insight into the structure evolution of adhesion interface with mobile promoter molecular, which are useful for future developments of continuum cohesive models for fracture of polymer–polymer interfaces. [Copyright &y& Elsevier]

Abstract: Interconnect technology of carbon nanotubes (CNTs) is essential for functional devices. However, difficulty in the fabrication of the interface between carbon nanotube (CNT) and CNT in axial direction, hindered the quality of connection and practical applications of electrical devices. Also, investigation of dynamic evolution of connection configuration about nanowelding is still lacking. In order to analyze the nanowelding configuration between CNTs in axial direction, the different connection cases are investigated using molecular dynamics simulation. Simulation results show that the nanowelding process could be accomplished at a lower temperature than the melting point of the bulk solder and the CNTs are connected with solder joints of nodule shape. It is also found that metal atoms are captured and dragged into nanotube to form the core filling structures of nanowires during nanowelding. Also, the connection configuration shows that Ag atoms diffuse along the outer walls of SWNTs with the dominant mechanism of capillary wetting, which would increase the contact length to improve the mechanical strength. [Copyright &y& Elsevier]

Abstract: By means of molecular dynamics (MD) and the modified analytic embedded-atom method (MAEAM), we investigate the effect of the γ′-phase volume fraction (γ′-VF) on the apparent interface energy, the critical thickness of the interface transition region (ITR) and the binding mechanism of the γ/γ′ interface. The results indicate that the apparent interface energy increases linearly with the ITR width increasing. Then, by extrapolating the ITR width to zero, we can obtain the interface energy which is equal to the limiting value of the apparent interface energy. The volume fraction of the γ′-phase has no influence on the interface energy and the critical thickness of the ITR. The interface energy is in reasonable agreement with the previous results and the critical value of the ITR width is about 1.7nm which also agrees well with the experimental and theoretical ones. Finally, the γ′-VF dependence of the apparent interface energy and the interface separation of the ITR with about 1.7nm thickness are analyzed in more detail. The results reveal that the crystalline configuration of Ni-base superalloy (NBSA) with (60–70%) γ′-VF is the stablest, which is similar to the previous investigations. [Copyright &y& Elsevier]

Using molecular-dynamics simulation, we have calculated the interfacial free energy γ between a hard-sphere fluid and hard spherical and cylindrical colloidal particles, as functions of the particle radius R and the fluid packing fraction η = pϭ³/6, where p and ϭ are the number density and hard-sphere diameter, respectively. These results verify that Hadwiger's theorem from integral geometry, which predicts that γ for a fluid at a surface, with certain restrictions, should be a linear combination of the average mean and Gaussian surface curvatures, is valid within the precision of the calculation for spherical and cylindrical surfaces up to η≈ 0.42. In addition, earlier results for γ for this system [Bryk et al, Phys. Rev. E 68, 031602 (2003)] using a geometrically based classical density functional theory are in excellent agreement with the current simulation results for packing fractions in the range where Hadwiger's theorem is valid. However, above η≈ 0.42, γ (R) shows significant deviations from the Hadwiger form indicating limitations to its use for high-density hard-sphere fluids. Using the results of this study together with Hadwiger's theorem allows one, in principle, to determine γ for any sufficiently smooth surface immersed in a hard-sphere fluid. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are used to explore the physical mechanisms of electrowetting and the limits of continuum theories. Nanoscale drops exhibit the same behavior seen in macroscopic experiments: The contact angle 6 follows continuum theory at low voltages and then saturates. Saturation limits applications of electrowetting and its origin is of great interest. In the simulations, saturation occurs when ions are pulled from the drop by large local fields. Saturation can be controlled by changing temperature, screening, or the energy binding ions to the fluid. We show a local force balance equation for 6 remains valid even after saturation and that the interface approaches the equilibrium contact angle within a few nanometers of the solid. [ABSTRACT FROM AUTHOR]

We provide a derivation allowing the calculation of thermal conductance at interfaces by equilibrium molecular dynamics simulations and illustrate our approach by studying thermal conduction mechanisms in Si/Ge superlattices. Thermal conductance calculations of superlattices with period thicknesses ranging from 0.5 to 60 nm are presented as well as the temperature dependence. Results have been compared to complementary Green-Kubo thermal conductivity calculations demonstrating that thermal conductivity of perfect superlattices can be directly deduced from interfacial conductance in the investigated period range. This confirms the predominant role of interfaces in materials with large phonon mean free paths. [ABSTRACT FROM AUTHOR]

Abstract: Nafion, a polymer electrolyte membrane of a fuel cell, can be modified by low energy Ar+ beam bombardment to increase its interfacial area with a catalyst. Recent experiments indicated that the sputtered sulfonate could lead to a decrease of hydrophilicity of Nafion when bombarded by a low energy Ar+ beam. To investigate the surface modification at the atomic level, molecular dynamic (MD) simulations and experiment were carried out. The effects of Ar+ at 0.5–3.0keV, and doses in the range of 1014–1015 ionscm−2 on the damage of the Nafion surface after bombardment were deduced from the simulations. This was assessed through both the chemical and physical changes of the Nafion side chain. The potential dissociation of the Cter bombardment was analyzed in terms of the elongated bond population. The percentage of the extended C the system was calculated to determine the possibility of sulfonate sputtering. Real-time determination of the amount of molecular species defragged under Ar+ ion bombardment by quadrupole mass spectroscopy (QMS) was used. The percentage of the amount of potentially broken Cfter bombardment derived from MD simulations was found in a correlation with sputtering of SO3 − fragments obtained from the experiments. The calculated results confirm the thresholds at 2.0keV as observed in experiment. [Copyright &y& Elsevier]

The water-TiO2(110) interface is as important as it is controversial. In our recent density functional theory (DFT) study [Liu et ai, Phys. Rev. Β 82,161415 (2010)], we showed (with several different exchange-correlation functionals) that water does not dissociate on the perfect TiO2(l 10) surface when care is taken to avoid artifacts resulting from the simulation cell used. Here, we report additional DFT-based molecular dynamics results that further support this view. We also briefly discuss some of the challenges involved in obtaining molecular-level understanding of the water-TiO2(110) interface. These are challenges that we, Wesolowski and co-workers, and many others have faced, challenges which have served to make this such an interesting and controversial system. [ABSTRACT FROM AUTHOR]

Abstract: We present a novel, three-dimensional, multiscale algorithm for simulations of water flow past a fullerene. We employ the Schwarz alternating overlapping domain method to couple molecular dynamics (MD) of liquid water around the C 540 buckyball with a Lattice–Boltzmann (LB) description for the Navier–Stokes equations. The proposed method links the MD and LB domains using a fully three-dimensional interface and coupling of velocity gradients. The present overlapping domain method implicitly preserves the flux of mass and momentum and bridges flux-based and Schwarz domain decomposition algorithms. We use this method to determine the slip length and hydrodynamic radius for water flow past a buckyball. [Copyright &y& Elsevier]

Abstract: Nonequilibrium molecular dynamics is applied to investigate thermal transport across two CNTs connected longitudinally by molecular linkers, which is a basic building-block for CNT network structures. We show the effect of different numbers, monomer types, and lengths of molecular linkers on the interfacial thermal conductance between CNTs and molecular linkers. We also analyze the density of vibrational normal modes to further understand the interfacial thermal conductance between different molecular linkers and CNTs. For most of the molecular linker type we simulated, the interfacial thermal conductance decreases with the increasing chain length. We find that aromatic-backbone structures are better than aliphatic-backbone structures to obtain higher interfacial thermal conductance with CNTs. Incorporating double bonds, oxygen atoms and amide groups into polymer chains shifts or redistributes of the density of vibrational normal modes, which in turn tunes the interfacial thermal conductance of molecular linker with CNTs. These results provide guidance for choosing molecular linkers to build up large-scale CNT-based network structures with tunable thermal properties. [Copyright &y& Elsevier]

Abstract: The interlayer sliding behavior of few-layer (3–8) graphene (FLG) is investigated using molecular dynamics simulations. A constant velocity is imposed on the top layer, inducing relative sliding between layers. With multiple interlayer interfaces, the sliding is found to present unique behavior, such as coherent sliding at multiple interfaces and a periodic layer-stacking transition, which is quite different from previously reported flake-on-substrate cases. It is observed that sliding is usually “stick–slip”, and can be divided into three stages, i.e. an initial stage, a developing stage and a stable sliding stage. A novel mechanism is proposed which explains the origin of the unique sliding behavior of FLG. [Copyright &y& Elsevier]

Abstract: To investigate whether polyoxometallate α-PW12O40 3− Keggin anions (noted PW3−) repulse each other in water, we calculated the changes in free energy ΔG(d) as a function of the P…P distance d (potential of mean force “PMF” calculations). As the anions approach each other, the free energy profiles are found to be quite flat, with a tiny minimum at ca. 11Å, showing that the anions can form “contact ion pairs” in the presence of either H3O+, UO2 2+ or Eu3+ counterions. The results obtained with different methodological variants (water models, PW3− charge models, sampling procedures) support our previous finding that PW3− ions can form dimers or oligomers in water (A. Chaumont and G. Wipff (2008) ). The importance of stabilizing bridging water molecules and solute granularity is demonstrated by comparing PW3− to S3− spherical analogues and to PW3+ cations (with all atomic charges of PW3− inverted). With these analogues, a somewhat repulsive behavior (ca +2 to 3kcal/mol) is observed at short distances. The role of water is further demonstrated by comparing PMFs in water and in methanol solution where there is no contact ion pair, but a free energy minimum at ca. 17Å, corresponding to an ion separated pair PW3−…Eu(MeOH)9 3+…PW3−. These findings are important for understanding processes like condensation and assembling of POMs and macro-ions in water or at aqueous interfaces. [Copyright &y& Elsevier]

The thermal conductance of interfaces plays a major role in defining the thermal properties of nanostructured materials in which heat transfer is predominantly phonon mediated. Ongoing research has improved the understanding of factors that govern interfacial phonon transport as well as the ability to predict thermal interface conductance. However, despite this progress, the ability to control interface conductance remains a major challenge. In this manuscript, we present a method to enhance and tune thermal interface conductance at vibrationally mismatched solid-solid interfaces. Enhancement is achieved through the insertion of an interfacial film with mediating vibrational properties, such that the vibrational mismatch at the interface is bridged, and consequently, the total interface conductance is enhanced. This phenomena is explored using nonequilibrium molecular dynamics simulations, where the effects of altering the interfacial film thickness, vibrational spectrum, and the temperature of the system are investigated. A systematic study of these pertinent design parameters explores the ability to enhance and tune phonon transport at both ideal (sharp) and nonideal (compositionally disordered) interfaces. Results show that interface conductance can be broadly enhanced by up to 5395 in comparison to the vibrationally mismatched baseline interface. Additionally, we find that compositional disorder at an interface does not imply a deterministic change in interface conductance, but instead, that the influence of compositional disorder depends on the characteristics of the disordered region itself. These results, in contrast to macroscopic thermal transport theory, imply that it is possible to increase thermal conductance associated with interface scattering by adding more material along the direction of heat flux. [ABSTRACT FROM AUTHOR]

Abstract: We recently performed constant energy molecular dynamics simulations of the endothermic decomposition of methane hydrate in contact with water to study phenomenologically the role of mass and heat transfer in the decomposition rate [S. Alavi, J.A. Ripmeester, J. Chem. Phys. 132 (2010) 144703]. We observed that with the progress of the decomposition front temperature gradients are established between the remaining solid hydrate and the solution phases. In this work, we provide further quantitative macroscopic and molecular level analysis of the methane hydrate decomposition process with an emphasis on elucidating microscopic details and how they affect the predicted rate of methane hydrate decomposition in natural methane hydrate reservoirs. A quantitative criterion is used to characterize the decomposition of the hydrate phase at different times. Hydrate dissociation occurs in a stepwise fashion with rows of sI cages parallel to the interface decomposing simultaneously. The correlations between decomposition times of subsequent layers of the hydrate phase are discussed. [Copyright &y& Elsevier]

Recent advances in the collection and interpretation of surface-sensitive vibrational spectroscopic measurements have made it possible to study the orientation of peptides and proteins in situ in a biologically relevant environment. However, interpretation of sum frequency generation (SFG) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) vibrational spectroscopy is hindered by the fact that orientation cannot be inferred without some prior knowledge of the protein structure. In this work, molecular dynamics simulations were used to study the interfacial orientation and structural deformation of the short β-sheet peptide tachyplesin I at the polystyrene/water interface. By combining these results with ATR-FTIR and SFG measurements, reasonable agreement was found with the simulation results, suggesting that tachyplesin I lies parallel to the surface, although the simulation results imply a broader distribution of peptide twist angles than could be characterized using available experimental measurements. The interfacial structure was found to be deformable even when disulfide bonds were preserved, and these local deviations from a purely extended β-sheet conformation may be of importance to future developments in the interpretation of SFG and ATR-FTIR spectra. [ABSTRACT FROM AUTHOR]

We have studied the self-assembly of hydrophobic nanoparticles at ionic liquid (IL)–water and IL–oil (hexane) interfaces using molecular dynamics (MD) simulations. For the 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6])/water system, the nanoparticles rapidly approached the IL–water interface and equilibrated more into the IL phase although they were initially in the water phase. In contrast, when the nanoparticles were dispersed in the hexane phase, they slowly approached the IL–hexane interface but remained primarily in the hexane phase. Consequently, the IL–hexane interface was rather undisturbed by the nanoparticles whereas the IL–water interface changed significantly in width and morphology to accommodate the presence of the nanoparticles. The equilibrium positions of the nanoparticles were also supported and explained by potential of mean force (PMF) calculations. Interesting ordering and charge distributions were observed at the IL–liquid interfaces. At the IL–hexane interface, the [BMIM] cations preferentially oriented themselves so that they were immersed more in the hexane phase and packed efficiently to reduce steric hindrance. The ordering likely contributed to a heightened IL density and a slightly positive charge at the IL–hexane interface. In contrast, the cations at the IL–water interface were oriented isotropically unless in the presence of nanoparticles, where the cations aligned across the nanoparticle surfaces. [ABSTRACT FROM AUTHOR]

We have studied the thermal conductance of tilt grain boundaries in graphene using nonequilibrium molecular dynamics simulations. When a constant heat flux is allowed to flow, we observe sharp jumps in temperature at the boundaries, characteristic of interfaces between materials of differing thermal properties. On the basis of the magnitude of these jumps, we have computed the boundary conductance of twin grain boundaries as a function of their misorientation angles. We find the boundary conductance to be in the range 1.5 × 1010to 4.5 × 1010W/(m2K), which is significantly higher than that of any other thermoelectric interfaces reported in the literature. Using the computed values of boundary conductances, we have identified a critical grain size of 0.1 μm below which the contribution of the tilt boundaries to the conductivity becomes comparable to that of the contribution from the grains themselves. Experiments to test the predictions of our simulations are proposed. [ABSTRACT FROM AUTHOR]

The atomic mixing and structural transformations in a Ag film-Cu substrate system irradiated by a femtosecond laser pulse are investigated in a simulation performed with a model that couples the classical molecular dynamics method with a continuum-level description of the laser excitation and subsequent relaxation of the conduction-band electrons. The higher strength of the electron-phonon coupling in Cu compared to Ag results in preferential sub-surface heating and melting of the Cu substrate. The melting is followed by fast cooling and rapid resolidification occurring under conditions of strong undercooling below the equilibrium melting temperatures of Cu and Ag. The rapid resolidification results in a complex structure of the interfacial region, where the lattice-mismatched interface is separated from the Ag-Cu mixing region by an intermediate pseudomorphic bcc Cu layer that grows epitaxially on the (001) face of the fcc Ag film during the final stage of the resolidification process. The new lattice-mismatched interface has a three-dimensional structure consisting of a periodic array of stacking fault pyramids outlined by stair-rod partial dislocations. The intermediate bcc layer and the stacking fault pyramid structure of the mismatched interface are likely to present a strong barrier for dislocation propagation, resulting in the effective hardening of the layered structure treated by the laser irradiation. The concentration profiles in the atomic mixing region are substantially wider compared to the width of the equilibrium Cu-Ag interface and have a pronounced asymmetric shape that reflects the preferential melting of the Cu substrate. [ABSTRACT FROM AUTHOR]

Abstract: We present the results of numerical simulations of the electrostatics and dynamics of water surrounding Kihara solutes described by a Lennard-Jones layer at the surface of a hard-sphere core. The dipolar response of the hydration layer peaks at the solute surface, significantly exceeding bulk water in polarity. This effect can be observed by atomic force microscopy. The dynamics of water shells are slow close to the surface, but become faster with the growing layer thickness and approach the bulk limit for layers 1nm thick. Slowing of the inner hydration shells strongly depends on the strength of solute–solvent attraction, offering a possibility of highly heterogeneous interfacial dynamics. [Copyright &y& Elsevier]

Abstract: We investigated structures derived from and ZSM-5-crystallites in different orientations and combinations. Gaps are introduced into the configurations in order to produce surfaces. However, interfaces can be formed by coalescence of surfaces. The structural and thermal properties of the thus generated interfaces and of the remaining surfaces are qualitatively discussed. Applying different sizes of the gaps between the structures allowed the monitoring of structural changes, partial pair-correlation functions and bond-angle distributions. Furthermore, we discuss the influence of the thermal or temperature distribution in the thus constructed materials. We report about the qualitative differences using both constant temperatures and temperature gradients. [Copyright &y& Elsevier]

The interface between nano-scale films is of relevance in many critical applications. Specifically, recent technological advances in semiconductor industry that utilize silicon-on-insulator devices have given importance to the understanding of thermal transport across Si\-SiO2 interface. Estimates of interfacial (Kapitza) resistance to the thermal transport across Si\-SiO2 films do not appear to exist at the present time. In this paper, we develop and carryout reverse non-equilibrium molecular dynamics simulations by imposing known heat flux to determine the Kapitza resistance between Si\-SiO2 thin films. For the Si\-SiO2 interface, the average Kapitza resistance for a \sim8∼\rm\AA thick oxide layer system was 0.503 \times10^-9∼m^2K/W and for a \sim11.5∼\rm\AA thick oxide layer system was 0.518\,\times 10^-9∼m^2K/W. These values were of the same order of magnitude as the Kapitza resistance values determined from the acoustic mismatch model and the diffuse mismatch model for the Si\-SiO2 interface. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are used to study brittle/ductile interfaces Ni/B2-NiAl under mechanical loading. Uniaxial tensile tests perpendicular to the interface are performed. It is shown that interfaces have influence on strain induced material failure by nucleation of defects. Crack propagation in the interface is investigated by applying load via fixed displacement boundary conditions. Determined crack velocities in the interface are found to be clearly above those in each of the materials. [ABSTRACT FROM AUTHOR]