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The exact, non perturbative, response theory developed within the field of non-equilibrium molecular dynamics, also known as TTCF (transient time correlation function), applies to quite general dynamical systems. Its key element is called the dissipation function because it represents the power dissipated by external fields acting on the particle system of interest, whose coupling with the environment is given by deterministic thermostats. This theory has been initially developed for time-independent external perturbations, and then it has been extended to time-dependent perturbations. It has also been applied to dynamical systems of different nature, and to oscillator models undergoing phase transitions, which cannot be treated with, e.g., linear response theory. The present work includes time-dependent stochastic perturbations in the theory using the Karhunen–Loève theorem. This leads to three different investigations of a given process. In the first, a single realization of the stochastic coefficients is fixed, and averages are taken only over the initial conditions, as in a deterministic process. In the second, the initial condition is fixed, and averages are taken with respect to the distribution of stochastic coefficients. In the last investigation, one averages over both initial conditions and stochastic coefficients. We conclude by illustrating the applicability of the resulting exact response theory with simple examples. [ABSTRACT FROM AUTHOR]

In this paper, we propose an alternative road to calculate the transport coefficients of fluids and the slip length inside nano-conduits in a Poiseuille-like geometry. These are all computationally demanding properties that depend on dynamic, thermal, and geometrical characteristics of the implied fluid and the wall material. By introducing the genetic programming-based method of symbolic regression, we are able to derive interpretable data-based mathematical expressions based on previous molecular dynamics simulation data. Emphasis is placed on the physical interpretability of the symbolic expressions. The outcome is a set of mathematical equations, with reduced complexity and increased accuracy, that adhere to existing domain knowledge and can be exploited in fluid property interpolation and extrapolation, bypassing timely simulations when possible. [ABSTRACT FROM AUTHOR]

Gibbs Ensemble Monte Carlo (GEMC) is a molecular simulation method commonly used for simulating phase equilibrium. This method has been proposed since 1987 and applied in many fields, such as geology, planetary science, chemical engineering, material science, etc. GEMC method combines canonical (NVT), isobaric-isothermal (NPT), and grand canonical (μVT) Monte Carlo techniques in a single simulation. The GEMC method was developed on the fundamental law of phase equilibrium that chemical potentials of each phase all equal. Two key factors affect the rationality and reliability of GEMC simulations: 1. particles can be efficiently moved in/out from certain phase during simulation; 2. samplings can represent the whole system well, in other words, samplings hold good ergodicity. In addition, various parallel methods have been developed to improve the simulation efficiency. In this review, an introduction to the theoretical fundamentals, improvements on particle movement and sampling protocols, acceleration techniques and some applications of the GEMC method will be presented. This is the first integrated review introducing the fundamentals, improvements and applications of the GEMC simulation method. [ABSTRACT FROM AUTHOR]

The current study focuses on non-equilibrium molecular dynamics (NEMD) simulations to investigate the slip properties of water flowing over different nanostructured surfaces. A simulation protocol is developed that applies constant shear stress throughout the fluid before measuring the slip length. Using pseudo-data, the reliability of this protocol in terms of both accuracy and noise of the results for high-slip and multiphase systems is demonstrated. In contrast to the NEMD techniques available in the literature, the protocol also enables a convenient way to compare the slip lengths of different surface coatings. The fluid slip lengths of surface coatings comprising carbon nanotubes on platinum are predicted using the proposed protocol with nitrogen gas trapped in the interstitial gaps. The role of these gas pockets in determining surface slip properties is investigated. The NEMD results from the proposed model compare well with a macroscopic theoretical model for nano-patterned surfaces. Finally, it is concluded that entrapped gas within nanostructures may offer significant drag reduction only if the gas surface coverage is above 95%. [ABSTRACT FROM AUTHOR]

We discuss (i) the measurement, (ii) interpretation and (iii) technical impact of temperature-dependent wall slip and its activation energy for polar and non-polar hydrocarbon Newtonian fluids moving relative to machined metal surfaces. A newly developed apparatus, the slip length tribometer (SLT), overcomes the drawbacks of existing measurement devices in terms of characterising relevant rough surfaces by measuring the slip length at different temperatures over a sufficiently large wetted area. The experimental data show that the bulk viscosity and slip length at the fluid-metal interface is independent of the shear rate up to 105 s-1, being consistent with recent results from molecular dynamics simulations by Mehrnia & Pelz (J. Mol. Liq., vol. 336, 2021, 116589). Furthermore, the activation energies for wall slip and bulk shear determined by means of the SLT differ by a factor of two, i.e. E = 0.5Eµ for non-polar hydrocarbon molecules sliding relative to metal walls. This difference is explained by a generalised Eyring model applied to wall slip. The paper closes with the impact of wall slip on Sommerfeld's similarity theory of tribology and the resulting Stribeck curve (Sommerfeld, Z. Math. Phys., vol. 50, 1904, pp. 97-155; Z. Tech. Phys., vol. 2. Jahrg., 1921; Mechanik der deformierbaren Medien, 1944, Akademische Verlagsgesellschaft Becker & Erler). For this purpose, Reynolds' equation generalised for wall slip is solved in combination with the experimentally determined constitutive relations for bulk shear and wall slip to predict typical characteristics of journal bearings. The results show that, for a typical journal bearing, where the ratio of slip length to average bearing clearance is of the order of 10-2, the influence of wall slip on both load-carrying capacity and dissipation is not negligible. This work combines nanofluidics and tribology in order to provide methods and knowledge for e.g. tailor-made fluids and interfaces. [ABSTRACT FROM AUTHOR]

In this study, we used molecular dynamics (MD) simulations to investigate the atomic ordering in the liquid aluminum (Al) adjacent to the amorphous substrate with smooth and rough surfaces. This study revealed that the liquid exhibited layering within about 5 atomic layers but no visible in-plane atomic ordering at the interface with the smooth amorphous surface, and neither layering nor in-plane atomic ordering with the rough surface of the amorphous substrate. However, the smooth amorphous surface induced some local ordered structure in the liquid at the interface by a structural templating mechanism, which promoted heterogeneous nucleation by creating a 2-dimensional (2D) nucleus in the third layer. The amorphous substrate with a rough surface had no effect on the nucleation in the liquid, leading to the occurrence of homogeneous nucleation with an undercooling 100 K larger than that of heterogeneous nucleation on the smooth amorphous substrate. This study confirmed that structural templating is a general mechanism for heterogeneous nucleation. [ABSTRACT FROM AUTHOR]

Colloid-sized particles (10 nm–10 μm in characteristic size) adsorb onto fluid interfaces, where they minimize their interfacial energy by straddling the surface, immersing themselves partly in each phase bounding the interface. The energy minimum achieved by relocation to the surface can be orders of magnitude greater than the thermal energy, effectively trapping the particles into monolayers, allowing them freedom only to translate and rotate along the surface. Particles adsorbed at interfaces are models for the understanding of the dynamics and assembly of particles in two dimensions and have broad technological applications, importantly in foam and emulsion science and in the bottom-up fabrication of new materials based on their monolayer assemblies. In this review, the hydrodynamics of the colloid motion along the surface is examined from both continuum and molecular dynamics frameworks. The interfacial energies of adsorbed particles is discussed first, followed by the hydrodynamics, starting with isolated particles followed by pairwise and multiple particle interactions. The effect of particle shape is emphasized, and the role played by the immersion depth and the surface rheology is discussed; experiments illustrating the applicability of the hydrodynamic studies are also examined. [ABSTRACT FROM AUTHOR]

A model based on molecular dynamics is suggested for description of the shape of nanobubble on the liquid-solid interface. The model results are in good agreement with the known experimental AFM measurements. Nanobubbles in water and in oil are studied. The evolution, moving and interactions of nanobubbles are considered. The influence of different external factors (temperature of the liquid, temperature of the substrate, the gradient of temperature, external pressure (depth of the liquid)) and internal characteristics (surface tension, density) on the nanobubbles evolution and behaviour is investigated. [ABSTRACT FROM AUTHOR]

In this study, we conducted a series of molecular dynamics simulations of gas accumulation on homogeneous graphite substrates with a focus on the structure of fluid in the region near the substrate. The fluid density distribution revealed that epitaxial layers and gas enrichment layer arise from the adsorption of fluid particles on solids. Our analysis suggests that interfacial gas molecules transform into a liquid-like state and water molecules tend to cluster within the thin region adjacent to the substrate as the strength of gas–solid interaction increases. We then calculated the argon-water interfacial tension using three different methods: the Young-Laplace equation, direct integration method, and force balance method. Our analysis suggests that higher pressure within the gaseous domain tends to reduce tension at the curved argon-water interface. Although widely adopted to describe tension at gas–liquid interfaces, our results indicate that Bakker's equation is not necessarily suitable for the calculation of tension at fluid-solid interfaces. Our results indicate that fluid-solid interfacial properties and the morphology of the gaseous domain are influenced by the liquid-like epitaxial gas layer and gas enrichment layer, which could be attributed to the highly energetic nature of gas molecules and the tendency of gasses to adsorb on solids. [ABSTRACT FROM AUTHOR]

Structure and properties of model colloidal suspensions in bulk solutions have been investigated using classical molecular dynamics simulations. The system consists of spherical particles (macroscopic colloids and microscopic counterions and/or coions) in a continuum solvent and is described using the primitive model. Variations in different properties with system parameters (colloid size, charge, concentration etc.) have been investigated. An effective one-component model [Adelman SA. J. Chem. Phys. 1976;64:724–731] of the multicomponent suspension is used to calculate the effective colloid–colloid potential, which develops an attractive minimum with increase in charge, size and concentration of colloid. [ABSTRACT FROM AUTHOR]

Simulating the dynamics of ions near polarizable nanoparticles (NPs) using coarse-grained models is extremely challenging due to the need to solve the Poisson equation at every simulation timestep. Recently, a molecular dynamics (MD) method based on a dynamical optimization framework bypassed this obstacle by representing the polarization charge density as virtual dynamic variables and evolving them in parallel with the physical dynamics of ions. We highlight the computational gains accessible with the integration of machine learning (ML) methods for parameter prediction in MD simulations by demonstrating how they were realized in MD simulations of ions near polarizable NPs. An artificial neural network–based regression model was integrated with MD simulation and predicted the optimal simulation timestep and optimization parameters characterizing the virtual system with 94.3% success. The ML-enabled auto-tuning of parameters generated accurate dynamics of ions for ≈ 10 million steps while improving the stability of the simulation by over an order of magnitude. The integration of ML-enhanced framework with hybrid Open Multi-Processing / Message Passing Interface (OpenMP/MPI) parallelization techniques reduced the computational time of simulating systems with thousands of ions and induced charges from thousands of hours to tens of hours, yielding a maximum speedup of ≈ 3 from ML-only acceleration and a maximum speedup of ≈ 600 from the combination of ML and parallel computing methods. Extraction of ionic structure in concentrated electrolytes near oil–water emulsions demonstrates the success of the method. The approach can be generalized to select optimal parameters in other MD applications and energy minimization problems. [ABSTRACT FROM AUTHOR]

Studies the dynamics of field-included director reorientation for a nematic phase comprised of flexible molecules. Alignment of directors produced by the competing magnetic and rotational torques; Change in the director orientation produced by stopping the rotation of a sample; Ratios of the quadrupolar splittings.

This paper reviews the current status of boiling heat transfer modelling, discusses the need for its improvement due to unresolved intriguing experimental findings and emergence of novel technical applications and outlines the directions for an advanced modelling approach. The state-of-the-art of computational boiling heat transfer studies is given for: macro-scale boiling models applied in two-fluid liquid-vapour interpenetrating media approach, micro-, meso-scale boiling computations by interface capturing methods, and nano-scale boiling simulations by molecular dynamics tools. Advantages, limitations and shortcomings of each approach, which originate from its grounding formulations, are discussed and illustrated on results obtained by the boiling model developed in our research group. Based on these issues, we stress the importance of adaptation of a multi-scale approach for development of an advanced boiling predictive methodology. A general road-map is outlined for achieving this challenging goal, which should include: improvement of existing methods for computation of boiling on different scales and development of conceptually new algorithms for linking of individual scale methods. As dramatically different time steps of integration for different boiling scales hinder the application of full multi-scale methodology on boiling problems of practical significance, we emphasise the importance of development of another algorithm for the determination of sub-domains within a macro-scale boiling region, which are relevant for conductance of small-scale simulations. [ABSTRACT FROM AUTHOR]

The micro air nucleus widely distributed in the ocean is a necessary condition for the cavitation of hydraulic machinery in seawater. In order to study the stability of air nucleus in seawater and cavitation inception, the computational domain of water molecules with air nucleus was studied using the method of molecular dynamics simulation, and the transient characteristics of air nucleus in liquid water were obtained. The key factors influencing nuclei stability were analyzed. The results showed that air nucleus with a certain mass could maintain the dynamic equilibrium in liquid water. The internal density of air nuclei had a critical value that allowed the nuclei to stably exist in water. The air nuclei mass was the decisive factor in its equilibrium volume in water, and the two were positively correlated. The internal density of air nuclei was negatively correlated with the nuclei radius when the nuclei was stable in water. Liquid surface tension was an important factor affecting the stability of the air nuclei. The larger the initial radius of nuclei, the smaller the water pressure, and the more likely the cavitation occurs. [ABSTRACT FROM AUTHOR]

The work presents a force field molecular dynamics study of nickel-coated graphene sheet bending at temperatures 300 and 1300 K. Nickel film is represented by nickel atoms located above the centres of carbon hexagons. Parameters for carbon–nickel interaction are fitted with regard to the accurate density functional calculations. Unstrained or flat configuration of the nickel–graphene system is found to be energetically unfavourable for the considered temperatures. Two types of curvatures are taken into account. Positive curvature is characterised by the nickel atoms located closer to the bending axis than the carbon ones, and negative curvature corresponds to the reverse atomic positions. It is found that the equilibrium radius for the curved nickel–graphene complex with the negative curvature is less than the corresponding value for the positive one. However, in both cases the equilibrium curvature radius of nickel-coated graphene is of the order of several nanometres. It is shown that both temperature and bending directions (zigzag and armchair) weakly affect the bending energy and do not change the equilibrium radius. Bending behaviour of the system is defined by the carbon-nickel interaction rather than the individual properties of isolated graphene and nickel films. [ABSTRACT FROM AUTHOR]

Heterogeneous systems composed of hydrophobic nanoporous materials and water are capable, depending on their characteristics, of efficiently dissipating (dampers) or storing ("molecular springs") energy. However, it is difficult to predict their properties based on macroscopic theories--classical capillarity for intrusion and classical nucleation theory (CNT) for extrusion--because of the peculiar behavior of water in extreme confinement. Here we use advanced molecular dynamics techniques to shed light on these nonclassical effects, which are often difficult to investigate directly via experiments, owing to the reduced dimensions of the pores. The string method in collective variables is used to simulate, without artifacts, the microscopic mechanism of water intrusion and extrusion in the pores, which are thermally activated, rare events. Simulations reveal three important nonclassical effects: the nucleation free-energy barriers are reduced eightfold compared with CNT, the intrusion pressure is increased due to nanoscale confinement, and the intrusion/extrusion hysteresis is practically suppressed for pores with diameters below 1:2 nm. The frequency and size dependence of hysteresis exposed by the present simulations explains several experimental results on nanoporous materials. Understanding physical phenomena peculiar to nanoconfined water paves the way for a better design of nanoporous materials for energy applications; for instance, by decreasing the size of the nanopores alone, it is possible to change their behavior from dampers to molecular springs. [ABSTRACT FROM AUTHOR]

A method has been proposed for determining interfacial free energy from the data of molecular dynamics simulation. The method is based on the thermodynamic integration procedure and is distinguished by applicability to both planar interfaces and those characterized by a high curvature. The workability of the method has been demonstrated by the example of determining the surface tension for critical nuclei of water droplets upon condensation of water vapor. The calculation has been performed at temperatures of 273-373 K and a pressure of 1 atm, thus making it possible to determine the temperature dependence of the surface tension for water droplets and compare the results obtained with experimental data and the simulation results for a 'planar' vapor-liquid interface. [ABSTRACT FROM AUTHOR]

Using all-atom molecular dynamics (MD) simulations at constant water chemical potential in combination with basic theoretical arguments, we study hydration-induced interactions between two overall charge-neutral yet polar planar surfaces with different wetting properties. Whether the water film between the two surfaces becomes unstable below a threshold separation and cavitation gives rise to long-range attraction, depends on the sum of the two individual surface contact angles. Consequently, cavitation-induced attraction also occurs for a mildly hydrophilic surface interacting with a very hydrophobic surface. If both surfaces are very hydrophilic, hydration repulsion dominates at small separations and direct attractive force contribution can--if strong enough--give rise to wet adhesion in this case. In between the regimes of cavitation-induced attraction and hydration repulsion we find a narrow range of contact angle combinations where the surfaces adhere at contact in the absence of cavitation. This dry adhesion regime is driven by direct surface-surface interactions. We derive simple laws for the cavitation transition as well as for the transition between hydration repulsion and dry adhesion, which favorably compare with simulation results in a generic adhesion state diagram as a function of the two surface contact angles. [ABSTRACT FROM AUTHOR]

The stability of colloidal suspensions is crucial in a wide variety of processes, including the fabrication of photonic materials and scaffolds for biological assemblies. The ionic strength of the electrolyte that suspends charged colloids is widely used to control the physical properties of colloidal suspensions. The extensively used two-body Derjaguin-Landau-Verwey-Overbeek (DLVO) approach allows for a quantitative analysis of the effective electrostatic forces between colloidal particles. DLVO relates the ionic double layers, which enclose the particles, to their effective electrostatic repulsion. Nevertheless, the double layer is distorted at high macroion volume fractions. Therefore, DLVO cannot describe the many-body effects that arise in concentrated suspensions. We show that this problem can be largely resolved by identifying effective point charges for the macroions using cell theory. This extrapolated point charge (EPC) method assigns effective point charges in a consistent way, taking into account the excluded volume of highly charged macroions at any concentration, and thereby naturally accounting for high volume fractions in both salt-free and added-salt conditions. We provide an analytical expression for the effective pair potential and validate the EPC method by comparing molecular dynamics simulations of macroions and monovalent microions that interact via Coulombic potentials to simulations of macroions interacting via the derived EPC effective potential. The simulations reproduce the macroion-macroion spatial correlation and the virial pressure obtained with the EPC model. Our findings provide a route to relate the physical properties such as pressure in systems of screened Coulomb particles to experimental measurements. [ABSTRACT FROM AUTHOR]

Liquid water can become metastable with respect to its vapor in hydrophobic confinement. The resulting dewetting transitions are often impeded by large kinetic barriers. According to macroscopic theory, such barriers arise from the free energy required to nucleate a critical vapor tube that spans the region between two hydrophobic surfaces—tubes with smaller radii collapse, whereas larger ones grow to dry the entire confined region. Using extensive molecular simulations of water between two nanoscopic hydrophobic surfaces, in conjunction with advanced sampling techniques, here we show that for intersurface separations that thermodynamically favor dewetting, the barrier to dewetting does not correspond to the formation of a (classical) critical vapor tube. Instead, it corresponds to an abrupt transition froman isolated cavity adjacent to one of the confining surfaces to a gap-spanning vapor tube that is already larger than the critical vapor tube anticipated by macroscopic theory. Correspondingly, the barrier to dewetting is also smaller than the classical expectation. We show that the peculiar nature of water density fluctuations adjacent to extended hydrophobic surfaces—namely, the enhanced likelihood of observing low-density fluctuations relative to Gaussian statistics—facilitates this nonclassical behavior. By stabilizing isolated cavities relative to vapor tubes, enhanced water density fluctuations thus stabilize novel pathways, which circumvent the classical barriers and offer diminished resistance to dewetting. Our results thus suggest a key role for fluctuations in speeding up the kinetics of numerous phenomena ranging from Cassie–Wenzel transitions on superhydrophobic surfaces, to hydrophobically driven biomolecular folding and assembly. [ABSTRACT FROM AUTHOR]

The process of material removal of single crystal copper with rough surfaces subjected to nanoscale scratching is studied in the present paper. We explore the key material removal mechanism by means of the observed variation of material removal under different surface roughnesses, tool speeds, scratching directions, tip shapes, feeds, double tip, and single tip. The investigation reveals that a higher peak on the surface reduces the local area roughness, and a higher valley enhances the stability of surface structure. The plastic deformation by means of dislocation loop transfers from the surface of substrate to the interior of workpiece with the rough or smooth surface during scratching process. A higher scratching velocity results in the increasing surface smoothness and reducing the impact on the rough surface atoms. The scratching along the critical angle 45° between scratching direction and surface texture orientation makes the surrounding atoms produce the minimal variation structure, helps to improve the structural stability, and plays an important role in protecting the scratching surface. The double-tip and single-tip scratching under different scratching feeds makes the rough surface perpendicular to the scratching direction substantially covered by chips or side flow. For different tip shapes, a cone diamond tip causes less plastic deformation in the subsurface than a prismatic diamond tip due to using different diamond tips with a contact area unequal. [ABSTRACT FROM AUTHOR]

This paper investigates the stability of nitrogen nanobubbles under different concentrations of nitrogen molecules by molecular dynamics simulations. It is found that the stability of nanobubbles is very sensitive to the concentration of nitrogen molecules in water. A sharp transition between disperse states and assemble states of nitrogen molecules is observed when the concentration of nitrogen molecules is changed. The relevant critical concentration of nitrogen molecules needed by the existing nitrogen nanobubbles is analyzed. [ABSTRACT FROM AUTHOR]

Since most experimental observations are performed at constant temperature and pressure, the isothermal-isobaric (NPT) ensemble has been widely used in molecular simulations. Nevertheless, the NPT ensemble has only recently been placed on a rigorous foundation. The proper formulation of the NPT ensemble requires a "shell" particle to uniquely identify the volume of the system, thereby avoiding the redundant counting of configurations. Here, we review our recent work in incorporating a shell particle into molecular dynamics simulation algorithms to generate the correct NPT ensemble averages. Unlike previous methods, a piston of unknown mass is no longer needed to control the response time of the volume fluctuations. As the volume of the system is attached to the shell particle, the system itself now sets the time scales for volume and pressure fluctuations. Finally, we discuss a number of tests that ensure the equations of motion sample phase space correctly and consider the response time of the system to pressure changes with and without the shell particle. Overall, the shell particle algorithm is an effective simulation method for studying systems exposed to a constant external pressure and may provide an advantage over other existing constant pressure approaches when developing nonequilibrium molecular dynamics methods. [ABSTRACT FROM AUTHOR]

This paper reports on simulations of electrokinetic transport in an electrolyte nanofilm in contact with a negatively charged substrate with discontinuous counter-charged patches. All-atom molecular dynamics simulations of an aqueous solution of chloride placed on a silica slab were conducted. The response of the system is investigated as it is subjected to different axial electric fields and as the surface electrostatics is modified by implementing different positive charge densities on the patches. Chloride and water density profiles reveal that the charged patches cause an enrichment of hydrated chloride ions near the surface. Therefore, the system with patches proved to be successful in trapping ions. Moreover, the number of chloride ions trapped on the surface increases as higher charge densities are imposed on the patches. It is observed that higher axial electric fields remove chloride ions from the surface; however, the amount removed decreases as higher charge densities are imposed on the patches. Velocity profiles show a stagnant water layer near the patches. The water and ion accumulation suggest the formation of a Stern layer with thickness of ca. 0.4 nm on the patches. Higher chloride density near the surface and increased surface-water affinity induced by the presence of charged patches result in smaller electroosmotic velocities. The simulation results compare well with velocities calculated using a continuum approach. This study shows that in a charged electrolyte solution in contact with a substrate, axial and normal ion distributions and consequently electroosmotic velocities can be controlled by modifying the electrostatics of the substrate. [ABSTRACT FROM AUTHOR]

In order to model the adhesive contact across different length scales, a multiscale approach is developed and used to study the adhesive contact behaviors between a rigid cylinder and an elastic face-centered cubic (FCC) substrate. The approach combines an atomistic treatment of the interfacial region with an elastic mechanics method description of the continuum region. The two regions are connected by a coupling region where nodes of the continuum region are refined to atoms of the atomistic region. Moreover, the elastic constants of FCC crystals are obtained directly from the Lennard-Jones potential to describe the elastic response characteristics of the continuum region, which ensures the consistency of material proprieties between atomistic and continuum regions. The multiscale approach is examined by comparing it with the pure MD simulation, and the results indicate that the multiscale modeling approach agrees well with the MD method in studying the adhesive contact behaviors. [ABSTRACT FROM AUTHOR]

Numerical simulation of Poiseuille flow of liquid Argon in a nanochannel using the non-equilibrium molecular dynamics simulation (NEMD) is performed. The nanochannel is a three-dimensional rectangular prism geometry where the concerned numbers of Argon atoms are 2,700, 2,550 and 2,400 at 102, 108 and 120 K. Poiseuille flow is simulated by embedding the fluid particles in a uniform force field. An external driving force, ranging from 1 to 11 PN (Pico Newton), is applied along the flow direction to inlet fluid particles during the simulation. To obtain a more uniform temperature distribution across the channel, local thermostating near the wall are used. Also, the effect of other mixing rules (Lorenthz–Berthelot and Waldman–Kugler rules) on the interface structure are examined by comparing the density profiles near the liquid/solid interfaces for wall temperatures 108 and 133 K for an external force of 7 PN. Using Kong and Waldman–Kugler rules, the molecules near the solid walls were more randomly distributed compared to Lorenthz–Berthelot rule. These mean that the attraction between solid–fluid atoms was weakened by using Kong rule and Waldman–Kugler rule rather than the Lorenthz–Berthelot rule. Also, results show that the mean axial velocity has symmetrical distribution near the channel centerline and an increase in external driving force can increase maximum and average velocity values of fluid. Furthermore, the slip length and slip velocity are functions of the driving forces and they show an arising trend with an increase in inlet driving force and no slip boundary condition is satisfied at very low external force (<1 PN). [ABSTRACT FROM AUTHOR]

This review is focused on molecular momentum transport at fluid-solid interfaces mainly related to microfluidics and nanofluidics in micro-/nano-electromechanical systems (MEMS/NEMS). This broad subject covers molecular dynamics behaviors, boundary conditions, molecular momentum accommodations, theoretical and phenomenological models in terms of gas-solid and liquid-solid interfaces affected by various physical factors, such as fluid and solid species, surface roughness, surface patterns, wettability, temperature, pressure, fluid viscosity and polarity. This review offers an overview of the major achievements, including experiments, theories and molecular dynamics simulations, in the field with particular emphasis on the effects on microfluidics and nanofluidics in nanoscience and nanotechnology. In Section 1 we present a brief introduction on the backgrounds, history and concepts. Sections 2 and 3 are focused on molecular momentum transport at gas-solid and liquid-solid interfaces, respectively. Summary and conclusions are finally presented in Section 4. [ABSTRACT FROM AUTHOR]

Large-scale molecular dynamics simulations are increasingly applied to the study of wetting dynamics. They have the advantage of being able not only to model the macroscopic world with reasonable accuracy, but also to reveal details at the microscopic level. Here we review the principles of modeling, using molecular dynamics, and comment on the specific methods used in our laboratory in comparison with others. By way of example, we explain how we extract physical parameters from simulations of partially wetting liquids on homogeneous substrates in a variety of geometries. We then show how the results have been applied to build our understanding of the underlying mechanisms. [ABSTRACT FROM AUTHOR]

In this paper molecular dynamics simulations are performed to study the accumulation behaviour of N2 and H2 at water/graphite interface under ambient temperature and pressure. It finds that both N2 and H2 molecules can accumulate at the interface and form one of two states according to the ratio of gas molecules number to square of graphite surface from our simulation results: gas films (pancake-like) for a larger ratio and nanobubbles for a smaller ratio. In addition, we discuss the stabilities of nanobubbles at different environment temperatures. Surprisingly, it is found that the density of both kinds of gas states can be greatly increased, even comparable with that of the liquid N2 and liquid H2. The present results are expected to be helpful for the understanding of the stable existence of gas film (pancake-like) and nanobubbles. [ABSTRACT FROM AUTHOR]

In this work, the GLOB model, an effective and reliable computational approach well suited for ab initio and QM/MM molecular dynamics simulations of complex molecular systems in solution, has been applied to study two representative open-shell systems, the cobalt(II) ion and the glycine radical in aqueous solution, with special reference to their structural and magnetic properties. The main structural features of the solvent cage around the cobalt ion and the hydrogen bonding patterns around the neutral and zwitterionic forms of the glycine radical have been investigated in some detail. The general good agreement with experiments supports the use of the present model to investigate more challenging and biological/technological relevant open-shell systems. [ABSTRACT FROM AUTHOR]

Grand-canonical transition-matrix Monte Carlo simulation is applied to analyze the effect of molecular association on the vapor-liquid coexistence and interfacial behavior of square-well based dimerizing fluids. Finite-size scaling techniques are implemented in conjunction with histogram reweighting to determine the infinite-system surface tension from a series of finite-size simulations. The effect of strength of association and size of association site on coexistence densities, pressure, surface tension, and monomer fraction is presented. Some qualitative features of the dependence of monomer fraction and surface tension on association strength are found to disagree with behavior expected from previous studies using the statistical associating fluid theory (SAFT). Comparison with experimental data shows that molecular models must incorporate an explicit association interaction in order to describe the surface-tension behavior of a real dimerizing fluid (acetic acid). [ABSTRACT FROM AUTHOR]

A method for the evaluation of higher order elementary diagrams is proposed. It utilizes symbolic algebra programming and automatic code generation. The method is used to calculate elementary diagrams for hard spheres up to the fifth. The obtained values are fitted to suitable functions of interparticle distance. The resulting truncated density expansion is compared with recent values of the bridge function calculated directly from the simulated pair distribution function. The method can be straightforwardly extended to hard-body systems and their mixtures. [ABSTRACT FROM AUTHOR]

The bridge function of the hard sphere fluid has been calculated from our new highly accurate Monte Carlo and molecular dynamics simulation data on the radial distribution function using the (inverted) Ornstein-Zernike equation. Both the systematic errors (finite size, grid size, tail) and statistical errors are analysed in detail and ways to suppress them are proposed. Uncertainties in the resulting values of B(r) are about 0.001. In contrast with many previous findings the bridge function is both positive and negative. [ABSTRACT FROM AUTHOR]

The structure of a hard sphere fluid confined by model slit and cylindrical pores is investigated. Results from grand canonical Monte Carlo (GCMC) simulations and from the hypernetted chain/mean spherical approximation (HNC/MSA) equation are reported. GCMC results are compared with those from the HNC/MSA equation, and agreement is good. The effect of confinement on liquids at the same chemical potentials is that the absorption of the hard sphere fluid into the pores decreases with increasing confinement, i.e., when going from planar to cylindrical geometry or by narrowing the pores. The adsorption on the pore walls has, in general, the opposite behaviour. For high bulk concentrations and certain values of cylindrical pore diameter the concentration profile is higher at the centre of the pore than next to the pore wall. A very strong, but continuous, transition occurs in the concentration profile, as a function of the cylinder's diameter. These results could be of some interest in catalysis studies. [ABSTRACT FROM AUTHOR]

Describes the structural, dynamical and electrical properties of the interface between two immiscible liquids. Structure and dynamics of the neat interface; Chemistry at the liquid-liquid interface; Microscopic behavior of molecules at the interfacial region.

The Tolman length and interfacial tension of partially miscible symmetric binary Lennard–Jones (LJ) fluids (A, B) was revealed by performing a large-scale molecular dynamics (MD) simulation with a sufficient interfacial area and cutting distance. A unique phenomenon was observed in symmetric binary LJ fluids, where two surfaces of tension existed on both sides of an equimolar dividing surface. The range of interaction εAB between the different liquids and the temperature in which the two LJ fluids partially mixed was clarified, and the Tolman length exceeded 3 σ when εAB was strong at higher temperatures. The results show that as the temperature or εAB increases, the Tolman length increases and the interfacial tension decreases. This very long Tolman length indicates that one should be very careful when applying the concept of the liquid–liquid interface in the usual continuum approximation to nanoscale droplets and capillary phase separation in nanopores. [ABSTRACT FROM AUTHOR]

We have developed a molecular mean-field theory—fourth-order Poisson–Nernst–Planck–Bikerman theory—for modeling ionic and water flows in biological ion channels by treating ions and water molecules of any volume and shape with interstitial voids, polarization of water, and ion-ion and ion-water correlations. The theory can also be used to study thermodynamic and electrokinetic properties of electrolyte solutions in batteries, fuel cells, nanopores, porous media including cement, geothermal brines, the oceanic system, etc. The theory can compute electric and steric energies from all atoms in a protein and all ions and water molecules in a channel pore while keeping electrolyte solutions in the extra- and intracellular baths as a continuum dielectric medium with complex properties that mimic experimental data. The theory has been verified with experiments and molecular dynamics data from the gramicidin A channel, L-type calcium channel, potassium channel, and sodium/calcium exchanger with real structures from the Protein Data Bank. It was also verified with the experimental or Monte Carlo data of electric double-layer differential capacitance and ion activities in aqueous electrolyte solutions. We give an in-depth review of the literature about the most novel properties of the theory, namely Fermi distributions of water and ions as classical particles with excluded volumes and dynamic correlations that depend on salt concentration, composition, temperature, pressure, far-field boundary conditions etc. in a complex and complicated way as reported in a wide range of experiments. The dynamic correlations are self-consistent output functions from a fourth-order differential operator that describes ion-ion and ion-water correlations, the dielectric response (permittivity) of ionic solutions, and the polarization of water molecules with a single correlation length parameter. [ABSTRACT FROM AUTHOR]

Curved interfaces between coexisting vapor and liquid phases are ubiquitous in nature, and the question of whether the Henry's law is applicable for highly curved vapor/liquid interfaces remains unsolved. Using stable surface nanobubbles that have highly curved interfaces as examples, we investigate the viability of Henry's law with molecular simulations and thermodynamic analysis. We show that the prediction of Henry's law displays an increasingly deviation from the simulation results as nanobubble curvature increases. Such a curvature dependence of Henry's law constant is ascribed to the nonideality of gas in liquid phase because of the required gas supersaturation for stabilizing nanobubbles. Based on the effect, we develop a relationship for determining Henry's law constant from the level of gas supersaturation, as well as a robust relationship between gas supersaturation and nanobubble radius. [ABSTRACT FROM AUTHOR]

Anesthetics are extremely important in modern surgery to greatly reduce the patient’s pain. The understanding of anesthesia at molecular level is the preliminary step for the application of anesthetics in clinic safely and effectively. Inert gases, with low chemical activity, have been found to cause anesthesia for centuries, but the mechanism is unclear yet. In this review, we first summarize the progress of theories about general anesthesia, especially for inert gas narcosis, and then propose a new hypothesis that the aggregated rather than the dispersed inert gas molecules are the key to trigger the narcosis to explain the steep dose-response relationship of anesthesia. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations show that the gas dissolved in water can be adsorbed at a hydrophobic interface and accumulates thereon. Initially, a water depletion layer appears on the hydrophobic interface. Gas molecules then enter the depletion layer and form a high-density gas-enriched layer. Finally, the gas-enriched layer accumulates to form a nanobubble. The radian of the nanobubble increases with time until equilibrium is reached. The equilibrium state arises through a Brenner–Lohse dynamic equilibrium mechanism, whereby the diffusive outflux is compensated by an influx near the contact line. Additionally, supersaturated gas also accumulates unsteadily in bulk water, since it can diffuse back into the water and is gradually adsorbed by a solid substrate. [ABSTRACT FROM AUTHOR]

An analytical method based on statistical mechanics is proposed to predict ultra-thin adsorbed films of physical fluids with molecular diversity formed on smooth surfaces. The model is representative of molecular interactions at the smooth summits of surface asperities in the nano-scale. At this physical scale the constraining effect of the solid barriers promotes discretization of the fluid volume into molecular layers. These layers are usually ejected from the contact in a stepwise manner. The integrated effect of intermolecular forces as well as their interactions with the contiguous surfaces is responsible for the discontinuous drainage of the fluid. However, at the same time, the adsorption energy of the molecular species strives to form a molecular monolayer upon the boundary solids. The net result of these complex interactions is an ultra-thin adsorbed film, whose shear characteristics depends on a competition between the repulsive solvation pressure and the energy of molecular adsorption. It is shown that very thin low shear strength films are formed in this manner. This would depend on the molecular concentration and the wall adsorption energy. An important implication is that boundary adherent films should be viewed as a result of surface-fluid combination for which the choice of concentration and fraction content of particular species are crucial. [ABSTRACT FROM AUTHOR]