Bjørn Hafskjold

Nonequilibrium molecular-dynamics simulation of net evaporation and net condensation, and evaluation of the gas-kinetic boundary condition at the interphase

Algorithms for simulating steady net evaporation and net condensation with molecular dynamics are presented. The evaporation and condensation coefficients are calculated, showing that they are not equal outside equilibrium. The distribution function at the interphase boundary is evaluated. There is a drift away from the interphase in the distribution function for the evaporated molecules and a drift velocity towards the interphase for the reflected molecules, both for net evaporation and for net condensation.

Simulating the effect of surfactant structure on bending moduli of monolayers

We have used dissipative particle dynamics to simulate amphiphilic monolayers on the interface between oil and water. An ultralow interfacial tension is imposed by means of Monte Carlo to resemble the amphiphilic films that separate oil and water regions in microemulsions. We calculate the bending modulus by analyzing the undulation spectrum. By varying the surfactant chain length and topology we investigate the effect of surfactant structure and composition of the monolayer on the bending moduli. We find that increasing the thickness has a larger effect than increasing the density of the layer. This follows from the observations that at a given interfacial tension, the bending modulus increases with chain length and is larger for linear than branched surfactants. The increase with chain length is approximately linear, which is slower than the theoretical predictions at a fixed area. We also investigated a binary mixture of short and long surfactants compared to pure layers of the same average chain length. We find a roughly linear decrease in bending modulus with mole fraction of short surfactants. Furthermore, the mixed film has a lower bending modulus than the corresponding pure film for all mole fractions. Linking the bending moduli to the structure of the surfactants is an important step in predicting the stability of microemulsions.

Molecular-level Calculation Scheme for Pressure in Inhomogeneous Systems of Flat and Spherical Layers

Expressions for the microscopic pressure tensor, suitable for use in molecular dynamics (MD) and Monte Carlo simulations, are presented. The expressions apply to heterogeneous systems consisting of particles interacting with pair-wise additive potentials. The normal and transverse components of the pressure tensor as defined by Irving and Kirkwood [Irving, A.J.H. and Kirkwood, J.G. J. Chem. Phys. , 18 (1950) 817] were used to derive explicit equations for the coarse-grained pressure in planar and spherical symmetries. Molecular dynamics simulation results at v /near planar and spherical surfaces confirmed that mechanical balance is attained at equilibrium. Furthermore, it was found that the computed coarse-grained pressure in a local volume was more precise than the pressure on a surface for a given simulation length.

Thermal diffusion factors for the Lennard-Jones/spline system

A steady-state, molecular-dynamics method is used to calculate the thermal diffusion factor α for binary isotopic mixtures at a single, supercritical temperature. The variation of α with density, c...

Can such Long Time Steps Really be used in Dissipative Particle Dynamics Simulations

Commonly used time steps in dissipative particle dynamics (DPD) simulations are too large and lead to systematic errors in the computed properties. The main source of errors is the inaccurate integration of the conservative force. This error can be reduced to some extent by constructing a smoother force without any abrupt change at the cut-off distance, but the improvement is marginal. Alternatively, we tried smooth forces that also lead to the same conclusion. It is possible to find combinations of parameters for the random and dissipative forces that make errors cancel, but the combinations will depend on the systems thermodynamic state and on the particular force model. The only safe procedure is to use small time steps, i.e. comparable with those used in MD simulations. Alternatively, an improved integration algorithm should be used for the conservative force.

Nonequilibrium Molecular Dynamics Simulations of Coupled Heat and Mass Transport in Binary Fluid Mixtures in Pores

Molecular dynamics simulations were carried out for a binary fluid mixture in a slit pore. The fluid was an argon-like Lennard–Jones/spline model. The pore wall was represented by the Steele model for a layered graphite structure. The pore had a heat source in one end and a heat sink in the other, resulting in a lateral temperature gradient, a Soret effect, and a thermal creep flow along the pore wall. Potential models with various depths were used to examine the effect of wetting and adsorption on the thermal creep flow. The main results were as follows, (a) A relatively strong creep flow was generated parallel to the wall by the temperature gradient. For strongly attracting fluid–wall potentials, the flow occurred from the cold to the hot end of the pore near the wall (except for the very narrow pore) and oppositely in the center of the pore. For a purely repulsive potential, the flow was weak and mostly in the opposite direction, (b) The thermal diffusion coefficient was comparable to that in bulk fluid at the same overall density, except when the creep flow was strong, in which case the thermal diffusion was blurred by the convective mixing.

Computer Simulations of Thermal Diffusion in Binary Fluid Mixtures

Compared to other transport properties, thermal diffusion has not been much studied by computer simulations. During the last decade, however, a couple of different simulation methods have been applied to a few systems, aiming both at determination of numerical values of the Soret coefficient and at a better understanding of the phenomenon. Based on such simulations, we will present some explanations for why there is a Soret effect in binary fluids.

Estimation of single electrode heats

Abstract An approximate expression for single electrode heats have been derived and tested. Peltier coefficients of a metal electrode in contact with a molten salt mixture were first derived using irreversible thermodynamics of surfaces. The expression for the Peltier coefficient for the electrolyte side of the interface has the value of a one-component electrolyte as a basis and includes correction terms for multicomponent mixtures. Analyses of results reported in the literature for salt mixtures of AgX–BX and CuX–BX show how the correction terms vary when the electrode surface is chosen as the frame of reference for the transports. Here X represents a halide ion and B is an alkali metal. The expression is used to estimate electrode heats for the B|BX electrode within 10–20% accuracy. The error is mainly due to neglect of the transported heat in the electrolyte.

A COMPARISON OF NON-EQUILIBRIUM MOLECULAR DYNAMICS AND NPT MONTE CARLO METHODS FOR MIXING PROPERTIES AND PARTIAL MOLAR QUANTITIES

Non-equilibrium molecular dynamics (NEMD) and NPT Monte Carlo simulations were carried out for two non-ideal Lennard-Jones/spline mixtures at supercritical temperatures. The systems were chosen so that one of them showed positive and the other negative excess volumes and enthalpies on mixing. A strong composition gradient was generated in the NEMD simulations, such that the entire composition range from pure component 1 to pure component 2 was covered in the molecular dynamics cell. A series of Monte Carlo simulations was made, covering the same composition range. The temperatures and pressures used in the MC simulations were chosen equal to the results from the NEMD simulations. The excess volumes and enthalpies on mixing from the two simulations showed excellent agreement, leading to the conclusion that the NEMD simulations were performed under local equilibrium despite the lack of overall equilibrium in the system. Excess and partial molar quantities were computed by the two methods, sand the data were...

Non-equilibrium molecular dynamics simulations of the transient Ludwig-Soret effect in a binary Lennard-Jones/spline mixture.

Abstract.A binary isotope mixture of Lennard-Jones/spline particles at equilibrium was perturbed by a sudden change in the systems boundary temperatures. The systems response was determined by non-equilibrium molecular dynamics (NEMD). Three transient processes were studied: 1) The propagation of a pressure (shock) wave, 2) heat diffusivity and conduction, and 3) thermal diffusion (the Ludwig-Soret effect). These three processes occur at different time scales, which makes it possible to separate them in one single NEMD run. The system was studied in liquid, supercritical, and dense gas states with various forms and strengths of the thermal perturbation. The results show that heat was initially transported by two separate mechanisms: 1) heat diffusion as described by the transient heat equation and 2) as a consequence of a pressure wave. The pressure wave travelled faster than the speed of sound, generating a shock wave in the system. Local equilibrium was found in the transient phase, even with very strong perturbations and in the shock front. Although the mass separation due to the Ludwig-Soret effect developed much slower than the pressure and temperature fields in the system at large, it was found that the Soret coefficient could be accurately determined from the initial phase of the transient and close to the heat source. This opens the possibility of a new way to analyse results from transient experiments and thereby minimize effects of gravity and convection due to buoyancy.Graphical abstract