761 research outputs found
Simultaneous description of bulk and interfacial properties of fluids by the Mie potential
The vapor-liquid equilibrium (VLE) of the Mie potential, where the dispersive
exponent is constant (m = 6) while the repulsive exponent n is varied between 9
and 48, is systematically investigated by molecular simulation. For systems
with planar vapor-liquid interfaces, long-range correction expressions are
derived, so that interfacial and bulk properties can be computed accurately.
The present simulation results are found to be consistent with the available
body of literature on the Mie fluid which is substantially extended. On the
basis of correlations for the considered thermodynamic properties, a
multicriteria optimization becomes viable. Thereby, users can adjust the three
parameters of the Mie potential to the properties of real fluids, weighting
different thermodynamic properties according to their importance for a
particular application scenario. In the present work, this is demonstrated for
carbon dioxide for which different competing objective functions are studied
which describe the accuracy of the model for representing the saturated liquid
density, the vapor pressure and the surface tension. It is shown that models
can be found which describe simultaneously the saturated liquid density and
vapor pressure with good accuracy, and it is discussed to what extent this
accuracy can be upheld as the model accuracy for the surface tension is further
improved
Contact angle of sessile drops in Lennard-Jones systems
Molecular dynamics simulation is used for studying the contact angle of
nanoscale sessile drops on a planar solid wall in a system interacting via the
truncated and shifted Lennard-Jones potential. The entire range between total
wetting and dewetting is investigated by varying the solid--fluid dispersive
interaction energy. The temperature is varied between the triple point and the
critical temperature. A correlation is obtained for the contact angle in
dependence of the temperature and the dispersive interaction energy. Size
effects are studied by varying the number of fluid particles at otherwise
constant conditions, using up to 150 000 particles. For particle numbers below
10 000, a decrease of the contact angle is found. This is attributed to a
dependence of the solid-liquid surface tension on the droplet size. A
convergence to a constant contact angle is observed for larger system sizes.
The influence of the wall model is studied by varying the density of the wall.
The effective solid-fluid dispersive interaction energy at a contact angle of
90 degrees is found to be independent of temperature and to decrease linearly
with the solid density. A correlation is developed which describes the contact
angle as a function of the dispersive interaction, the temperature and the
solid density. The density profile of the sessile drop and the surrounding
vapor phase is described by a correlation combining a sigmoidal function and an
oscillation term
MolMod – an open access database of force fields for molecular simulations of fluids
The MolMod database is presented, which is openly accessible at http://molmod.boltzmann-zuse.de and contains intermolecular force fields for over 150 pure fluids at present. It was developed and is maintained by the Boltzmann-Zuse Society for Computational Molecular Engineering (BZS). The set of molecular models in the MolMod database provides a coherent framework for molecular simulations of fluids. The molecular models in the MolMod database consist of Lennard-Jones interaction sites, point charges, and point dipoles and quadrupoles, which can be equivalently represented by multiple point charges. The force fields can be exported as input files for the simulation programmes ms2 and ls1 mardyn, GROMACS, and LAMMPS. To characterise the semantics associated with the numerical database content, a force field nomenclature is introduced that can also be used in other contexts in materials modelling at the atomistic and mesoscopic levels. The models of the pure substances that are included in the database were generally optimised such as to yield good representations of experimental data of the vapour–liquid equilibrium with a focus on the vapour pressure and the saturated liquid density. In many cases, the models also yield good predictions of caloric, transport, and interfacial properties of the pure fluids. For all models, references to the original works in which they were developed are provided. The models can be used straightforwardly for predictions of properties of fluid mixtures using established combination rules. Input errors are a major source of errors in simulations. The MolMod database contributes to reducing such errors.BMBF, 01IH16008E, Verbundprojekt: TaLPas - Task-basierte Lastverteilung und Auto-Tuning in der PartikelsimulationEC/H2020/694807/EU/Enrichment of Components at Interfaces and Mass Transfer in Fluid Separation Technologies/ENRICOEC/H2020/760907/EU/Virtual Materials Market Place (VIMMP)/VIMM
Boon and Bane: On the role of Adjustable Parameters in Simulation Models
We claim that adjustable parameters play a crucial role in building and applying simulation models. We analyze that role and illustrate our findings using examples from equations of state in thermodynamics. In building simulation models, two types of experiments, namely, simulation and classical experiments, interact in a feedback loop, in which model parameters are adjusted. A critical discussion of how adjustable parameters function shows that they are boon and bane of simulation. They help to enlarge the scope of simulation far beyond what can be determined by theoretical knowledge, but at the same time undercut the epistemic value of simulation models
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