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

Modeling of Nanoindentation in Ni-Graphene Nanocomposites: A Molecular Dynamics Sensitivity Study

Using molecular dynamics simulation, we perform nanoindentation simulations on a Ni-graphene model system, in which a graphene flake coats the grain boundary of a Ni bi-crystal. Material strengthening or weakening by inclusion of graphene is discussed with the help of the force needed to indent to a specified depth. By varying the depth of the graphene flake with respect to the indentation depth we identify the distance up to which graphene influences the indentation behavior. In addition, we vary the details of the modeling of the graphene flake in the matrix metal and determine their influence on the performance of the nanocomposite. Our results indicate that the modeling results are robust against variations in the modeling of the graphene flake

Water Evaporation and Conformational Changes from Partially Solvated Ubiquitin

Using molecular dynamics simulation, we study the evaporation of water molecules off partially solvated ubiquitin. The evaporation and cooling rates are determined for a molecule at the initial temperature of 300 K. The cooling rate is found to be around 3 K/ns, and decreases with water temperature in the course of the evaporation. The conformation changes are monitored by studying a variety of intermediate partially solvated ubiquitin structures. We find that ubiquitin shrinks with decreasing hydration shell and exposes more of its hydrophilic surface area to the surrounding

A Fluid Dynamics Calculation of Sputtering from a Cylindrical Thermal Spike

The sputtering yield, Y, from a cylindrical thermal spike is calculated using a two dimensional fluid dynamics model which includes the transport of energy, momentum and mass. The results show that the high pressure built-up within the spike causes the hot core to perform a rapid expansion both laterally and upwards. This expansion appears to play a significant role in the sputtering process. It is responsible for the ejection of mass from the surface and causes fast cooling of the cascade. The competition between these effects accounts for the nearly linear dependence of $Y$ with the deposited energy per unit depth that was observed in recent Molecular Dynamics simulations. Based on this we describe the conditions for attaining a linear yield at high excitation densities and give a simple model for this yield.Comment: 10 pages, 9 pages (including 9 figures), submitted to PR

Ejection of Glycine Molecules Adsorbed on a Water Ice Surface by Swift-heavy Ion Irradiation

Organic molecules may be adsorbed on the ice surfaces of comets or moons. We study the desorption process induced by swift-heavy ion irradiation using a molecular dynamics simulation. Focusing on the amino acid glycine adsorbed on water ice as a prototypical example, we model a 2 MeV sulfur ion impact as it might be typical of magnetospheric ion impact on the surface of Europa. We find that molecules are ejected intact within a radius of up to 25 Å around the ion impact point. Within a core region of around 10 Å, glycine molecules are destroyed and mainly fragments are emitted. Prominent fragments produced are cyanide CN-, carbon monoxide CO, cyanate OCN-, and carbon dioxide CO2, in agreement with experimental studies. In addition, radiolysis of water ice generates the radicals H+, H3O+, and HO- as well as the gases H2, O2, and some H2O2. While the smaller fragments easily obtain velocities above 2 km s-1 - the escape velocity from Europa - most ejected glycine molecules obtain smaller velocities and will thus not leave the moon permanently. Our results thus provide a detailed example that shows to what extent intact emission of organic molecules from Europa's surface by ion irradiation is possible and may be used for modeling the height distribution of ejecta in Europa's exosphere.Fil: Anders, Christian. Technische Universität Kaiserslautern; AlemaniaFil: Bringa, Eduardo Marcial. Universidad de Mendoza; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Urbassek, Herbert M.. Technische Universität Kaiserslautern; Alemani

Adsorption of Diclofenac and PFBS on Hair Keratin Dimer

Environmental pollution by man-made toxic and persistent organic compounds, found throughout the world in surface and groundwater, has various negative effects on aquatic life systems and even humans. Therefore, it is important to develop and improve water treatment technologies capable of removing such substances from wastewater or purifying drinking water. The two substances investigated are the widely used painkiller diclofenac and a member of the class of "forever chemicals", perfluorobutane sulfonate. Both are known to have serious negative effects on living organisms, especially under long-term exposure, and are detectable in human hair, suggesting adsorption to a part of the hair fiber complex. In this study, a human hair keratin dimer is investigated for its ability to absorb diclofenac and perfluorobutane sulfonate. Initial predictions for binding sites are obtained via molecular docking and subjected to molecular dynamics simulations for more than $1$ $\mathrm{\mu s}$. The binding affinities obtained by the linear interaction energy method are high enough to motivate further research on human hair keratins as a sustainable, low-cost, and easily allocatable filtration material.Comment: 19 pages, 8 figures, 2 table

Modeling of Martensitic Transformations in Pure Iron by a Phase Field Approach Using Information from Atomistic Simulation

A phase field approach for martensitic transformations is introduced. The parameters are determined due to results from molecular dynamic simulations for pure iron. The continuum model is provided with the atomistic input data to examine the evolution of microstructure in 2D, both under the influence of external load and for interface motion through the transformation induced eigenstrain. Therefore, different configurations of the two phases are used. In addition, the energy evolution of the system is studied in detail during the transformation process. The numerical implementation of the model is performed with finite elements while an implicit time integration scheme is applied for the transient terms

Nanoindentation tests of heavy-ion-irradiated Au foams - Molecular dynamics simulation

Irradiation by light ions may change the mechanical properties of nanofoams. Using molecular-dynamics simulation, we study the effect of irradiating a Au foam (porosity, 50%, and ligament diameter, 3 nm) with heavy ions: here, 10 keV Au ions up to a dose of 4 × 1016 m-2. We demonstrate that in consequence, the ligament morphology changes in the irradiated region, caused by local melting. The changes in mechanical properties are monitored by simulated nanoindentation tests. We find that the foam hardness is only around 1/3 of the hardness of a bulk Au crystal. Irradiation increases the hardness of the foam by around 10% in the central irradiated area. The plastic zone extends to only 1.5 ac, where ac denotes the contact radius; this value is unchanged under irradiation. The hardness increase after irradiation is attributed to two concurring effects. To begin with, irradiation induces melting and annealing of the ligaments, leading to their coarsening and alleviating surface stress, which in turn increases the dislocation nucleation threshold. In addition, irradiation introduces a stacking fault forest that acts as an obstacle to dislocation motion.Fil: Ruestes, Carlos Javier. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Anders, Christian. University Kaiserslautern; AlemaniaFil: Bringa, Eduardo Marcial. Universidad de Mendoza; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Urbassek, Herbert M.. University Kaiserslautern; Alemani