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

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

Stopping of porous projectiles in granular targets

Using granular mechanics, we determine the stopping force acting on spherical granular projectiles impinging on a flat granular bed. We find that the stopping force is proportional to the impact energy, as in Poncelet's law. For fixed velocity, it is proportional to the projectile cross-sectional area rather than to its volume. These dependences only hold in the early stages of stopping, before the projectile has been strongly fragmented. Analogies to the stopping of atomic clusters in compact matter are pointed out.Fil: Planes, María Belén. Universidad de Mendoza. Facultad de Ingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Millán, Emmanuel Nicolás. Universidad Nacional de Cuyo. Instituto para las Tecnologías de la Información y las Comunicaciones; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Urbassek, Herbert M.. University Kaiserslautern; AlemaniaFil: Bringa, Eduardo Marcial. Universidad de Mendoza. Facultad de Ingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

A Monte Carlo code for the collisional evolution of porous aggregates (CPA)

Context. The collisional evolution of submillimeter-sized porous dust aggregates is important in many astrophysical fields. Aims. We have developed a Monte Carlo code to study the processes of collision between mass-asymmetric, spherical, micron-sized porous silica aggregates that belong to a dust population. Methods. The Collision of Porous Aggregates (CPA) code simulates collision chains in a population of dust aggregates that have different sizes, masses, and porosities. We start from an initial distribution of granular aggregate sizes and assume some collision velocity distribution. In particular, for this study we used a random size distribution and a Maxwell-Boltzmann velocity distribution. A set of successive random collisions between pairs of aggregates form a single collision chain. The mass ratio, filling factor, and impact velocity influence the outcome of the collision between two aggregates. We averaged hundreds of thousands of independent collision chains to obtain the final, average distributions of aggregates. Results. We generated and studied four final distributions (F), for size (n), radius (R), porosity, and mass-porosity distributions, for a relatively low number of collisions. In general, there is a profuse generation of monomers and small clusters, with a distribution F (R) ∝ R-6 for small aggregates. Collisional growth of a few very large clusters is also observed. Collisions lead to a significant compaction of the dust population, as expected. Conclusions. The CPA code models the collisional evolution of a dust population and incorporates some novel features, such as the inclusion of mass-asymmetric aggregates (covering a wide range of aggregate radii), inter-granular friction, and the influence of porosity.Fil: Millán, Emmanuel Nicolás. Universidad Nacional de Cuyo. Instituto para las Tecnologías de la Informacion y las Comunicaciones; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Planes, María Belén. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentina. Universidad de Mendoza. Facultad de Ingenieria; ArgentinaFil: Urbassek, Herbert M.. University Kaiserslautern; AlemaniaFil: Bringa, Eduardo Marcial. Universidad de Mendoza. Facultad de Ingenieria; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentin

Effect of subsurface voids on the nanoindentation of Fe crystals

Subsurface voids may strongly affect the response of materials to nanoindentation. We explore these effects for a bcc single-crystalline Fe sample using molecular dynamics simulation. Deformation occurs mainly by nucleation and propagation of dislocations. As dislocations impinge into the voids, these suffer a reduction in volume, consistent with mass transfer mechanisms. Our results show that voids act as highly efficient absorbers of dislocations, effectively limiting the extension of the plastic zone. Surprisingly, mechanical properties are marginally affected by the presence of voids in the range of sizes and spatial distributions tested, except for voids a few nanometers below the surface. Deformation twinning is observed as a transient effect in some cases; however, for voids close enough to the indentation area, no twinning was found.Fil: Hofer, Juan Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Ruestes, Carlos Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Interdisciplinario de Ciencias Básicas. - Universidad Nacional de Cuyo. Instituto Interdisciplinario de Ciencias Básicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Bringa, Eduardo Marcial. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Universidad de Mendoza. Facultad de Ingeniería; Argentina. Universidad Mayor; ChileFil: Urbassek, Herbert M.. University Kaiserslautern; Alemani

Interplay of plasticity and phase transformation in shock wave propagation in nanocrystalline iron

Strong shock waves create not only plasticity in Fe, but also phase transform the material from its bcc phase to the high-pressure hcp phase. We perform molecular-dynamics simulations of large, 8-million atom nanocrystalline Fe samples to study the interplay between these two mechanisms. We compare results for a potential that describes dislocation generation realistically but excludes phase change with another which in addition faithfully features the bcc → hcp transformation. With increasing shock strength, we find a transition from a two-wave structure (elastic and plastic wave) to a three-wave structure (an additional phase-transformation wave), in agreement with experiment. Our results demonstrate that the phase transformation is preceded by dislocation generation at the grain boundaries (GBs). Plasticity is mostly given by the formation of dislocation loops, which cross the grains and leave behind screw dislocations. We find that the phase transition occurs for a particle velocity between 0.6 and 0.7 km s−1. The phase transition takes only about 10 ps, and the transition time decreases with increasing shock pressure.Fil: Gunkelman, Nina. University of Kaiserlautern; AlemaniaFil: Tramontina Videla, Diego Ramiro. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Bringa, Eduardo Marcial. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Urbassek, Herbert M.. University of Kaiserlautern; Alemani

Bouncing window for colliding nanoparticles: Role of dislocation generation

Available macroscopic theories—such as the Johnson-Kendall-Roberts (JKR) model—predict sphericalparticles to stick to each other at small collision velocitiesv; above the bouncing velocity,vb, they bounce. Westudy the details of the bouncing threshold using molecular dynamics simulation for crystalline nanoparticleswhere atoms interact via the Lennard-Jones potential. We show that the bouncing velocity strongly dependson the nanoparticle orientation during collision; for some orientations, nanoparticles stick at all velocities.The dependence of bouncing on orientation is caused by energy dissipation during dislocation activity. Thebouncing velocity decreases with increasing nanoparticle radius in reasonable agreement with JKR theory. Fororientations for which bouncing exists, nanoparticles stick again at a higher velocity, the fusion velocity,vf,such that bouncing only occurs in a finite range of velocities—the bouncing window. The fusion velocity israther independent of the nanoparticle radius.Fil: Nietiadi, Maureen L.. Universität Kaiserslautern; AlemaniaFil: Millán, Emmanuel Nicolás. Universidad Nacional de Cuyo. Instituto para las Tecnologías de la Informacion y las Comunicaciones; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Bringa, Eduardo Marcial. Universidad de Mendoza; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Urbassek, Herbert M.. Universität Kaiserslautern; Alemani

The elastic-plastic transition in nanoparticle collisions

When nanoparticles (NPs) collide with low velocities, they interact elastically in the sense that-besides their fusion caused by their mutual van-der-Waals attraction-no defects are generated. We investigate the minimum velocity, vc, necessary for generating defects and inducing plasticity in the NP. The determination of this elastic-plastic threshold is of prime importance for modeling the behavior of granular matter. Using the generic Lennard-Jones interaction potential, we find vc to increase strongly with decreasing radius. Current models do not agree with our simulations, but we provide a model based on dislocation emission in the contact zone that quantitatively describes the size dependence of the elastic-plastic transition.Fil: Millán, Emmanuel Nicolás. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Tramontina Videla, Diego Ramiro. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Urbassek, Herbert M.. University Kaiserslautern; AlemaniaFil: Bringa, Eduardo Marcial. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentin

Impulsive generation of 100 dislocation loops in BCC iron

The conditions for the formation of 100 dislocation loops in body-centered cubic (BCC) iron were investigated via molecular dynamics simulations using a simplified model intended to mimic conditions in high energy collision cascades, focusing on the possible coherent displacement of atoms at the boundary of a subcascade. We report on the formation of 100 dislocation loops due to the fast displacement of a few hundred atoms with a coherent acceleration, in agreement with previous results for much larger cascade simulations. We analyze in detail the resulting atomic velocities and pressures, and find that they cannot be described within the usual formalism for a shock regime, since the pressure pulse only lasts less than 1 ps and does not match expected values from a Hugoniot shock. Our simulations include two interatomic potentials: Mendelev, which is extensively used for radiation damage simulations, and Ackland, which has been used for shock simulations because it can reproduce the experimentally observed transition from BCC to hexagonal close-packed structure at around 25 GPa, at high deformation rates. They both show similar evolution of defects, also indicating departure from a shock regime which is extremely different for these potentials.Fil: Bertoni, Andrés Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Interdisciplinario de Ciencias Básicas. - Universidad Nacional de Cuyo. Instituto Interdisciplinario de Ciencias Básicas; ArgentinaFil: Deluigi, Orlando Raul. Universidad de Mendoza; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Dos Santos Mendez, Gonzalo Joaquín. Universidad de Mendoza; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Pérez Diaz, M.. Universidad Nacional de Cuyo; ArgentinaFil: Bringa, Eduardo Marcial. Universidad Mayor; Chile. Universidad de Mendoza; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentin

Shock-induced plasticity in nanocrystalline iron: Large-scale molecular dynamics simulations

Large-scale nonequilibrium molecular dynamics (MD) simulations of shock waves in nanocrystalline iron show evidence of plasticity before the polymorphic transformation takes place. The atomistic structure in the shock direction shows an elastic precursor, plastic deformation, and shock-induced phase transformation from bcc to hcp iron. In this Rapid Communication, large-scale MD models show that the shock response of iron is highly related to the ramp time of the applied shocks. For long ramp times we observe significant plastic relaxation and formation of microstructure defects. Pressure-induced phase transformations in iron are accompanied by stress relaxation achieving almost fully relaxed three-dimensional hydrostatic final states. The evolution of the stress relaxation is in agreement with theory and experiments. Analysis of the x-ray diffraction patterns calculated from the atomistic structure using the Debye equation revealed pronounced anisotropy of the line broadening that is caused by stacking faults in hcp Fe and by dislocations in bcc Fe.Fil: Luu, Hoang Thien. Clausthal University of Applied Technology; AlemaniaFil: Ravelo, Ramón. University of Texas at El Paso; Estados UnidosFil: Rudolph, Martin. TU Bergakademie Freiberg; AlemaniaFil: Bringa, Eduardo Marcial. Universidad de Mendoza; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Germann, Timothy C.. Los Alamos National High Magnetic Field Laboratory; Estados UnidosFil: Rafaja, David. TU Bergakademie Freiberg; AlemaniaFil: Gunkelmann, Nina. Clausthal University of Applied Technology; Alemani