Anisotropic surface stresses of a solid/fluid interface: Molecular dynamics calculations for the copper/methane interface.

The full tensorial surface stress of an interface between a face-centered cubic crystal (copper) and an isotropic liquid (methane) is computed for two crystal orientations {100} and {110} using molecular dynamics simulations. The bulk crystal orientation {100} is symmetric, whereas the {110} orientation is not. Finite size effects, which can be important in the case of an interface between an isotropic solid and a liquid, are studied in detail for the two crystal orientations. We first show that the symmetry of the surface stress tensor is that of the bulk crystal orientation. In the case of the asymmetric crystal orientation {110}, the relative difference between the components of the surface stress is substantial (∼50%). Finally, we show that finite size effects persist to much larger sizes in the case of the {100} orientation compared to the case of the {110} interface, for instance, through an artificial breakdown of the symmetry of the surface stress tensor.

Interatomic Potential in the Nonequilibrium Warm Dense Matter Regime.

We present a new measurement of lattice disassembly times in femtosecond-laser-heated polycrystalline Au nanofoils. The results are compared with molecular dynamics simulations incorporating a highly optimized, embedded-atom-method interatomic potential. For absorbed energy densities of 0.9-4.3 MJ/kg, the agreement between the experiment and simulation reveals a single-crystal-like behavior of homogeneous melting and corroborates the applicability of the interatomic potential in the nonequilibrium warm dense matter regime. For energy densities below 0.9 MJ/kg, the measurement is consistent with nanocrystal behavior where melting is initiated at the grain boundaries.

Calculation of a solid/liquid surface tension: A methodological study.

The surface tension of a model solid/liquid interface constituted of a graphene sheet surrounded by liquid methane has been computed using molecular dynamics in the Kirkwood-Buff formalism. We show that contrary to the fluid/fluid case, the solid/liquid case can lead to different structurations of the first fluid layer, leading to significantly different values of surface tension. Therefore we present a statistical approach that consists in running a series of molecular simulations of similar systems with different initial conditions, leading to a distribution of surface tensions from which an average value and uncertainty can be extracted. Our results suggest that these distributions converge as the system size increases. Besides we show that surface tension is not particularly sensitive to the choice of the potential energy cutoff and that long-range corrections can be neglected contrary to what we observed in the liquid/vapour interfaces. We have not observed the previously reported commensurability effect.

Theoretical study of the nucleation/growth process of carbon clusters under pressure.

We used molecular dynamics and the empirical potential for carbon LCBOPII to simulate the nucleation/growth process of carbon clusters both in vacuum and under pressure. In vacuum, our results show that the growth process is homogeneous and yields mainly sp(2) structures such as fullerenes. We used an argon gas and Lennard-Jones potentials to mimic the high pressures and temperatures reached during the detonation of carbon-rich explosives. We found that these extreme thermodynamic conditions do not affect substantially the topologies of the clusters formed in the process. However, our estimation of the growth rates under pressure are in much better agreement with the values estimated experimentally than our vacuum simulations. The formation of sp(3) carbon was negligible both in vacuum and under pressure which suggests that larger simulation times and cluster sizes are needed to allow the nucleation of nanodiamonds.

Uniaxial Hugoniostat: a method for atomistic simulations of shocked materials.

An new equilibrium molecular-dynamics method (the uniaxial Hugoniostat) is proposed to study the energetics and deformation structures in shocked crystals. This method agrees well with nonequilibrium molecular-dynamics simulations used to study shock-wave propagation in solids and liquids.