We investigate finite-size effects on diffusion in confined fluids using molecular dynamics simulations and hydrodynamic calculations. Specifically, we consider a Lennard-Jones fluid in slit pores without slip at the interface and show that the use of periodic boundary conditions in the directions along the surfaces results in dramatic finite-size effects, in addition to that of the physically relevant confining length. As in the simulation of bulk fluids, these effects arise from spurious hydrodynamic interactions between periodic images and from the constraint of total momentum conservation. We derive analytical expressions for the correction to the diffusion coefficient in the limits of both elongated and flat systems, which are in excellent agreement with the molecular simulation results except for the narrowest pores, where the discreteness of the fluid particles starts to play a role. The present work implies that the diffusion coefficients for wide nanopores computed using elongated boxes suffer from finite-size artifacts which had not been previously appreciated. In addition, our analytical expression provides the correction to be applied to the simulation results for finite (possibly small) systems. It applies not only to molecular but also to all mesoscopic hydrodynamic simulations, including Lattice-Boltzmann, Multiparticle Collision Dynamics or Dissipative Particle Dynamics, which are often used to investigate confined soft matter involving colloidal particles and polymers.

We use molecular dynamics to investigate how the structure, diffusion and hydrodynamic properties of clay interfaces with aqueous solution depend on the nature of the clay, the nature of the counterions and the salt concentration in the solution. Specifically, we study water-filled nanopores between uncharged (pyrophyllite) and charged (montmorillonite and beidellite, with susbtitutions located in the octahedral and tetrahedral layers, respectively) clays, with sodium or cesium as counterions, in the absence and in the presence of added salt. We discuss how the balance between solvation and attraction of the cations to the surface results in various distributions between innerand outer-sphere complexes and how this influences the dynamics of water near the surface, as well as the hydrodynamic flow in the presence of an external force. In the latter case, the discussion based on mapping the molecular velocity profiles to a continuous description (parabolic Poiseuille flow) shows that the larger effects come from the presence/absence of charge in the mineral, as well as the localization of substitutions within the clay layer. The salt concentration and the nature of the counterions have a comparatively less important impact far from the surface-even though some differences are observed in its close vicinity, which are not properly captured by the continuous description.

Superionic conductivity in certain polymorphs of Ag2S has inspired numerous concepts for materials applications, but the relationship between the structure and the mobility of silver ions remains poorly explored. Here, we report ab initio molecular dynamics simulations for low- (acanthite) and high-temperature (argentite) Ag2S polymorphs that reveal the dynamical processes giving rise to the superionic behavior in the latter. Similarities between their sulfur sublattices enable simulations of silver ion diffusivities and pathways on essentially an equal footing. For the higher temperature polymorph, calculated temperature-dependent mean square displacements and activation energies by the nudged elastic band method show good correspondence with expectations from the experiment. In the superionic state, silver atoms diffuse in a liquid-like behavior with no preferred diffusion pathways, within the relatively stable body-centered cubic sulfur framework. In contrast, conduction in acanthite appears to depend more on the mobilities of electronic charge carriers.

Understanding the electrochemical properties of mineral−water interfaces tends to rely upon electrical double layer (EDL) models, but these models are based on the assumption that electrostatic equilibrium is constantly maintained. In reality, interfacial reactions, ion diffusion, and their electrochemical signatures are based in nonequilibrium conditions of locally or globally imbalanced electrical fields where current EDL models have limited purview. Here, we performed molecular dynamics (MD) simulations of the orthoclase (001) surface in contact with a 1 M NaCl aqueous solution under various electric fields, to explore the interplay between EDL structure and dynamics when perturbed by electric fields of different directions and strengths, by confinement, and by different distributions of structural surface charge. The simulations showed that confinement between two opposing (001) surfaces led to the development of an induced field when the applied field was perpendicular to the surfaces and, as a result, to ionic diffusion coefficients that were independent of electric field strength. In contrast, when the applied field was parallel to the surfaces, confinement resulted in ionic diffusion coefficients that were more strongly dependent on the magnitude of the electric field than in bulk water. Differences in the density and distribution of aluminol groups on the two surfaces had a significant impact on how the interfacial structure and dynamics varied in the presence of an electric field. Notably, these differences resulted in an electro-osmotic flow with opposite directions at the two surfaces under parallel applied electric field. Overall, the MD simulations highlighted the importance of considering atomic-level structure and heterogeneities when developing models of the electrochemical properties of mineral−water interfaces.

Mass transport along grain boundaries in alloys depends not only on the atomic structure of the boundary, but also its chemical make-up. In this work, we use molecular dynamics to examine the effect of Cr alloying on interstitial and vacancy-mediated transport at a variety of grain boundaries in Ni. We find that, in general, Cr tends to reduce the rate of mass transport, an effect which is greatest for interstitials at pure tilt boundaries. However, there are special scenarios in which it can greatly enhance atomic mobility. Cr tends to migrate faster than Ni, though again this depends on the structure of the grain boundary. Further, grain boundary mobility, which is sometimes pronounced for pure Ni grain boundaries, is eliminated on the time scales of our simulations when Cr is present. We conclude that the enhanced transport and grain boundary mobility often seen in this system in experimental studies is the result of non-equilibrium effects and is not intrinsic to the alloyed grain boundary. These results provide new insight into the role of grain boundary alloying on transport that can help in the interpretation of experimental results and the development of predictive models of materials evolution.

The RExSi1–x alloys are elaborated for three eutectic compositions. They are studied by ATD, electrical, and magnetic measurements. These alloys exhibit a good thermal stability, an important atomic relaxation, and a two‐step crystallization to attain the crystalline alloys located on each side of the eutectic. The RE‐rich alloys have a metallic or nearly metallic behaviour; the RE‐poor alloys present a semiconductor‐like conductivity. It is impossible to say if there is a magnetic order at low temperature for small x. On the contrary the asperomagnetic order is evident for RE‐rich alloys. The thermal behaviour of the magnetization in weak field is similar to that for the other parameters (coercive field, remanent magnetization, loop energy, field saturating the loop) characterising the hysteresis loop.

No abstract

No abstract