Classical molecular dynamics simulations of amorphous silica surfaces

We have adapted classical molecular dynamics to study the structural and dynamical properties of amorphous silica surfaces. Concerning the structure, the density profile exhibits oscillations perpendicularly to the surface as observed in liquid metal surfaces and the pair correlation functions as well as the angle distributions show features (absent in the interior of the films) that can be attributed to the presence of 2-fold rings which are perpendicular to the surface. From the mean-squared displacement of the non bridging oxygen atoms we find that in the interior region they move perpendicular to the surface while they move parallel to it in the surface region.Comment: 7 pages,5 figures - To be published in J. Phys. C.

Crystallization in a model glass: influence of the boundary conditions

Using molecular dynamics calculations and the Voronoi tessellation, we study the evolution of the local structure of a soft-sphere glass versus temperature starting from the liquid phase at different quenching rates. This study is done for different sizes and for two different boundary conditions namely the usual cubic periodic boundary conditions and the isotropic hyperspherical boundary conditions for which the particles evolve on the surface of a hypersphere in four dimensions. Our results show that for small system sizes, crystallization can indeed be induced by the cubic boundary conditions. On the other hand we show that finite size effects are more pronounced on the hypersphere and that crystallization is artificially inhibited even for large system sizes.Comment: 11 pages, 2 figure

Computer investigation of the energy landscape of amorphous silica

The multidimensional topography of the collective potential energy function of a so-called strong glass former (silica) is analyzed by means of classical molecular dynamics calculations. Features qualitatively similar to those of fragile glasses are recovered at high temperatures : in particular an intrinsic characteristic temperature $T_c\simeq 3500$K is evidenced above which the system starts to investigate non-harmonic potential energy basins. It is shown that the anharmonicities are essentially characterized by a roughness appearing in the potential energy valleys explored by the system for temperatures above $T_c$.Comment: 5 pages; accepted for publication in PR

Structure and dynamics of a model glass: influence of long-range forces

We vary the amplitude of the long-range Coulomb forces within a classical potential describing a model silica glass and study the consequences on the structure and dynamics of the glass, via molecular dynamics simulations. This model allows us to follow the variation of specific features such as the First Sharp Diffraction Peak and the Boson Peak in a system going continuously from a fragile (no Coulomb forces) to a strong (with Coulomb forces) glass. In particular we show that the characteristic features of a strong glass (existence of medium range order, bell-shaped ring size distribution, sharp Boson peak) appear as soon as tetrahedral units are formed.Comment: 5 pages, 4 figures. To be published in J.Phys.: C

Glucose availability and sensitivity to anoxia of isolated rat peripheral nerve

The contrast between resistance to ischemia and ischemic lesions in peripheral nerves of diabetic patients was explored by in vitro experiments. Isolated and desheathed rat peroneal nerves were incubated in the following solutions with different glucose availability: 1) 25 mM glucose, 2) 2.5 mM glucose, and 3) 2.5 mM glucose plus 10 mM 2-deoxy-D-glucose. Additionally, the buffering power of all of these solutions was modified. Compound nerve action potential (CNAP), extracellular pH, and extracellular potassium activity (aKe) were measured simultaneously before, during, and after a period of 30 min of anoxia. An increase in glucose availability led to a slower decline in CNAP and to a smaller rise in aKe during anoxia. This resistance to anoxia was accompanied by an enhanced extracellular acidosis. Postanoxic recovery of CNAP was always complete in 25 mM HCO3(-)-buffered solutions. In 5 mM HCO3- and in HCO3(-)-free solutions, however, nerves incubated in 25 mM glucose did not recover functionally after anoxia, whereas nerves bathed in solutions 2 or 3 showed a complete restitution of CNAP. We conclude that high glucose availability and low PO2 in the combination with decreased buffering power and/or inhibition of HCO3(-)-dependent pH regulation mechanisms may damage peripheral mammalian nerves due to a pronounced intracellular acidosis

Backward correlations and dynamic heterogeneities: a computer study of ion dynamics

We analyse the correlated back and forth dynamics and dynamic heterogeneities, i.e. the presence of fast and slow ions, for a lithium metasilicate system via computer simulations. For this purpose we define, in analogy to previous work in the field of glass transition, appropriate three-time correlation functions. They contain information about the dynamics during two successive time intervals. First we apply them to simple model systems in order to clarify their information content. Afterwards we use this formalism to analyse the lithium trajectories. A strong back-dragging effect is observed, which also fulfills the time-temperature superposition principle. Furthermore, it turns out that the back-dragging effect is long-ranged and exceeds the nearest neighbor position. In contrast, the strength of the dynamic heterogeneities does not fulfill the time-temperature superposition principle. The lower the temperature, the stronger the mobility difference between fast and slow ions. The results are then compared with the simple model systems considered here as well as with some lattice models of ion dynamics.Comment: 12 pages, 10 figure