The dynamics of glass-forming systems shows a multitude of features that are absent innormal liquids, such as non-exponential relaxation and a strong temperature-dependence ofthe relaxation time. Connecting these dynamic properties to the microscopic structure of thesystem is challenging because of the presence of the structural disorder. Here we usecomputer simulations of a metallic glass-former to establish such a connection. By probingthe temperature and wave-vector dependence of the intermediate scattering function wefindthat the relaxation dynamics of the glassy melt is directly related to the local arrangement oficosahedral structures: Isolated icosahedra give rise to a liquid-like stretched exponentialrelaxation whereas clusters of icosahedra lead to a compressed exponential relaxation that isreminiscent to the one found in a solid. Our results show that in metallic glass-formers thesetwo types of relaxation processes can coexist and give rise to a dynamics that is surprisinglycomple

Structuring and degrading polymer gels via catalytic reactions. Réunion plénière du GDR SLAMM (Solliciter LA Matière Molle)

Understanding the fracture behavior of glasses at the atomic scale is of fundamental impor-tance for improving the mechanical properties of these materials. Using an interaction potentialthat has been parametrized via ab initio calculations [1,2], we have carried out molecular dy-namics simulations in order to investigate the influences of system size, sample geometry, andstrain rate on the fracture behavior of sodium silicate glasses. In contrast to earlier simulationstudies on fracture, in which the bulk glass sample has often been put directly under stress, wehave applied here uniaxial tension to a glass sample that has stress-free surfaces, i.e. we adopta setup that is close to the one used in experiments on fracture studies. Our results show thatthe used interatomic potential captures reasonably well the brittle fracture on the nanoscale ofsilica glass and the enhanced ductility when sodium oxide is added to the glass network. Wehave found that below a critical strain rate of around 0.5/ns the stress-strain curve remainsbasically unchanged. By investigating the formation, growth and coalescence of cavities in thestrained glass samples we find that in sodium silicate glasses the formation of cavities is beingmore pronounced than in silica glass. The analysis of atomic energy, local stress and strain, andatomic displacements have confirmed the presence and the development of mechanically weakzones in the glass network, resulting in paths along which the cracks advance.A. Carré, S. Ispas, J. Horbach, and W. Kob, ”Developing empirical potentials from ab initiosimulations: The case of amorphous silica,” Comput. Mater. Sci., vol. 124, pp. 323–334, Nov.2016.S. Sundararaman, L. Huang, S. Ispas, and W. Kob, ”New optimization scheme to obtain inter-action potentials for oxide glasses”, to be sumitted (2017)

We propose a new scheme to parametrize effective pair potentials that can be used to simulate oxideglasses. As input data for the optimization we use the radial distribution functions of the liquid andthe vibrational density of state of the glass, both obtained from ab initio simulations, as well asexperimental data on the pressure and/or composition dependence of the density and the elasticmoduli of the glass [1].For the case of silica we find that this new scheme allows to find potentials that are significantlyaccurate than previous ones even if the functional form is the same, thus demonstrating that evensimple two-body potentials can be superior to more complex three-body potentials. We have testedthe new potential by calculating the pressure dependence of the elastic moduli and find a goodagreement with the corresponding experimental data.For binary alkali (lithium, sodium, potassium) silicate glasses, the new potentials allow to reproducethe composition dependence of both density and elastic moduli. Further, we examine thecapabilities of these potentials for studying ternary compositions containing two alkali oxides, andwe find that they are reliable even if they have been developed for binary compositions.Simplicity of the functional form also makes these potentials computationally more efficient thanpotentials with more complex functional forms, and hence more suitable for simulations involvinglarge length and/or time scales scales.[1] S. Sundararaman, L. Huang, S. Ispas, and W. Kob, ”New optimization scheme to obtaininteraction potentials for oxide glasses”, J. Chem. Phys. 148 194504 (2018)

We use computer simulations to probe the thermodynamic and dynamic properties of a glass-former that undergoes an ideal glass-transition because ofthe presence of randomly pinned particles.We find that even deep in the equilibrium glass state the system relaxes to some extent because ofthe presence of localized excitations that allow the systemto access different inherent structures,giving thus rise to a non-trivial contribution to the entropy. By calculating with high accuracy thevibrational part of the entropy, we show that also in the equilibrium glass state thermodynamicsand dynamics give a coherent picture and that glasses shouldnot be seen as a disordered solid inwhich the particles undergo just vibrational motion but instead as a system with a highly nonlinearinternal dynamics.

We propose a new scheme to parametrize effective pair potentials that can be used tosimulate oxide glasses. As input data for the optimization we use the radial distributionfunctions of the liquid and the vibrational density of state of the glass, both obtained fromab initio simulations, as well as experimental data on the pressure and/or composition de-pendence of the density and the elastic moduli of the glass [1].For the case of silica we find that this new scheme allows to find potentials that are signifi-cantly accurate than previous ones even if the functional form is the same, thus demonstratingthat even simple two-body potentials can be superior to more complex three-body poten-tials. We have tested the new potential by calculating the pressure dependence of the elasticmoduli and find a good agreement with the corresponding experimental data.For binary alkali (lithium, sodium, potassium) silicate glasses, the new potentials allowto reproduce the composition dependence of both density and elastic moduli. Further, weexamine the capabilities of these potentials for studying ternary compositions containing twoalkali oxides, and we find that they are reliable even if they have been developed for binarycompositions.Simplicity of the functional form also makes these potentials computationally more efficientthan potentials with more complex functional forms, and hence more suitable for simulationsinvolving large length and/or time scales scales.S. Sundararaman, L. Huang, S. Ispas, and W. Kob, ”New optimization scheme to obtaininteraction potentials for oxide glasses”, submitted (2018)

Upon mechanical loading, granular materials yield and undergo plastic deformation. The nature of plastic deformation is essential for the development of the macroscopic constitutive models and the understanding of shear band formation. However, we still do not fully understand the microscopic nature of plastic deformation in disordered granular materials. Here we used synchrotron X-ray tomography technique to track the structural evolutions of three-dimensional granular materials under shear. We establish that highly distorted coplanar tetrahedra are the structural defects responsible for microscopic plasticity in disordered granular packings. The elementary plastic events occur through flip events which correspond to a neighbor switching process among these coplanar tetrahedra (or equivalently as the rotation motion of 4-ring disclinations). These events are discrete in space and possess specific orientations with the principal stress direction.