🍩 Database of Original & Non-Theoretical Uses of Topology

(found 2 matches in 0.002777s)

  1. Revealing Key Structural Features Hidden in Liquids and Glasses (2019)

    Hajime Tanaka, Hua Tong, Rui Shi, John Russo
    Abstract A great success of solid state physics comes from the characterization of crystal structures in the reciprocal (wave vector) space. The power of structural characterization in Fourier space originates from the breakdown of translational and rotational symmetries. However, unlike crystals, liquids and amorphous solids possess continuous translational and rotational symmetries on a macroscopic scale, which makes Fourier space analysis much less effective. Lately, several studies have revealed local breakdown of translational and rotational symmetries even for liquids and glasses. Here, we review several mathematical methods used to characterize local structural features of apparently disordered liquids and glasses in real space. We distinguish two types of local ordering in liquids and glasses: energy-driven and entropy-driven. The former, which is favoured energetically by symmetry-selective directional bonding, is responsible for anomalous behaviours commonly observed in water-type liquids such as water, silicon, germanium and silica. The latter, which is often favoured entropically, shows connections with the heterogeneous, slow dynamics found in hard-sphere-like glass-forming liquids. We also discuss the relationship between such local ordering and crystalline structures and its impact on glass-forming ability.

  2. Tuning Cavitation and Crazing in Polymer Nanocomposite Glasses Containing Bimodal Grafted Nanoparticles at the Nanoparticle/Polymer Interface (2019)

    Rui Shi, Hu-Jun Qian, Zhong-Yuan Lu
    Abstract It is widely accepted that adding nanoparticles (NPs) into polymer matrices can dramatically alter the mechanical properties of the material, and that the properties at the NP/polymer interface play a vital role. By performing coarse-grained molecular dynamics simulations, we study the stress–strain behaviour of polymer/NP composites (PNCs) in a glassy state under a triaxial tensile deformation, in which the NPs are well dispersed in the system via bimodal grafting. A ‘HOMO’ system, in which the short grafted chains are chemically identical to the matrix polymer, and a ‘HETERO’ system, in which the short grafted chains interact weakly with the matrix, are investigated. Our simulations demonstrate that the HOMO system behaves very similarly to the pure polymer system, with quick cavitation and a drop in stress after the yielding point, corresponding to a craze deformation process. While in the HETERO system, weak interactions between the short grafts and the matrix polymer induce a low local modulus, therefore, rather homogeneous void formation and consequently a slower cavitation process are observed at the surface of the well dispersed NPs during the tensile deformation. As a result, the depletion effect at the NP surface eventually leads to NP re-assembly at large strains. Moreover, the HETERO system undergoes a shear-deformation-tended tensile process rather than the craze deformation found in the HOMO system. At the same time, the HETERO system is more ductile, with a much slower drop in stress after yielding than the HOMO system. In addition, the homogeneous generation of voids at small strain in the HETERO system can be utilized in the fabrication of polymer films with desirable separation abilities for gases or small molecules. We hope that these simulation results will be helpful for the property regulation of PNC materials containing polymer grafted NPs.