Energy barriers and cooperative motion at the surface of freestanding glassy polystyrene films.

We investigate the near-surface relaxation of freestanding atactic polystyrene films with molecular dynamics simulations. As in previous coarse-grained simulations, relaxation times for backbone segments and phenyl rings are linked to their bulk relaxation times via a power-law coupling relation. Variation of the coupling exponent with distance from the surface is consistent with depth-dependent activation barriers. We also quantify a reduction in dynamical heterogeneity at the interface, which can be interpreted in the framework of cooperative models for glassy dynamics.

Identifying structural flow defects in disordered solids using machine-learning methods.

We use machine-learning methods on local structure to identify flow defects-or particles susceptible to rearrangement-in jammed and glassy systems. We apply this method successfully to two very different systems: a two-dimensional experimental realization of a granular pillar under compression and a Lennard-Jones glass in both two and three dimensions above and below its glass transition temperature. We also identify characteristics of flow defects that differentiate them from the rest of the sample. Our results show it is possible to discern subtle structural features responsible for heterogeneous dynamics observed across a broad range of disordered materials.

[Reduced availability of potential cornea donors: reasons and suggestions]. / Limitierte Verfügbarkeit von potenziellen Hornhaut-Spendern: Gründe und Verbesserungsvorschläge.

AIM: The purpose of the study was to analyse the number of potential cornea donors in relationship to the number of actual donors. Furthermore, the reasons for the discrepancy between the number of the potential and actual donors were analysed. METHODS: Over a period of 6 months data were collected from 6 hospitals and the institute for forensic medicine of the university hospital. The reasons why only a few actual donors from a big pool of potential donors remain for transplantation were investigated. RESULTS: Circumstances related to the medical staff were causal for refusal in 37 % of the cases. In 23 % the cause for refusal was related to the relatives (23 %). Furthermore, in 16 organisational problems caused a failure in cornea donation. 18 % were excluded due to contraindications. A total rate of 6 % actual donors was achieved. CONCLUSIONS: The final consent rate was only 6 % out of all potential donors. Organisational failure was only 16 % in contrast to 60 % refusal due to causes relating to medical staff and relatives. Therefore, further education of physicians and the public is needed.

Plastic response and correlations in athermally sheared amorphous solids.

The onset of irreversible deformation in low-temperature amorphous solids is due to the accumulation of elementary events, consisting of spatially and temporally localized atomic rearrangements involving only a few tens of atoms. Recently, numerical and experimental work addressed the issue of spatiotemporal correlations between these plastic events. Here, we provide further insight into these correlations by investigating, via molecular dynamics (MD) simulations, the plastic response of a two-dimensional amorphous solid to artificially triggered local shear transformations. We show that while the plastic response is virtually absent in as-quenched configurations, it becomes apparent if a shear strain was previously imposed on the system. Plastic response has a fourfold symmetry, which is characteristic of the shear stress redistribution following the local transformation. At high shear rate we report evidence for a fluctuation-dissipation relation, connecting plastic response and correlation, which seems to break down if lower shear rates are considered.

Yield conditions for deformation of amorphous polymer glasses.

Shear yielding of glassy polymers is usually described in terms of the pressure-dependent Tresca or von Mises yield criteria. We test these criteria against molecular dynamics simulations of deformation in amorphous polymer glasses under triaxial loading conditions that are difficult to realize in experiments. Difficulties and ambiguities in extending several standard definitions of the yield point to triaxial loads are described. Two definitions, the maximum and offset octahedral stresses, are then used to evaluate the yield stress for a wide range of model parameters. In all cases, the onset of shear is consistent with the pressure-modified von Mises criterion, and the pressure coefficient is nearly independent of many parameters. Under triaxial tensile loading, the mode of failure changes to cavitation, and the von Mises criterion no longer applies.

Time-dependent elastic response to a local shear transformation in amorphous solids.

The elastic response of a two-dimensional amorphous solid to induced local shear transformations, which mimic the elementary plastic events occurring in deformed glasses, is investigated via molecular-dynamics simulations. We show that for different spatial realizations of the transformation, despite relative fluctuations of order one, the long-time equilibrium response averages out to the prediction of the Eshelby inclusion problem for a continuum elastic medium. We characterize the effects of the underlying dynamics on the propagation of the elastic signal. A crossover from a propagative transmission in the case of weakly damped dynamics to a diffusive transmission for strong damping is evidenced. In the latter case, the full time-dependent elastic response is in agreement with the theoretical prediction, obtained by solving the diffusion equation for the displacement field in an elastic medium.