Short- and medium-range orders in as-quenched and deformed SiO2 glasses: An atomistic study

International audienceUsing a truncated BKS interatomic potential, we compare the coordination number, ring size distribution and distributions of activation energies in silica glasses of different densities processed with two different routes: either directly quenched from the high temperature melt at different densities, or deformed from an ambient temperature, low density glass through cycles of quasistatic compression-decompression. We find that the coordination number and ring size distribution vary simultaneously in both quenched and deformed glasses and are irreversible in compressed glasses unloaded to zero pressure, but are surprisingly close to reversible (with a hysteresis) when the decompressions are continued back to the initial density. At a given density, the pressure is significantly lower in quenched glasses than in compressed glasses but the coordination number and ring size distribution are unexpectedly similar, showing that their evolution during compression is mostly a steric effect. Also, distributions of activation energies in quenched and deformed glasses are significantly different, with the deformed glasses showing the classical overabundance of low activation energies characteristic of far-from-equilibrium systems

Inverse Meyer-Neldel behavior for activated processes in model glasses

International audienceThe activation-relaxation technique is used to explore the distribution of escape times from metastable minima of a model metallic glass, estimated using the harmonic transition state theory. We investigate in particular the distribution of attempt frequencies and find that these prefactors are distributed over a range of almost ten orders of magnitude, in stark contrast with the common assumption that they are given by a typical vibrational frequency. Moreover, the typical attempt frequencies appear to decrease with increasing barrier energy, a behavior inverse to the so-called Meyer-Neldel rule observed in many processes with apparent Arrhenius behavior. When combined with a multiple-hop analysis that takes into account the multiplicity of possible transitions, a direct Meyer-Neldel behavior is recovered, albeit with a reduced characteristic prefacto

Potential Energy Landscape of Glasses: Distributions of Activation Energies.

International audienc

Risk Stratification of COVID-19 Patients Using Ambulatory Oxygen Saturation in the Emergency Department

Introduction: It is difficult to determine illness severity for coronavirus disease 2019 (COVID-19) patients, especially among stable-appearing emergency department (ED) patients. We evaluated patient outcomes among ED patients with a documented ambulatory oxygen saturation measurement.Methods: This was a retrospective chart review of ED patients seen at New York University Langone Health during the peak of the COVID-19 pandemic in New York City. We identified ED patients who had a documented ambulatory oxygen saturation. We studied the outcomes of high oxygen requirement (defined as >4 liters per minute) and mechanical ventilation among admitted patients and bounceback admissions among discharged patients. We also performed logistic regression and compared the performance of different ambulatory oxygen saturation cutoffs in predicting these outcomes.Results: Between March 15–April 14, 2020, 6194 patients presented with fever, cough, or shortness of breath at our EDs. Of these patients, 648 (11%) had a documented ambulatory oxygen saturation, of which 165 (24%) were admitted. Notably, admitted and discharged patients had similar initial vital signs. However, the average ambulatory oxygen saturation among admitted patients was significantly lower at 89% compared to 96% among discharged patients (p<0.01). Among admitted patients with an ambulatory oxygen saturation, 30% had high oxygen requirements and 8% required mechanical ventilation. These rates were predicted by low ambulatory oxygen saturation (p<0.01). Among discharged patients, 50 (10%) had a subsequent ED visit resulting in admission. Although bounceback admissions were predicted by ambulatory oxygen saturation at the first ED visit (p<0.01), our analysis of cutoffs suggested that this association may not be clinically useful.Conclusion: Measuring ambulatory oxygen saturation can help ED clinicians identify patients who may require high levels of oxygen or mechanical ventilation during admission. However, it is less useful for identifying which patients may deteriorate clinically in the days after ED discharge and require subsequent hospitalization.

Off-Lattice Kinetic Monte Carlo Methods

International audienceExact modeling of the dynamics of chemical and material systems over experimentally relevant time scales still eludes us even with modern computational resources. Fortunately, many systems can be described as rare event systems where atoms vibrate around equilibrium positions for a long time before a transition is made to a new atomic state. For those systems, the kinetic Monte Carlo (KMC) algorithm provides a powerful solution. In traditional KMC, mechanism and rates are computed beforehand, limiting moves to discretized positions and largely ignoring strain. Many systems of interest, however, are not well-represented by such lattice-based models. Moreover, materials often evolve with complex and concerted mechanisms that cannot be anticipated before the start of a simulation. In this chapter, we describe a class of algorithms, called offlattice or adaptive KMC, which relaxes both limitations of traditional KMC, with atomic configurations represented in the full configuration space and reaction events are calculated on-the-fly, with the possible use of catalogs to speed up calculations. We discuss a number of implementations of off-lattice KMC developed by different research groups, emphasizing the similarities between the approaches that open modeling to new classes of problems