Cooling-Rate Effects in Sodium Silicate Glasses: Bridging the Gap between Molecular Dynamics Simulations and Experiments

Although molecular dynamics (MD) simulations are commonly used to predict the structure and properties of glasses, they are intrinsically limited to short time scales, necessitating the use of fast cooling rates. It is therefore challenging to compare results from MD simulations to experimental results for glasses cooled on typical laboratory time scales. Based on MD simulations of a sodium silicate glass with varying cooling rate (from 0.01 to 100 K/ps), here we show that thermal history primarily affects the medium-range order structure, while the short-range order is largely unaffected over the range of cooling rates simulated. This results in a decoupling between the enthalpy and volume relaxation functions, where the enthalpy quickly plateaus as the cooling rate decreases, whereas density exhibits a slower relaxation. Finally, we demonstrate that the outcomes of MD simulations can be meaningfully compared to experimental values if properly extrapolated to slower cooling rates

Investigating the Driving Force of Glass Relaxation for Flexible and Over-Constrained Sodium Silicate Glasses by Molecular Dynamics Simulations

Topological constraint theory classifies network glasses into three categories, viz., flexible, isostatic, and stressed–rigid, where flexible glasses comprise fewer independent constraints than atomic degrees of freedom and stressed–rigid glasses have more topological constraints than atomic degrees of freedom. For flexible glasses, based on MD simulations of a sodium silicate glass with varying cooling rate (from 0.001 to 100 K/ps), we show that thermal history primarily affects the medium-range order structure, while the short-range order is largely unaffected over the range of cooling rates simulated. This results in a decoupling between the enthalpy and volume relaxation functions, where the enthalpy quickly plateaus as the cooling rate decreases, whereas density exhibits a slower relaxation. We also show that relaxation occurs through the transformation of small silicate rings into larger ones. We demonstrate that this mechanism is driven by the fact that small rings (< 6-membered) are topologically over-constrained and experience some internal stress. At the atomic level, such stress manifests itself by a competition between radial and angular constraints, wherein the weaker bond-bending constraints yield to the stronger bond-stretching ones. For over-constrained glasses, they are expected to exhibit some internal stress due to the competition among the redundant constraints. However, the nature and magnitude of this internal stress remain poorly characterized. Here, based on molecular dynamics simulations of a stressed–rigid sodium silicate glass, we present a new technique allowing us to directly compute the internal stress present within a glass network. We show that the internal stress comprises two main contributions: (i) a residual entropic stress that depends on the cooling rate and (ii) an intrinsic topological stress resulting from the over-constrained nature of the glass. Overall, these results provide a microscopic picture for the structural instability of over-constrained glasses

Investigating the Driving Force of Glass Relaxation for Flexible and Over-Constrained Sodium Silicate Glasses by Molecular Dynamics Simulations

Topological constraint theory classifies network glasses into three categories, viz., flexible, isostatic, and stressed–rigid, where flexible glasses comprise fewer independent constraints than atomic degrees of freedom and stressed–rigid glasses have more topological constraints than atomic degrees of freedom. For flexible glasses, based on MD simulations of a sodium silicate glass with varying cooling rate (from 0.001 to 100 K/ps), we show that thermal history primarily affects the medium-range order structure, while the short-range order is largely unaffected over the range of cooling rates simulated. This results in a decoupling between the enthalpy and volume relaxation functions, where the enthalpy quickly plateaus as the cooling rate decreases, whereas density exhibits a slower relaxation. We also show that relaxation occurs through the transformation of small silicate rings into larger ones. We demonstrate that this mechanism is driven by the fact that small rings (< 6-membered) are topologically over-constrained and experience some internal stress. At the atomic level, such stress manifests itself by a competition between radial and angular constraints, wherein the weaker bond-bending constraints yield to the stronger bond-stretching ones. For over-constrained glasses, they are expected to exhibit some internal stress due to the competition among the redundant constraints. However, the nature and magnitude of this internal stress remain poorly characterized. Here, based on molecular dynamics simulations of a stressed–rigid sodium silicate glass, we present a new technique allowing us to directly compute the internal stress present within a glass network. We show that the internal stress comprises two main contributions: (i) a residual entropic stress that depends on the cooling rate and (ii) an intrinsic topological stress resulting from the over-constrained nature of the glass. Overall, these results provide a microscopic picture for the structural instability of over-constrained glasses

virtual output queues

Performance analysis of large multicast switches with multicas

Interaction Research on the Antiviral Molecule Dufulin Targeting on Southern Rice Black Streaked Dwarf Virus P9-1 Nonstructural Protein

ern rice black streaked dwarf virus (SRBSDV) causes severe harm to rice production. Unfortunately, studies on effective antiviral drugs against SRBSDV and interaction mechanism of antiviral molecule targeting on SRBSDV have not been reported. This study found dufulin (DFL), an ideal anti-SRBSDV molecule, and investigated the interactions of DFL targeting on the nonstructural protein P9-1. The biological sequence information and bonding characterization of DFL to four kinds of P9-1 protein were described with fluorescence titration (FT) and microscale thermophoresis (MST) assays. The sequence analysis indicated that P9-1 had highly-conserved C- and N-terminal amino acid residues and a hypervariable region that differed from 131 aa to 160 aa. Consequently, wild-type (WT-His-P9-1), 23 C-terminal residues truncated (TR-ΔC23-His-P9-1), 6 N-terminal residues truncated (TR-ΔN6-His-P9-1), and Ser138 site-directed (MU-138-His-P9-1) mutant proteins were expressed. The FT and MST assay results indicated that DFL bounded to WT-His-P9-1 with micromole affinity and the 23 C-terminal amino acids were the potential targeting site. This system, which combines a complete sequence analysis, mutant protein expression, and binding action evaluating system, could further advance the understanding of the interaction abilities between antiviral drugs and their targets

Algal uptake of hydrophilic and hydrophobic dissolved organic nitrogen in the eutrophic lakes

Dissolved organic nitrogen (DON) derived from sediments plays an active role in biogeochemical cycling of nutrients in aquatic ecosystems. Sediments from four eutrophic lakes were studied using three-dimensional fluorescence excitation-emission matrix (3DEEM) spectra and supelite XAD-8 macro porous resin separation to investigate the bioavailability of hydrophilic and hydrophobic DON to algae (Microcystis flos-aquae (Wittr.) Kirchner). The results showed that the average loss of DON was <6.0% after dividing DON into hydrophilic and hydrophobic components, demonstrating the utility of XAD-8 resin separation in the study of DON components from lake sediments. The 3DEEM analysis showed that hydrophobic and hydrophilic DON comprised humic- and protein-like materials, respectively. During the incubation period, the bioavailability of hydrophilic DON, which accounted for 59.3%-80.4% of total DON, stimulated algal growth, suggesting that hydrophilic DON was the primary source of organic nitrogen for algae. In contrast, hydrophobic DON increased algal density by only 31.8% of that observed for hydrophilic DON, and had a small (accounted for 20.0%-26.6% of total DON) effect on algal growth over the short-term. The significant differences in algal growth between the two types of DON suggested that they should be considered separately in the eutrophic lake restorations. (C) 2018 Elsevier Ltd. All rights reserved

Temporal variation in zooplankton and phytoplankton community species composition and the affecting factors in Lake Taihu-a large freshwater lake in China

Monitoring diverse components of aquatic ecosystems is vital for elucidation of diversity dynamics and processes, which alter freshwater ecosystems, but such studies are seldom conducted. Phytoplankton and zooplankton are integral components which play indispensable parts in the structure and ecological service function of water bodies. However, few studies were made on how zooplankton and phytoplankton community may respond simultaneously to change of circumstance and their mutual relationship. Therefore, we researched synchronously the phytoplankton communities as well as zooplankton communities based on monthly monitoring data from September 2011 to August 2012 in heavily polluted areas and researched their responses to variation in environmental parameters and their mutual relationship. As indicated by Time-lag analysis (TLA), the long-term dynamics of phytoplankton and zooplankton were undergoing directional variations, what's more, there exists significant seasonal variations of phytoplankton and zooplankton communities as indicated by Non-Metric Multidimensional scaling (NMDS) methods. Also, Redundancy Analysis (RDA) demonstrated that environmental indicators together accounted for 25.6% and 50.1% variance of phytoplankton and zooplankton, respectively, indicating that environmental variations affected significantly on the temporal dynamics of phytoplankton as well as zooplankton communities. What's more, variance partioning suggested that the major environmental factors influencing variation structures of zooplankton communities were water temperature, concentration of nitrogen, revealing the dominating driving mechanism which shaped the communities of zooplankton. It was also found that there was significant synchronization between zooplankton biomass and phytoplankton biomass (expressed as Chl-a concentration), which suggested that zooplankton respond to changes in dynamic structure of phytoplankton community and can initiate a decrease in phytoplankton biomass through grazing in a few months. (C) 2018 Elsevier Ltd. All rights reserved