关键词： DNA   elasticity   molecular dynamics   single-stranded break   stiffness   Young modulus  

摘要： Abstract: Early detection and repair of damaged DNA is important for cell functioning and survival. The recently proposed mechanism of intranucleosomal loop formation suggests the relaxation of DNA supercoiling accumulated during transcription through damaged chromatin. The degree of DNA relaxation is affected by the mechanical properties and structure of the double helix. In this work, the consequences from the introduction of a single-stranded break on the mechanical properties of a DNA fragment are studied using molecular dynamics. It is concluded that the introduction of a single-stranded break leads to decreased stiffness and higher elasticity of the damaged DNA molecule as compared to the intact one. This, in turn, may lead to relief in the supercoiling of the defective DNA and to the enzyme arrest. © 2020, Allerton Press, Inc.

关键词： anesthesia   BOLD fMRI   cerebral blood volume   forepaw stimulation   isoflurane   ketamine   vascular reactivity  

摘要： Mouse functional MRI (fMRI) has been of great interest due to the abundance of transgenic models. Due to a mouse's small size, spontaneous breathing has often been used. Because the vascular physiology affecting fMRI might not be controlled normally, its effects on functional responses were investigated with optical intrinsic signal (OIS) imaging and 9.4 T BOLD fMRI. Three conditions were tested in C57BL/6 mice: spontaneous breathing under ketamine and xylazine anesthesia (KX), mechanical ventilation under KX, and mechanical ventilation under isoflurane. Spontaneous breathing under KX induced an average pCO(2) of 83 mmHg, whereas a mechanical ventilation condition achieved a pCO(2) of 37-41 mmHg within a physiological range. The baseline diameter of arterial and venous vessels was only 7%-9% larger with spontaneous breathing than with mechanical ventilation under KX, but it was much smaller than that in normocapnic isoflurane-anesthetized mice. Three major functional studies were performed. First, CBV-weighted OIS and arterial dilations to 4-second forepaw stimulation were rapid and larger at normocapnia than hypercapnia under KX, but very small under isoflurane. Second, CBV-weighted OIS and arterial dilations by vasodilator acetazolamide were measured for investigating vascular reactivity and were larger in the normocapnic condition than in the hypercapnic condition under KX. Third, evoked OIS and BOLD fMRI responses in the contralateral mouse somatosensory cortex to 20-second forepaw stimulation were faster and larger in the mechanical ventilation than spontaneous breathing. BOLD fMRI peaked at the end of the 20-second stimulation under hypercapnic spontaneous breathing, and at 9 seconds under mechanical ventilation. The peak amplitude of BOLD fMRI was 2.2% at hypercapnia and 3.4% at normocapnia. Overall, spontaneous breathing induces sluggish reduced hemodynamic and fMRI responses, but it is still viable for KX anesthesia due to its simplicity, noninvasiveness, a

关键词： Selective laser melting   Aluminum alloy   Scandium   Precipitation kinetics   In situ small angle X-ray scattering  

摘要： The dynamic metallurgical characteristics of the selective laser melting (SLM) process offer fabricated materials with non-equilibrium microstructures compared to their cast and wrought counterparts. To date, few studies on the precipitation kinetics of SLM processed heat-treatable alloys have been reported, despite the importance of obtaining such detailed knowledge for optimizing the mechanical properties. In this study, for the first time, the precipitation behavior of an SLM fabricated Al-Mn-Sc alloy was systematically investigated over the temperature range of 300-450 degrees C. The combination of in-situ synchrotron-based ultra-small angle X-ray scattering (USAXS), small angle X-ray scattering (SAXS) and X-ray diffraction (XRD) revealed the continuous evolution of Al6Mn and Al3Sc precipitates upon isothermal heating in both precipitate structure and morphology, which was confirmed by ex-situ transmission electron microscopy (TEM) studies. A pseudo-delay nucleation and growth phenomenon of the Al3Sc precipitates was observed for the SLM fabricated Al-Mn-Sc alloy. This phenomenon was attributed to the pre-formed Sc clusters in the as-fabricated condition due to the intrinsic heat treatment effect induced by the unique layer-by-layer building nature of SLM. The growth kinetics for the Al6Mn and Al3Sc precipitates were established based on the in-situ X-ray studies, with the respective activation energies determined to be (74 +/- 4) kJ/mol and (63 +/- 9) kJ/mol. The role of the precipitate evolution on the final mechanical properties was evaluated by tensile testing, and an observed discontinuous yielding phenomenon was effectively alleviated with increased aging temperatures. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

关键词： profiling   parametric designing   power loss   turbine blades   axial-flow turbomachinery  

摘要： A parametric method for designing the cross sections of axial turbine machinery blade airfoils is described. Analytical Bezier curves are used for representing the profile generatrices and the skeleton line. Their shapes are uniquely determined by a set of 13 parameters that are related to the turbine cascade's aerodynamic and strength characteristics. These parameters determine the main points through which the profile generatrices must pass at the specified tangent line angles. In general terms, the blade shape's design calculation involves the following sequential stages: calculating the geometrical constraints based on 13 parameters, determining the skeleton line shape, and calculating the profile suction and pressure sides. By using the proposed method, it is possible to control the blade shape taking into account the imposed geometrical conditions, including the surface curvature distributions. A distinguishing feature of the proposed method is its adaptability: the degree of Bezier curves is determined during calculation by means of multidimensional minimization. This feature imparts flexibility to the method in terms of the possibility to specify the parameters in wide ranges of their values, thus making the method versatile in nature. An algorithm for iteratively determining the curve shape through calculating the skeleton line is presented. The proposed approach is supplemented by a procedure for automatically parameterizing the profiles specified as unevenly distributed pointwise coordinates of their generatrices. This opens up the possibility to represent the already existing blade shapes in the form of 13 parameters and to optimize them.

关键词： Phytoplankton  

摘要： Mechanistic phytoplankton functional group (PFG) models are used to develop water quality targets designed to mitigate cyanobacteria blooms, but it remains unclear whether PFG models adequately simulate cyanobacteria dynamics as most are evaluated against observations of chlorophyll-alpha instead of PFG biomass. To address this challenge, we analyzed an application of CE-QUAL-ICM, a 3D mechanistic PFG model used by water managers and modelers. Global Sensitivity Analysis was employed to assess the sensitivity of modeled chlorophyll-alpha, cyanobacteria biomass, and eukaryotic phytoplankton biomass to 42 uncertain input factors in CE-QUAL-ICM's PFG growth and loss functions. Results revealed that parameterization of CE-QUAL-ICM captured bloom variation but underpredicted bloom peaks, and simulated chlorophyll-a with greater skill than PFG biomass. Additionally, when run across realistic ranges of PFG parameter values, model outputs were highly sensitive to chlorophyll-to-carbon ratios and phosphorus uptake parameters, indicating that these factors should be the focus of targeted parameterization efforts.

关键词： 2D nanomaterials   C3N   Structural defects   Molecular dynamics  

摘要： Remarkable properties that free-standing 2D nanomaterials offer have allured academic and industrial interest in them beyond graphene. One of these materials is cyanoethynyl, which is a 2D monocrystalline carbon nitride material with a C3N stoichiometry. Investigations have confirmed its high thermal stability at elevated temperatures and ferromagnetism at low temperatures, yet very little is known about its mechanical properties. This paper reports a molecular dynamics (MD) insight into this gap. The simulations are validated against density functional theory calculations. The tensile properties of defect-free C3N sheets at/under different temperatures and strain rates are investigated. The influences of single vacancy (SV), Stone-Waals (SW), and clustered vacancy (CV) defects on these properties are also studied. The findings are compared against those reported for graphene and hexagonal boron nitride (hBN). The results show that the strength of C3N decreases with increasing temperature and/or decreasing the strain rate. They also indicate that CV defects deteriorate the mechanical properties of C3N to a greater degree than SV and SW defects. At the same defect concentration, C3N shows a lower sensitivity in its stress and strain at failure to the defects compared with graphene and hBN. For example, 3% SV can reduce the failure stress of graphene and hBN by 75% and 73%, respectively, while this reduction is 54% for C3N. For the failure strain, these reductions are 72%, 63%, and 48% for graphene, hBN, and C3N, respectively.

关键词： Cerebral aneurysm   haemodynamics   computational fluid dynamics   quasi-mechanistic model   Krieger model  

摘要： Cerebral aneurysm is a fatal neurovascular disorder. Computational fluid dynamics simulation of aneurysm haemodynamics is one of the most important research tools which provide increasing potential for clinical applications. However, computational fluid dynamics modelling of such delicate neurovascular disorder involves physical complexities that cannot be easily simplified. Recently, it was shown that the Newtonian simplification used to close the shear stress tensor of the Navier-Stokes equation is not sufficient to explore aneurysm haemodynamics. This article explores the differences between the latter simplification, non-Newtonian power-law model and a newly proposed quasi-mechanistic model. The modified Krieger model, which treats blood as a suspension of plasma and particles, was implemented in computational fluid dynamics context here for the first time and is made available to the readers in a C# code in the supplementary material of this article. Two middle-cerebral artery and two anterior-communicating artery aneurysms, all ruptured, were utilized here as case studies. It was shown that the modified Krieger model had higher sensitivity for wall shear stress calculations in comparison with the other two models. The modified Krieger model yielded lower wall shear stress values consistently in comparison with the other two models. Moreover, the modified Krieger model has generally predicted higher pressure in the aneurysm models. Based on published aneurysm rupture studies, it is believed that ruptured aneurysms are usually correlated with lower wall shear stress values than unruptured ones. Therefore, this work concludes that the modified Krieger model is a potential candidate for providing better clinical relevance to aneurysm computational fluid dynamics simulations.

关键词： Cemented paste backfill   Hydration kinetics   Mixing   Chemical composition   Mechanical property  

摘要： Mixing is a crucial step in the application of any cement-based material. To have a clear understanding of the effect of mixing on cemented paste backfill (CPB) properties is necessary for the preparation of high-quality CPB. This paper researches the effect of mixing time on fresh and hardened properties of CPB, such as hydration kinetics, mechanical property, micro-structural characterization, and ion elution by using inductively coupled plasma mass spectrometry (ICP). The results show that the hydration kinetics and mechanical property of the CPB are sensitive to the mixing time. The mixing time mainly affects the acceleration period, and an improved shearing condition is beneficial to the improvement of the mechanical strength of CPB, especially the early strength, while excessive mixing causes deterioration to mechanical property. These observations can be explained by changes in physical and chemical composition of fresh state CPB due to different mixing programs. (C) 2020 Elsevier Ltd. All rights reserved.

关键词： Dynamics  

摘要： A general and rigorous quantum method is proposed for studying capture dynamics between two diatomic molecules in full dimensionality. By solving the time-independent Schrodinger equation with proper boundary conditions, this method is ideally suited for studying quantum dynamics of cold and ultracold reactions. To illustrate its applicability, the capture dynamics between ultracold KRb molecules is characterized in full six dimensions for the first time using a first-principles based long-range interaction potential. The calculated capture rates for collisions involving distinguishable and indistinguishable (KRb)-K-40-Rb-87 molecules are in good agreement with the experiment and exhibit clear Wigner threshold behaviors. Predictions for ultracold collisions between internally excited (KRb)-K-40-Rb-87 suggest minor changes in the loss rate, consistent with experimental observations in similar systems.

关键词： Zr-based bulk metallic glasses   Glass-forming ability   Crystallization kinetics   Mechanical properties  

摘要： In present study, the effects of the substitution of Ag for Fe on the glass-forming ability (GFA), crystallization kinetics, and mechanical properties of Zr60Cu25Al10Fe5-xAgx (x = 0, 1, 2, 3, 4, and 5 at%) bulk metallic glasses (BMGs) were investigated. Under the same experimental conditions, the critical diameter for glass formation increased significantly by appropriately substituting Ag for Fe. The activation energies, including E-g, E-x, and E-p, all increased significantly, which indicated that Ag addition reduced the sensitivity of the amorphous alloys to temperature, as well as increased the energy barriers of atom rearrangements, nucleation and growth, i.e., the stability of the BMG enhanced. In addition, the plasticity of the alloys was enhanced thanks to the substitution. The plastic strain reached 3.7% when the content of Ag was 4 at%, which is nearly 10 times higher than the original composition (0.4%). (C) 2020 Elsevier B.V. All rights reserved.