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Gas Flow Models of Shale: A Review
No full textConventional flow models based on Darcy's flow physics fail to model shale gas production data accurately. The failure to match field data and laboratory-scale evidence of non-Darcy flow has led researchers to propose various gas-flow models for the shale reservoirs. There is extensive evidence that suggests the size of the pores in shale is microscopic in the range of a few to hundreds of nanometers (also known as nanopores). These small pores are mostly associated with the shale's organic matter portion, resulting in a dual pore system that adds to the gas flow complexity. Unlike Darcy's law, which assumes that a dominant viscous flux determines a rock's permeability, shale's permeability leads to other flow processes besides viscous flow such as gas slippage and Knudsen diffusion. This paper reviews the dominant gas-flow processes in a single nanopore based on theoretical models and molecular dynamics simulations, and Lattice Boltzmann modeling. We extend the review to pore network models used to study the gas permeability of shale
Members of Parliament are minimally accountable for their issue stances (and they know it)
No full textFor incumbents to be accountable for their issue stances, voters must sanction incumbents whose positions are ‘out of step’ with their own. We test the electoral accountability of British legislators for their stance on Brexit. We find that there is very limited issue accountability. Individuals who disagreed with their representative’s stance on Brexit were three percentage points less likely to vote for them. The aggregate consequences of these individual effects are limited. A one-standard deviation increase in the proportion of constituents agreeing with their incumbent’s Brexit stance is associated with an increase of 0.56 percentage points in incumbent vote share. These effects are ~1.5 times larger when the main challenger has a different Brexit stance to the incumbent. A follow-up survey of Members of Parliament (MPs) shows that MPs’ estimates of the effects of congruence are similar in magnitude. Our findings suggest that issue accountability is conditional in nature and limited in magnitude even for an issue such as Brexit which shouldbe maximally amenable to such effects
Social norms and evolutionary tax compliance
No full textThe paper studies tax evasion in an evolutionary setting. In addition to standard variables such as the fine individuals may have to pay if found guilty or the probability of being audited, agents' inclination to engage in tax evasion may also be affected by social interactions. Moreover, expected payoffs may include reputational costs or rewards awarded by society after an individual is audited. The paper shows how (i) social norms may play a very important role in defining the long run evolution of tax evasion and, consequently, that (ii) policymakers should consider reforms that would increase social awareness and information rather than more (financially and politically) expensive traditional auditing instruments; in addition, (iii) fiscal/auditing policies should be carefully tailored to the particular economic and social setting in place in a country.<br/
Sparse Spatial Attention Network for Semantic Segmentation
No full textThe spatial attention mechanism captures long-range dependencies by aggregating global contextual information to each query location, which is beneficial for semantic segmentation. In this paper, we present a sparse spatial attention network (SSANet) to improve the efficiency of the spatial attention mechanism without sacrificing the performance. Specifically, a sparse non-local (SNL) block is proposed to sample a subset of key and value elements for each query element to capture long-range relations adaptively and generate a sparse affinity matrix to aggregate contextual information efficiently. Experimental results show that the proposed approach outperforms other context aggregation methods and achieves state-of-the-art performance on the Cityscapes and the PASCAL Context datasets
The Philosophical Basis of Seeing and Touching Structural Concepts
No full textStructural concepts are fundamentals of civil engineering for students to learn, for lecturers to teach and for engineers to use. Many students however find it difficult to understand structural concepts due to their abstract nature. Seeing and Touching Structural Concepts has been developed as an approach to help civil engineering students gain intuitive (without the need for analysis) understanding of structural concepts taking them from structural elements to whole structures and from theory to practice. The paper explains the philosophical basis of the approach which involves the fundamental ideas and principles underlying the approach and its evolution. This has included the development of over 60 physical models produced in pairs, one with and one without involving a structural concept, to illustrate the effects and the significance of the structural concepts. Over 70 associated cases using structural concepts creatively have been identified from engineering practice. A website, www.structuralconcepts.org, has been developed and two associated books have been published in English and Chinese. Five criteria underpinning the approach of Seeing and Touching Structural Concepts are expressed as Seeking new connections, Exploring new meanings, Evolving into intuitive understanding, Making wide and creative applications and Simplicity (SEEMS), which are demonstrated using examples and synthesised case studies
The predictive ability of the 313-variant-based polygenic risk score for contralateral breast cancer risk prediction in women of European ancestry with a heterozygote BRCA1 or BRCA2 pathogenic variant
No full textPurpose To evaluate the association between a previously published 313-variant-based breast cancer (BC) polygenic risk score (PRS313) and contralateral breast cancer (CBC) risk, in BRCA1 and BRCA2 pathogenic variant heterozygotes. Methods We included women of European ancestry with a prevalent first primary invasive BC (BRCA1=6,591 with 1,402 prevalent CBC cases; BRCA2=4,208 with 647 prevalent CBC cases) from CIMBA, a large international retrospective series. Cox regression analysis was performed to assess the association between overall and ER-specific PRS313 and CBC risk. Results For BRCA1 heterozygotes the estrogen receptor (ER)-negative PRS313 showed the largest association with CBC risk, HR per SD=1.12, 95%CI [1.06-1.18], C-index=0.53; for BRCA2 heterozygotes, this was the ER-positive PRS313, HR=1.15, 95%CI [1.07-1.25], C-index=0.57. Adjusting for family history, age at diagnosis, treatment or pathological characteristics for the first BC did not change association effect sizes. For women developing first BC <age 40 years, the cumulative PRS313 5th and 95th percentile 10-year CBC risks were 22% and 32% for BRCA1 and 13% and 23% for BRCA2 heterozygotes, respectively. Conclusion The PRS313 can be used to refine individual CBC risks for BRCA1/2 heterozygotes of European ancestry, however the PRS313 needs to be considered in the context of a multifactorial risk model to evaluate whether it might influence clinical-decision-making
<sup>23</sup>Na NMR T<sub>1</sub> relaxation measurements as a probe for diffusion and dynamics of sodium ions in salt-glycerol mixtures
No full textMixtures of sodium salts with oxygen-containing molecules are useful from the perspective of applications such as sodium ion batteries but also because they fill the gap between deep eutectic solvents and molten salt hydrates. In a previous work, the physical properties (such as diffusion coefficients, conductivity, viscosity and glass transition temperature) of four salts, namely Na2B4O7.10H2O, NaOAc.3H2O, NaBr, and NaOAc, were measured with glycerol. Pulsed-field gradient (PFG) NMR was also used to measure self-diffusion coefficients of 1H-bearing species. However, the technique was not able to measure diffusion of sodium ions due to the very fast NMR relaxation rate of such species, resulting in loss of PFG NMR signal. In the current work, this study is expanded to using 23Na T1 relaxation measurements, which under certain assumptions, can be translated into diffusion coefficients. Analysis of the physical properties is then correlated with self-diffusion coefficient measurements to elucidate information about structure and ionic mobility. It is shown that NaOAc.3H2O, NaBr and NaOAc fit models for ionic conductivity and diffusion, which are consistent with ionic liquids where charge transport is limited by ionic mobility rather than the number of charge carriers. The waters of hydration of NaOAc.3H2O do not appear to form a separate phase but instead are strongly coordinated to the cation. In contrast, Na2B4O7.10H2O appears to form a water-rich phase with enhanced sodium mobility
Hot-Carrier Cooling in High-Quality Graphene is Intrinsically Limited by Optical Phonons
No full textMany promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand the relaxation dynamics after photoexcitation. These dynamics contain a sub-100 femtosecond thermalization phase, which occurs through carrier-carrier scattering and leads to a carrier distribution with an elevated temperature. This is followed by a picosecond cooling phase, where different phonon systems play a role: graphene acoustic and optical phonons, and substrate phonons. Here, we address the cooling pathway of two technologically relevant systems, both consisting of high-quality graphene with a mobility >10,000 cm2V􀀀1s􀀀1 and environments that do not efficiently take up electronic heat from graphene: WSe2-encapsulated graphene and suspended graphene. We study the cooling dynamics using ultrafast pump-probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to substrate phonons is relatively inefficient in these systems, suggesting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a timescale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the high-energy tail of the hot-carrier distribution emit optical phonons. This creates a permanent heat sink, as carriers efficiently re-thermalize. We develop a macroscopic model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights will guide the development of graphene-based optoelectronic devices
Negative catalysis or non-Bell-Evans-Polanyi reactivity by metalloenzymes: Examples from mononuclear heme and non-heme iron oxygenases
No full textThere are many examples in bioinorganic chemistry, where metalloenzymes produce different reaction products than analogous biomimetic model complexes despite them having the same transition metal and first-coordination sphere environment. As a result, a lot of research has been devoted to the understanding of the effect of the first- and second coordination sphere of catalytic reaction centers. Thermodynamically, catalysis should follow the Bell-Evans-Polanyi principle, where the product with the largest driving force gives the lowest reaction barrier and consequently highest reaction rate and dominant reaction products. However, there are many examples in the literature, where the dominant products of an enzymatic reaction do not correspond to the reaction process with the largest exothermicity. In general, catalysis that follows the Bell-Evans-Polanyi principle is designated positive catalysis, whereas when products are obtained from non-Bell-Evans-Polanyi reactivity, it is defined as negative catalysis. In enzymes; however, the first- and second-coordination sphere determines whether positive or negative catalysis happens but many intricate details on how selectivities are reversed are still unknown. In this review paper, we cover recent advances on enzyme understanding where negative catalysis is dominant over positive catalysis. We show that the enzyme can achieve this feat, e.g., through the positioning of substrate and oxidant that then gives the desired product distributions. In particular, often the substrate is positioned such that the desired group of the substrate is positioned as close as possible to the oxidant, while unwanted activation channels are blocked by electrostatic perturbations or shielded by protein residues. Furthermore, recent work has shown that reactivity patterns in enzymes can also be influenced by long-range electrostatic interactions like an electric dipole field that can weaken or strengthen chemical bonds through long-range polarization effects. As a result of these electric field effects and long-range electrostatic interactions, enzymes can react through negative catalysis, where a thermodynamically less likely process gives the dominant products. In this review paper, we define and describe positive and negative catalysis in enzymes and compare structure and reactivity with biomimetic and homogeneous catalysts for a number of mononuclear iron-type enzymes including the cytochromes P450 and non-heme iron dioxygenases