On the influence of the intermolecular potential on the wetting properties of water on silica surfaces

We study the wetting properties of water on silica surfaces using molecular dynamics (MD) simulations. To describe the intermolecular interaction between water and silica atoms, two types of interaction potential models are used: the standard Br\'odka and Zerda (BZ) model, and the Gulmen and Thompson (GT) model. We perform an in-depth analysis of the influence of the choice of the potential on the arrangement of the water molecules in partially filled pores and on top of silica slabs. We find that at moderate pore filling ratios, the GT silica surface is completely wetted by water molecules, which agrees well with experimental findings, while the commonly used BZ surface is less hydrophilic and is only partially wetted. We interpret our simulation results using an analytical calculation of the phase diagram of water in partially filled pores. Moreover, an evaluation of the contact angle of the water droplet on top of the silica slab reveals that the interaction becomes more hydrophilic with increasing slab thickness and saturates around 2.5-3 nm, in agreement with the experimentally found value. Our analysis also shows that the hydroaffinity of the surface is mainly determined by the electrostatic interaction, but that the van der Waals interaction nevertheless is strong enough that it can turn a hydrophobic surface into a hydrophilic surface.Comment: Article: 9 pages, 7 Figures. There is also a supplementary information file: 2 pages, 3 Figure

Why Do Computers Depreciate?

The value of installed computers falls rapidly and therefore computers have a very high user cost. The paper provides a complete account of the non-financial user cost of personal computers -- decomposing it into replacement cost change, obsolescence, instantaneous depreciation, and age-related depreciation. The paper uses data on the resale price of computers and a hedonic price index for new computers to achieve this decomposition. Once obsolescence is taken into account, age-related depreciation -- which is often identified as deterioration -- is estimated to be negligible. While the majority of the loss in value of used computers comes from declines in replacement cost, this paper shows the second most important source of decline in value is obsolescence. Obsolescence is accelerated by the decline in replacement cost of computers. Cheaper computing power drives developments in software and networks that make older computers less productive even though their original functionality remains intact.

Signatures of topological branched covers

Let $X^4$ and $Y^4$ be smooth manifolds and $f: X\to Y$ a branched cover with branching set $B$. Classically, if $B$ is smoothly embedded in $Y$, the signature $\sigma(X)$ can be computed from data about $Y$, $B$ and the local degrees of $f$. When $f$ is an irregular dihedral cover and $B\subset Y$ smoothly embedded away from a cone singularity whose link is $K$, the second author gave a formula for the contribution $\Xi(K)$ to $\sigma(X)$ resulting from the non-smooth point. We extend the above results to the case where $Y$ is a {\it topological} four-manifold and $B$ is locally flat, away from the possible singularity. Owing to the presence of non-locally-flat points on $B$, $X$ in this setting is a stratified pseudomanifold, and we use the Intersection Homology signature of $X$, $\sigma_{IH}(X)$. For any knot $K$ whose determinant is not $\pm 1$, a homotopy ribbon obstruction is derived from $\Xi(K)$, providing a new technique to potentially detect slice knots that are not ribbon

A ROSAT Survey of Contact Binary Stars

Contact binary stars are common variable stars which are all believed to emit relatively large fluxes of x-rays. In this work we combine a large new sample of contact binary stars derived from the ROTSE-I telescope with x-ray data from the ROSAT All-Sky Survey (RASS) to estimate the x-ray volume emissivity of contact binary stars in the galaxy. We obtained x-ray fluxes for 140 contact binaries from the RASS, as well as 2 additional stars observed by the XMM-Newton observatory. From these data we confirm the emission of x-rays from all contact binary systems, with typical luminosities of approximately 1.0 x 10^30 erg s^-1. Combining calculated luminosities with an estimated contact binary space density, we find that contact binaries do not have strong enough x-ray emission to account for a significant portion of the galactic x-ray background.Comment: 19 pages, 5 figures, accepted by A

The Sustainable Development Goal on Water and Sanitation

Target 7c of the Millennium Development Goals (MDG 7c) aimed to halve the population that had no sustainable access to water and basic sanitation before 2015. According to the data collected by the Joint Monitoring Programme in charge of measuring progress towards MDG 7c, 2.6 billion people gained access to safe water and 2.3 billion people to basic sanitation. Despite these optimistic figures, many academics have criticised MDG 7c. We provide an overview of this critique by performing a systematic literature review of 61 studies conducted over the MDG implementation period (2002-2015) and shortly after. Our objective is to contribute to the debate on the operationalisation of the Sustainable Development Goal on water and sanitation (SDG 6). The academic debate on MDG 7c mainly focused on the effectiveness of the indicators for safe water and sanitation and on the political dynamics underlying the selection of these indicators. SDG 6 addresses some of the concerns raised on the indicators for safe water and sanitation but fails to acknowledge the politics of indicator setting. We are proposing additional indicators and reflect on the limitations of using only quantitative indicators to measure progress towards SDG 6

Elecxit: The Cost of Bilaterally Uncoupling British-EU Electricity Trade

The UK’s withdrawal from the European Union could mean that it leaves the EU Single Market for electricity (Elecxit). This paper develops methods to study the longer-term consequences of this electricity market disintegration, and in particular the end of market coupling. Before European electricity markets were coupled, different market closing times forced traders to commit to cross-border trading volumes based on anticipated market prices. Interconnector capacity was often under-used, and power sometimes flowed from high- to low-price areas. A model of these market frictions is developed, empirically verified on 2009 data (before market coupling) and applied to estimate the costs of market uncoupling in 2030. A less efficient market and the abandonment of some planned interconnectors would raise generation costs by €560m a year (1.5%) compared to remaining in the Single Electricity Market. Sixty percent (€300m) of these welfare losses occur in Great Britain

A Catalog of 1022 Bright Contact Binary Stars

In this work we describe a large new sample of contact binary stars extracted in a uniform manner from sky patrol data taken by the ROTSE-I telescope. Extensive ROTSE-I light curve data is combined with J, H, and K band near-infrared data taken from the Two Micron All-Sky Survey (2MASS) to add color information. Contact binaries candidates are selected using the observed period-color relation. Candidates are confirmed by visual examination of the light curves. To enhance the utility of this catalog, we derive a new J-H period-color-luminosity relation and use this to estimate distances for the entire catalog. From these distance estimates we derive an estimated contact binary space density of (1.7 +/- 0.6) x 10^-5 pcs^-3.Comment: 26 pages, 12 figures, accepted for publication in A

Business models for active buildings

Active Buildings that allow users to adjust their demands on the grid to the needs of the energy system could greatly assist the transition to net zero, but will not be widely adopted unless the businesses involved can make money from doing so. We describe the construction, flexibility and information supply chains of activities needed to make these buildings work. Drawing on the results of an expert workshop, we set out four possible business models deserving further investigation. Developers may find it profitable to build or upgrade energy-efficient buildings with the monitoring and control equipment needed to adjust demand and energy storage as required, selling them soon after completion. Aggregators monitor the state of the building and communicate with the energy system to adjust the building’s demand while maintaining comfort levels, in return for suitable payments. Energy service companies may sell energy-as-a-service and own the equipment instead of a consumer who wishes to minimize their upfront costs, and the idea of an active, energy-efficient, building may be attractive to the tenants of the new group of all-inclusive rental companies, and hence to those companies. Our discussion shows that each is an evolution of an existing (successful) business model, but that further work will be needed to evaluate their profitability when applied to Active Buildings