Non-equilibrium phase behavior and friction of confined molecular films under shear: a non-equilibrium molecular dynamics study

The phase behavior of a confined liquid at high pressure and shear rate, such as is found in elastohydrodynamic lubrication, can influence the traction characteristics in machine operation. Generic aspects of this behavior are investigated here using Non-equilibrium Molecular Dynamics (NEMD) simulations of confined Lennard-Jones (LJ) films under load with a recently proposed wall-driven shearing method without wall atom tethering [C. Gattinoni et al., Phys. Rev. E 90, 043302 (2014)]. The focus is on thick films in which the nonequilibrium phases formed in the confined region impact on the traction properties. The nonequilibrium phase and tribological diagrams are mapped out in detail as a function of load, wall sliding speed, and atomic scale surface roughness, which is shown can have a significant effect. The transition between these phases is typically not sharp as the external conditions are varied. The magnitude of the friction coefficient depends strongly on the nonequilibrium phase adopted by the confined region of molecules, and in general does not follow the classical friction relations between macroscopic bodies, e.g., the frictional force can decrease with increasing load in the Plug-Slip (PS) region of the phase diagram owing to structural changes induced in the confined film. The friction coefficient can be extremely low (∼0.01) in the PS region as a result of incommensurate alignment between a (100) face-centered cubic wall plane and reconstructed (111) layers of the confined region near the wall. It is possible to exploit hysteresis to retain low friction PS states well into the central localization high wall speed region of the phase diagram. Stick-slip behavior due to periodic in-plane melting of layers in the confined region and subsequent annealing is observed at low wall speeds and moderate external loads. At intermediate wall speeds and pressure values (at least) the friction coefficient decreases with increasing well depth of the LJ potential between the wall atoms, but increases when the attractive part of the potential between wall atoms and confined molecules is made larger

Equilibrium fluctuations of liquid state static properties in a subvolume by molecular dynamics

Engineering & Physical Science Research Council (EPSRC) EP/G026114/1; Engineering & Physical Science Research Council (EPSRC) EPSRC Fellowship Smith; Engineering & Physical Science Research Council (EPSRC) EP/N025954/1

A spectroscopic cell for fast pressure jumps across the glass transition line

We present a new experimental protocol for the spectroscopic study of the dynamics of glasses in the aging regime induced by sudden pressure jumps (crunches) across the glass transition line. The sample, initially in the liquid state, is suddenly brought in the glassy state, and therefore out of equilibrium, in a four-window optical crunch cell which is able to perform pressure jumps of 3 kbar in a time interval of ~10 ms. The main advantages of this setup with respect to previous pressure-jump systems is that the pressure jump is induced through a pressure transmitting fluid mechanically coupled to the sample stage through a deformable membrane, thus avoiding any flow of the sample itself in the pressure network and allowing to deal with highly viscous materials. The dynamics of the sample during the aging regime is investigated by Brillouin Light Scattering (BLS). For this purpose the crunch cell is used in conjunction with a high resolution double monochromator equipped with a CCD detector. This system is able to record a full spectrum of a typical glass forming material in a single 1 s shot. As an example we present the study of the evolution toward equilibrium of the infinite frequency longitudinal elastic modulus (M_infinity) of a low molecular weight polymer (Poly(bisphenol A-co-epichlorohydrin), glycidyl end capped). The observed time evolution of M_infinity, well represented by a single stretched exponential, is interpreted within the framework of the Tool-Narayanaswamy theory.Comment: 9 pages, 11 figure

A review of solar driven absorption cooling with photovoltaic thermal systems

The aim of this investigation is to evaluate the recent advances in the field of solar absorption cooling systems from the viewpoint of solar collector types. A review in the area of photovoltaic thermal (PVT) absorption cooling systems is conducted. This review includes experimental and computational work focusing on collector types and their efficiencies and performance indicators. Compared to vapour compression air conditioning systems, 50% of primary energy was saved by using solar absorption cooling systems and 10–35% maximum electrical efficiency of PVT was achieved. This review shows that Coefficient of Performance (COP) for solar cooling systems is in the range of 0.1–0.91 while the thermal collector efficiencies are in the range of 0.06–0.64. The average area to produce cooling for single effect absorption chillers for experimental and computational projects is 4.95 m2/kWc and 5.61 m2/kWc respectively. The specific area for flat plat collector (FPC) is in the range of 2.18–9.4 m2/kWc, while for evacuated tube collector (ETC) is in the range of 1.27–12.5 m2/kWc. For concentrated photovoltaic thermal collector (CPVT) and PVT, the average area to produce cooling for solar absorption chillers are 2.72 m2/kWc and 3.1 m2/kWc respectively

Improving thermal and electrical efficiency in photovoltaic thermal systems for sustainable cooling system integration

Research into photovoltaic thermal systems is important in solar technologies as photovoltaic thermal systems are designed to produce both electrical and thermal energy, this can lead to improved performance of the overall system. The performance of photovoltaic thermal systems is based on several factors that include photovoltaic thermal materials, design, ambient temperature, inlet and outlet fluid temperature and photovoltaic cell temperature. The aim of this study is to investigate the effect of photovoltaic thermal outlet water temperatures and solar cell temperature on both electrical and thermal efficiency for different range of inlet water temperature. To achieve this, a mathematical model of a photovoltaic thermal system was developed to calculate the anticipated system performance. The factors that affect the efficiency of photovoltaic thermal collectors were discussed and the outlet fluid temperature from the photovoltaic thermal is investigated in order to reach the highest overall efficiency for the solar cooling system. An average thermal and electrical efficiency of 65% and 13.7%, respectively, was achieved and the photovoltaic thermal mathematical model was validated with experimental data from literature

Control-volume representation of molecular dynamics

A Molecular Dynamics (MD) parallel to the Control Volume (CV) formulation of fluid mechanics is developed by integrating the formulas of Irving and Kirkwood, J. Chem. Phys. 18, 817 (1950) over a finite cubic volume of molecular dimensions. The Lagrangian molecular system is expressed in terms of an Eulerian CV, which yields an equivalent to Reynolds' Transport Theorem for the discrete system. This approach casts the dynamics of the molecular system into a form that can be readily compared to the continuum equations. The MD equations of motion are reinterpreted in terms of a Lagrangian-to-Control-Volume (\CV) conversion function $\vartheta_{i}$, for each molecule $i$. The \CV function and its spatial derivatives are used to express fluxes and relevant forces across the control surfaces. The relationship between the local pressures computed using the Volume Average (VA, Lutsko, J. Appl. Phys 64, 1152 (1988)) techniques and the Method of Planes (MOP, Todd et al, Phys. Rev. E 52, 1627 (1995)) emerges naturally from the treatment. Numerical experiments using the MD CV method are reported for equilibrium and non-equilibrium (start-up Couette flow) model liquids, which demonstrate the advantages of the formulation. The CV formulation of the MD is shown to be exactly conservative, and is therefore ideally suited to obtain macroscopic properties from a discrete system.Comment: 19 pages, 15 figure

Impact of excess NOx emissions from diesel cars on air quality, public health and eutrophication in Europe

Diesel cars have been emitting four to seven times more NOx in on-road driving than in type approval tests. These ‘excess emissions’ are a consequence of deliberate design of the vehicle’s after-treatment system, as investigations during the ‘Dieselgate’ scandal have revealed. Here we calculate health and environmental impacts of these excess NOx emissions in all European countries for the year 2013. We use national emissions reported officially under the UNECE Convention for Long-range Transport of Atmospheric Pollutants and employ the EMEP MSC-W Chemistry Transport Model and the GAINS Integrated Assessment Model to determine atmospheric concentrations and resulting impacts. We compare with impacts from hypothetical emissions where light duty diesel vehicles are assumed to emit only as much as their respective type approval limit value or as little as petrol cars of the same age. Excess NO2 concentrations can also have direct health impacts, but these overlap with the impacts from particulate matter (PM) and are not included here. We estimate that almost 10 000 premature deaths from PM2.5 and ozone in the adult population (age >30 years) can be attributed to the NOx emissions from diesel cars and light commercial vehicles in EU28 plus Norway and Switzerland in 2013. About 50% of these could have been avoided if diesel limits had been achieved also in on-road driving; and had diesel cars emitted as little NOx as petrol cars, 80% of these premature deaths could have been avoided. Ecosystem eutrophication impacts (critical load exceedances) from the same diesel vehicles would also have been reduced at similar rates as for the health effects

Statistical Analysis and Molecular Dynamics Simulations of the Thermal Conductivity of Lennard–Jones Solids Including Their Pressure and Temperature Dependencies

© 2020 The Authors. Published by Wiley-VCH GmbH Aspects of the thermal conductivity, λ, of a Lennard–Jones (LJ) solid along an isotherm and the sublimation line are studied using equilibrium molecular dynamics (MD) simulations. A reformulation of the Green–Kubo time correlation function expression for λ in the form of a probability distribution function (PDF) of single trajectory contributions (STC) exhibits the same characteristic statistical trends as found previously for liquids, even at high pressures and low temperatures. The analysis reveals that for short periods of time the thermal conductivity can be negative. This feature is evident along the sublimation line isobar and a low-temperature isotherm going to high densities. Along the isobar and isotherm lines, λ is to a good approximation a power law in temperature and density, respectively. This behavior is used in a more general thermodynamics-based analysis description of the state point dependence of the thermal conductivity. The heat flux autocorrelation function increasingly develops a damped oscillatory appearance as pressure increases or temperature decreases, consistent with the phonon formulation of thermal conductivity.Engineering and Physical Sciences Research Council. Grant Number: EP/N025954/

Varieties of labour administration in Europe and the consequences of the Great Recession

This article focuses on national public administration activities that relate to employment, social protection and industrial relations. The International Labour Organization (ILO) refers to these activities collectively as ‘labour administration’ and regards the bodies that conduct them within individual countries as together forming national systems of labour administration. This article explores the concept of ‘national system of labour administration’ and considers the potential contribution of comparative institutional analysis in understanding how national systems are organised and change over time. The article also compares the organisation of national labour administration systems in European Union (EU) countries and analyses how these systems have developed since the start of the economic crisis that erupted in 2008