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Algorithms and methods to couple and link multiple model types are often needed to tackle some of the most challenging problems in materials' simulations. Developing these represents both a complex and fascinating intellectual challenge, and a pressing need to tackle realistic systems in applied and industrial research. We summarize here the conclusions of the meeting, where we have identified the areas where coupling- and linking efforts would have the greatest impact, and which should be the policy recommendations to make to the Commission to facilitate this impact. This expert meeting, convened by the European Materials Modelling Council and hosted by CECAM, gathered some of the foremost experts across different domain areas, with the goal of identifying a roadmap for coupling and linking simulations that can act as a manifesto for the field, and inform future policy makers of the algorithmic and methodological developments needed and of their relevance and role in accelerating simulation-driven innovation., Please download and cite Version 2 as this comprises the complete list of co-authors.

Algorithms and methods to couple and link multiple model types are often needed to tackle some of the most challenging problems in materials' simulations. Developing these represents both a complex and fascinating intellectual challenge, and a pressing need to tackle realistic systems in applied and industrial research. We summarize here the conclusions of the meeting, where we have identified the areas where coupling- and linking efforts would have the greatest impact, and which should be the policy recommendations to make to the Commission to facilitate this impact. This expert meeting, convened by the European Materials Modelling Council and hosted by CECAM, gathered some of the foremost experts across different domain areas, with the goal of identifying a roadmap for coupling and linking simulations that can act as a manifesto for the field, and inform future policy makers of the algorithmic and methodological developments needed and of their relevance and role in accelerating simulation-driven innovation.

Water purification membranes comprising aligned, dense arrays of carbon nanotubes (CNTs) have been investigated for more than 10 years. Water transport 2–5 orders of magnitude greater than Hagen-Poiseuille predictions has been observed in CNTs of diameters 0.8–10 nm in a small number of experiments. While the measured flow rates in different experiments substantially disagree with each other, there is a clear opportunity for these membranes to impact filtration technologies. We propose a multiscale computational flow method that combines molecular dynamics (MD) simulations in critical locations of the membrane with a continuum flow resistance model. This provides the flow resistances in a nanotube membrane configuration to enable, for the first time, computationally-efficient macroscopic predictions of flows through laboratory-scale membranes. Our multiscale simulation results of water flow through CNTs are also used to calibrate the Hagen–Poiseuille–Weissberg equation with slip. This study reveals that the slip length, density and viscosity can vary with CNT diameter at sub-2-nm diameters, which would otherwise be challenging to compute using MD alone. Previously published experimental results show either clear agreement or clear disagreement with our multiscale predictions; more work is required to understand this variance for similar flow cases.

Bubble cavitation is important in technologies such as non-invasive cancer treatment and diagnosis, surface cleaning, and waste-water treatment. The cavitation threshold is the critical external tensile pressure that induces unstable growth of the bubble. Surface nanobubbles have been previously shown experimentally to be stable down to -6MPa, in disagreement with the Blake threshold, which is the classical cavitation model that predicts bulk bubbles with radii ~100nm should be unstable below -0:6MPa. Here we use Molecular Dynamics (MD) to simulate quasi-2D and 3D nitrogen surface nanobubbles immersed in water, subject to a range of pressure drops until unstable growth is observed. We propose and assess new cavitation threshold models, derived from mechanical equilibrium analyses for both the quasi-2D and 3D cavitating bubbles. The discrepancies from the Blake threshold are attributed to the pinned contact line, within which the surface nanobubbles grow with constant lateral contact diameter, and consequently a reduced radius of curvature. We conclude with discussion of previous experimental results on the cavitation of relatively large surface nanobubbles.

We report the results of molecular dynamics investigations into the behaviour of nanoscale water droplets on surfaces subjected to cyclic-frequency normal vibration. Our results show, for the first time, a range of vibration-induced phenomena, including the existence of the following different regimes: evaporation, droplet oscillation, and droplet lift-off. We also describe the effect of different surface wettabilities on evaporation. The outcomes of this work can be utilised in the design of future nano-engineered technologies that employ surface/bulk acoustic waves, such as water-based cooling systems for high-heat-generating processor chips, by tuning the vibration frequency and amplitude, as well as the surface wettability, to obtain the desired performance.

Introduction: The literature describes the high patient satisfaction rate after breast reduction. In this retrospective study, we use the BREAST-Q to analyse satisfaction with breast appearance and physical, psychosocial and sexual well-being of patients who underwent bilateral breast reduction (BBR) at Mater Dei Hospital (MDH). We also looked into whether age, co-morbidities and weight of breast tissue removed makes a difference to the overall satisfaction rate. Method: Permission to use the BREAST-Q questionnaire and translate it into Maltese was obtained from Mapi Reasearch Trust. The questionnaire was offered either in Maltese or in English, after an official translation was produced following a linguistic validation process. All patients who underwent BBR at MDH under the care of both consultant Plastic Surgeons were invited to complete the BREAST-Q questionnaire via a telephone call and asked to come to MDH to fill it in. Other patient specific information was obtained from their hospital notes. Results: We hope to demonstrate a better quality of life following surgery and aim to compare the results of this study to others carried out worldwide. In this way we can better understand the local situation and see where there is the room for improvement. Conclusion: In this world of evidence-based medicine, the BREAST-Q is ideal for a holistic approach in analysing patient satisfaction after BBR. Having local data at hand makes it easier for patients who are interested in undergoing the surgery to associate themselves with other local individuals., peer-reviewed

We report nonequilibrium molecular simulations of the vibration-induced heating of nanoscale-thick water layers on a metal substrate. In addition to experimentally confirmed acoustothermal evaporation, we observe hitherto unmapped nucleate and film boiling regimes, accompanied by the generation of unprecedented heat fluxes [∼O(109) W/m2]. We develop a universal scaling parameter to classify the heat-transfer regimes and to predict the thickness of the residual nonevaporating liquid layer. The results find broad application to systems involving drying, coatings, and sprays.

dsmcFoam+ is a direct simulation Monte Carlo (DSMC) solver for rarefied gas dynamics, implemented within the OpenFOAM software framework, and parallelised with MPI. It is open-source and released under the GNU General Public License in a publicly available software repository that includes detailed documentation and tutorial DSMC gas flow cases. This release of the code includes many features not found in standard dsmcFoam, such as molecular vibrational and electronic energy modes, chemical reactions, and subsonic pressure boundary conditions. Since dsmcFoam+ is designed entirely within OpenFOAM’s C++ object-oriented framework, it benefits from a number of key features: the code emphasises extensibility and flexibility so it is aimed first and foremost as a research tool for DSMC, allowing new models and test cases to be developed and tested rapidly. All DSMC cases are as straightforward as setting up any standard OpenFOAM case, as dsmcFoam+ relies upon the standard OpenFOAM dictionary based directory structure. This ensures that useful pre- and post-processing capabilities provided by OpenFOAM remain available even though the fully Lagrangian nature of a DSMC simulation is not typical of most OpenFOAM applications. We show that dsmcFoam+ compares well to other well-known DSMC codes and to analytical solutions in terms of benchmark results.

Although the modified Young’s equation is frequently applied to evaluate the line tension of droplets, debate concerning the value and even the sign of the line tension is ongoing. The reason for this is that the line tension defined in the modified Young’s equation is not a pure line tension but an apparent line tension, which includes the effects of the Tolman length and the stiffness coefficients. In this paper, we employ molecular dynamics (MD) to simulate three-dimensional water nanodroplets on platinum surfaces and determine their apparent line tensions by applying a linear fit to the relation of the cosine of the contact angle to the curvature of the contact line. The effects of the Tolman length and the position of the solid-liquid dividing interface on the measured line tension are investigated. On the one hand, our results elucidate the reason why MD results for line tensions are so scattered and also lend numerical support to Schimmele et al.’s theoretical predictions [“Conceptual aspects of line tensions,” J. Chem. Phys. 127, 164715 (2007)]. On the other hand, our MD simulation results demonstrate that the modified Young’s equation is a useful tool to predict the macroscopic contact angle based on a linear fit of the measured contact angles at the nanoscale. The apparent line tension is, however, sensitive to the chosen position of the solid-liquid dividing interface.

This paper introduces mdFoam+, which is an MPI parallelised molecular dynamics (MD) solver implemented entirely within the OpenFOAM software framework. It is open-source and released under the same GNU General Public License (GPL) as OpenFOAM. The source code is released as a publicly open software repository that includes detailed documentation and tutorial cases. Since mdFoam+ is designed entirely within the OpenFOAM C++ object-oriented framework, it inherits a number of key features. The code is designed for extensibility and flexibility, so it is aimed first and foremost as an MD research tool, in which new models and test cases can be developed and tested rapidly. Implementing mdFoam+ in OpenFOAM also enables easier development of hybrid methods that couple MD with continuum-based solvers. Setting up MD cases follows the standard OpenFOAM format, as mdFoam+ also relies upon the OpenFOAM dictionary-based directory structure. This ensures that useful pre- and post-processing capabilities provided by OpenFOAM remain available even though the fully Lagrangian nature of an MD simulation is not typical of most OpenFOAM applications. Results show that mdFoam+ compares well to another well-known MD code (e.g. LAMMPS) in terms of benchmark problems, although it also has additional functionality that does not exist in other open-source MD codes.

Nanotubes (NTs) with diameters less than 2 nm have been proposed for next-generation reverse osmosis membranes. At this molecular scale, the NTs are narrow enough to block salt ions and other contaminants, but still wide enough to allow water to flow along the NTs at seemingly unprecedented rates. Simulations for design of NT membranes can be challenging. On the one hand, the standard equations for water flow through pipes are not applicable at sub-2-nm scales due to the dominance of non-continuum phenomena; on the other hand, full molecular simulations are computationally intractable for flows up to laboratory or prototype scales. This article describes recent multiscale approaches to simulating flows through aligned NT membranes of various materials. These multiscale techniques offer a unique and economical solution that can shed light on sometimes conflicting experimental results and point the way to future engineering design of nanostructured membranes.

Filtering of particle-based simulation data can lead to reduced computational costs and enable more efficient information transfer in multi-scale modelling. This paper compares the effectiveness of various signal processing methods to reduce numerical noise and capture the structures of nano-flow systems. In addition, a novel combination of these algorithms is introduced, showing the potential of hybrid strategies to improve further the de-noising performance for time-dependent measurements. The methods were tested on velocity and density fields, obtained from simulations performed with molecular dynamics and dissipative particle dynamics. Comparisons between the algorithms are given in terms of performance, quality of the results and sensitivity to the choice of input parameters. The results provide useful insights on strategies for the analysis of particle-based data and the reduction of computational costs in obtaining ensemble solutions.

An open source implementation of chemistry modelling for the direct simulationMonte Carlo (DSMC) method is presented. Following the recent work of Bird [1] an approach known as the quantum kinetic (Q-K) method has been adopted to describe chemical reactions in a 5-species air model using DSMC procedures based on microscopic gas information. The Q-K technique has been implemented within the framework of the dsmcFoam code, a derivative of the open source CFD code OpenFOAM. Results for vibrational relaxation, dissociation and exchange reaction rates for an adiabatic bath demonstrate the success of the Q-K model implementation in dsmcFoam when compared with analytical solutions for both inert and reacting conditions. A comparison is also made between the Q-K and total collision energy (TCE) chemistry approaches for a hypersonic flow benchmark case.

We present an internal-flow multiscale method ('unsteady-IMM') for compressible, time-varying/unsteady flow problems in nano-confined high-aspect-ratio geometries. The IMM is a hybrid molecular-continuum method that provides accurate flow predictions at macroscopic scales because local microscopic corrections to the continuum-fluid formulation are generated by spatially and temporally distributed molecular simulations. Exploiting separation in both time and length scales enables orders of magnitude computational savings, far greater than seen in other hybrid methods. We apply the unsteady-IMM to a converging-diverging channel flow problem with various time-and length-scale separations. Comparisons are made with a full molecular simulation wherever possible; the level of accuracy of the hybrid solution is excellent in most cases. We demonstrate that the sensitivity of the accuracy of a solution to the macro-micro time-stepping, as well as the computational speed-up over a full molecular simulation, is dependent on the degree of scale separation that exists in a problem. For the largest channel lengths considered in this paper, a speed-up of six orders of magnitude has been obtained, compared with a notional full molecular simulation.

An open source implementation of chemistry modelling for the direct simulation\ud Monte Carlo (DSMC) method is presented. Following the recent work of Bird [1] an\ud approach known as the quantum kinetic (Q-K) method has been adopted to describe\ud chemical reactions in a 5-species air model using DSMC procedures based on\ud microscopic gas information. The Q-K technique has been implemented within the\ud framework of the dsmcFoam code, a derivative of the open source CFD code\ud OpenFOAM. Results for vibrational relaxation, dissociation and exchange reaction\ud rates for an adiabatic bath demonstrate the success of the Q-K model when compared\ud with analytical solutions for both inert and reacting conditions. A comparison is also\ud made between the Q-K and total collision energy (TCE) chemistry approaches for a\ud hypersonic flow benchmark case.