Detecting vapour bubbles in simulations of metastable water

International audienceThe investigation of cavitation in metastable liquids with molecular simulations requires an appropriate definition of the volume of the vapour bubble forming within the metastable liquid phase. Commonly used approaches for bubble detection exhibit two significant flaws: first, when applied to water they often identify the voids within the hydrogen bond network as bubbles thus masking the signature of emerging bubbles and, second, they lack thermodynamic consistency. Here, we present two grid-based methods, the M-method and the V-method, to detect bubbles in metastable water specifically designed to address these shortcomings. The M-method incorporates information about neighbouring grid cells to distinguish between liquid- and vapour-like cells, which allows for a very sensitive detection of small bubbles and high spatial resolution of the detected bubbles. The V-method is calibrated such that its estimates for the bubble volume correspond to the average change in system volume and are thus thermodynamically consistent. Both methods are computationally inexpensive such that they can be used in molecular dynamics and Monte Carlo simulations of cavitation. We illustrate them by computing the free energy barrier and the size of the critical bubble for cavitation in water at negative pressure

On the effect of thermal ageing in CFRP mechanical properties

Highly Accelerated Life Testing (HALT) of any product consist in applying several types of loads for accelerated reliability analysis in order to find weaknesses during both design and development stages. Different environmental stress conditions like high rate thermal cycling and vibration are generally introduced during HALT. In this work, thermal cycles have been performed between -50 and 150°C for aeronautic composite materials in non-reactive and oxidative atmospheres. The thermal fatigue tests applied to polymer fibre reinforced composites involve matrix oxidation and cracking due to the mismatch of the thermal expansion coefficient between the carbon fibres and the epoxy matrix. In order to correlate the composite degradation with mechanical properties, fatigue and quasi-static tensile tests have been carried out in reference and post-ageing conditions. Microstructural characterization using X-ray computed tomography and light optical microscopy have been performed as well for a better understanding on damage mechanisms. Combined effect of thermal cycling and mechanical load tests on CFRP structural parts (T profile) were also performed

Study of the triplet and pair structure of strong electrolytes modeled via truncated Coulomb interactions

9 pages, 10 figures, 1 table.The structure of 3:3 and 1:3 electrolyte solutions at various concentrations and several cation/anion size ratios has been analyzed in terms of triplet and pair correlation functions, by means of simulation and a triplet integral equation theory derived from the inhomogeneous Ornstein–Zernike equation. The interaction model consists of a truncated and shifted Coulomb plus the Ramanathan–Friedman repulsive core. Concentration and size and charge asymmetry are found to induce changes in the triplet structure beyond those predicted by the simple superposition approximation, which are, however, correctly reproduced by the triplet integral equation.This work was funded by the Spanish Dirección General de Enseñanza Superior e Investigación Científica under Grant No. BFM2001-1017-C03. S.J. acknowledges financial support from the Universidad Complutense de Madrid.Peer reviewe

Theory and simulation of central force model potentials: Application to homonuclear diatomic molecules

14 pags., 14 figs., 6 tabs.Structure and thermodynamics of fluids made of particles that interact via a central force model potential are studied by means of Monte Carlo simulations and integral equation theories. The Hamiltonian has two terms, an intramolecular component represented by a harmonic oscillatorlike potential and an intermolecular interaction of the Lennard-Jones type. The potential does not fulfill the steric saturation condition so it leads to a polydisperse system. First, we investigate the association (clustering) and thermodynamic properties as a function of the potential parameters, such as the intramolecular potential depth, force constant, and bond length. It is shown that the atomic hypernetted chain (HNC) integral equation provides a correct description of the model as compared with simulation results. The calculation of the HNC pseudospinodal curve indicates that the stability boundaries between the vapor and liquid phases are strongly dependent on the bond length and suggests that there might be a direct gas-solid transition for certain elongations. On the other hand, we have assessed the ability of the model to describe the thermodynamics and structure of diatomic liquids such as N2 and halogens. To this end we have devised a procedure to model the intramolecular potential depth to reproduce the complete association limit (i.e., an average number of bonds per particle equal to one). This constraint is imposed on the Ornstein-Zernike integral equation in a straightforward numerical way. The structure of the resulting fluid is compared with results from molecular theories. An excellent agreement between the HNC results for the associating fluid and the reference interaction site model (RISM)-HNC computations for the atom-atom model of the same fluid is obtained. There is also a remarkable coincidence between the simulation results for the molecular and the associating liquids, despite the polydisperse character of the latter. The stability boundaries in the complete association limit as predicted by the HNC integral equation have been computed for different bond lengths corresponding to real molecular liquids. These boundaries appear close to the experimental liquid branch of the vapor-liquid coexistence line of the molecular systems under consideration. © 1996 American Institute of Physics.This work was partially supported by Grants No. PB93- 0085 and PB94-0112 furnished by the Direccion General de Investigacion Cientıfica y Tecnologica ~DGICYT! of Spain. FB acknowledges a predoctoral fellowship awarded by the Universidad Complutense de Madrid

Dynamic stiffnesses of foundations

A general theory that described the B.I.E.M. in steady-state elastodynamics is developed. A comprehensive formulation for homogeneous and heterogeneous media is presented and also some results in practical cases as well as a general review of several other possibilities

B.I.E.M. in solid mechanics

In solid mechanics the weak formulation produces an integral equation ready for a discretization and with less restrictive requiremets than the standard field equations. Fundamentally the weak formulation is a expresion of a green formula. An alternative is to choose another green formula materializing a reciprocity relationship between the basis unknowns and an auxiliary family of functions. The degree of smoothness requiered to practice the discretization is then translated to the auxiliar functions. The subsequent discretization (constant, linear etc.)produces a set of equations on the boundary of the domain. For linear 3-D problems the BIEM appears then as a powerful alternative to FEM, because of the reduction to 2-D thanks to the features previously described

Recognition of emotional facial expressions in Alexithymia

Alexithymia is a personality trait which is associated with difficulties in identifying and verbalizing emotions. Previous studies have shown a significant association between alexithymia and a lack of ability to decode emotional facial expressions. Three groups of university students (N = 1645) were formed by splitting the sample based on Toronto Alexithymia Scale (TAS-20) scores. All participants performed an emotional expression recognition task, using the "Reading the Mind in the Eyes'" method. The experimenter presented images of the eye-region of the faces of actors and actresses. As expected, results showed that alexithymics performed worse on ability to infer others' emotional states expressed by the eyes. These results suggest that alexithymia is associated with impaired emotion recognition, that is most apparent when processing capacity is restricted, high-alexithymic individuals could develop less detailed perceptual representations of facial expression which might impair the process of drawing conclusions about its emotional significance