25,331 research outputs found

    Crystallization of the Wahnstr\"om Binary Lennard-Jones Liquid

    Full text link
    We report observation of crystallization of the glass-forming binary Lennard-Jones liquid first used by Wahnstr\"om [G. Wahnstr\"om, Phys. Rev. A 44, 3752 (1991)]. Molecular dynamics simulations of the metastable liquid on a timescale of microseconds were performed. The liquid crystallized spontaneously. The crystal structure was identified as MgZn_2. Formation of transient crystallites is observed in the liquid. The crystallization is investigate at different temperatures and compositions. At high temperature the rate of crystallite formation is the limiting factor, while at low temperature the limiting factor is growth rate. The melting temperature of the crystal is estimated to be T_m=0.93 at rho=0.82. The maximum crystallization rate of the A_2B composition is T=0.60+/-0.02.Comment: 4 pages, 4 figures; corrected typo

    Exponential distributions of collective flow-event properties in viscous liquid dynamics

    Get PDF
    We study the statistics of flow events in the inherent dynamics in supercooled two- and three-dimensional binary Lennard-Jones liquids. Distributions of changes of the collective quantities energy, pressure and shear stress become exponential at low temperatures, as does that of the event "size" S≡∑di2S\equiv\sum {d_i}^2. We show how the SS-distribution controls the others, while itself following from exponential tails in the distributions of (1) single particle displacements dd, involving a Lindemann-like length dLd_L and (2) the number of active particles (with d>dLd>d_L).Comment: Accepter version (PRL

    Strong pressure-energy correlations in liquids as a configuration space property: Simulations of temperature down jumps and crystallization

    Get PDF
    Computer simulations recently revealed that several liquids exhibit strong correlations between virial and potential energy equilibrium fluctuations in the NVT ensemble [U. R. Pedersen {\it et al.}, Phys. Rev. Lett. {\bf 100}, 015701 (2008)]. In order to investigate whether these correlations are present also far from equilibrium constant-volume aging following a temperature down jump from equilibrium was simulated for two strongly correlating liquids, an asymmetric dumbbell model and Lewis-Wahnstr{\"o}m OTP, as well as for SPC water that is not strongly correlating. For the two strongly correlating liquids virial and potential energy follow each other closely during the aging towards equilibrium. For SPC water, on the other hand, virial and potential energy vary with little correlation as the system ages towards equilibrium. Further proof that strong pressure-energy correlations express a configuration space property comes from monitoring pressure and energy during the crystallization (reported here for the first time) of supercooled Lewis-Wahnstr{\"o}m OTP at constant temperature

    Fine Structure of the 1s3p ^3P_J Level in Atomic ^4He: Theory and Experiment

    Full text link
    We report on a theoretical calculation and a new experimental determination of the 1s3p ^3P_J fine structure intervals in atomic ^4He. The values from the theoretical calculation of 8113.730(6) MHz and 658.801(6) MHz for the nu_{01} and nu_{12} intervals, respectively, disagree significantly with previous experimental results. However, the new laser spectroscopic measurement reported here yields values of 8113.714(28) MHz and 658.810(18) MHz for these intervals. These results show an excellent agreement with the theoretical values and resolve the apparent discrepancy between theory and experiment.Comment: 9 pages, 3 figure

    ‘VIOLET’: a fluorescence-based simulation exercise for training healthcare workers in the use of personal protective equipment

    Get PDF
    Background Healthcare workers caring for patients with high-consequence infectious diseases (HCIDs) require protection from pathogen exposure, for example by wearing personal protective equipment (PPE). Protection is acquired through the inherent safety of the PPE components, but also their safe and correct use, supported by adequate training and user familiarity. However, the evidence base for HCID PPE ensembles and any associated training is lacking, with subsequent variation between healthcare providers. Aim To develop an evidence-based assessment and training tool for evaluating PPE ensembles and doffing protocols, in the assessment of patients with suspected HCIDs. Methods VIOLET (Visualising Infection with Optimised Light for Education and Training) comprises a healthcare mannequin adapted to deliver simulated bodily fluids containing UV-fluorescent tracers. On demand and remotely operated, the mannequin projectile vomits (blue), coughs (red), has diarrhoea (yellow) and is covered in sweat (orange). Wearing PPE, healthcare staff participate in an HCID risk assessment and examination of the ‘patient’, thereby becoming exposed to these bodily fluids. Contamination of PPE is visualized and body-mapped under UV light before and after removal. Observational findings and participant feedback, around its use as a training exercise, is also recorded. Findings Significant contamination from different exposure events was seen, enabling evaluation of PPE and doffing procedures used. Observational data and participant feedback demonstrated its strengths and success as a training technique. Conclusion Simulation exercises using VIOLET provide evidence-based assessment of PPE ensembles, and are a valuable resource for training of healthcare staff in wearing and safe doffing of PPE

    Use of ultraviolet-fluorescence-based simulation in evaluation of personal protective equipment worn for first assessment and care of a patient with suspected high-consequence infectious disease

    Get PDF
    Background: Variations currently exist across the UK in the choice of personal protective equipment (PPE) used by healthcare workers when caring for patients with suspected high-consequence infectious diseases (HCIDs). Aim: To test the protection afforded to healthcare workers by current PPE ensembles during assessment of a suspected HCID case, and to provide an evidence base to justify proposal of a unified PPE ensemble for healthcare workers across the UK. Methods: One ‘basic level’ (enhanced precautions) PPE ensemble and five ‘suspected case’ PPE ensembles were evaluated in volunteer trials using ‘Violet’; an ultraviolet-fluorescence-based simulation exercise to visualize exposure/contamination events. Contamination was photographed and mapped. Findings: There were 147 post-simulation and 31 post-doffing contamination events, from a maximum of 980, when evaluating the basic level of PPE. Therefore, this PPE ensemble did not afford adequate protection, primarily due to direct contamination of exposed areas of the skin. For the five suspected case ensembles, 1584 post-simulation contamination events were recorded, from a maximum of 5110. Twelve post-doffing contamination events were also observed (face, two events; neck, one event; forearm, one event; lower legs, eight events). Conclusion: All suspected case PPE ensembles either had post-doffing contamination events or other significant disadvantages to their use. This identified the need to design a unified PPE ensemble and doffing procedure, incorporating the most protective PPE considered for each body area. This work has been presented to, and reviewed by, key stakeholders to decide on a proposed unified ensemble, subject to further evaluation

    Slow epidemic extinction in populations with heterogeneous infection rates

    Get PDF
    We explore how heterogeneity in the intensity of interactions between people affects epidemic spreading. For that, we study the susceptible-infected-susceptible model on a complex network, where a link connecting individuals ii and jj is endowed with an infection rate βij=λwij\beta_{ij} = \lambda w_{ij} proportional to the intensity of their contact wijw_{ij}, with a distribution P(wij)P(w_{ij}) taken from face-to-face experiments analyzed in Cattuto et  al.et\;al. (PLoS ONE 5, e11596, 2010). We find an extremely slow decay of the fraction of infected individuals, for a wide range of the control parameter λ\lambda. Using a distribution of width aa we identify two large regions in the a−λa-\lambda space with anomalous behaviors, which are reminiscent of rare region effects (Griffiths phases) found in models with quenched disorder. We show that the slow approach to extinction is caused by isolated small groups of highly interacting individuals, which keep epidemic alive for very long times. A mean-field approximation and a percolation approach capture with very good accuracy the absorbing-active transition line for weak (small aa) and strong (large aa) disorder, respectively

    Pressure-energy correlations in liquids. V. Isomorphs in generalized Lennard-Jones systems

    Get PDF
    This series of papers is devoted to identifying and explaining the properties of strongly correlating liquids, i.e., liquids with more than 90% correlation between their virial W and potential energy U fluctuations in the NVT ensemble. Paper IV [N. Gnan et al., J. Chem. Phys. v131, 234504 (2009)] showed that strongly correlating liquids have "isomorphs", which are curves in the phase diagram along which structure, dynamics, and some thermodynamic properties are invariant in reduced units. In the present paper, using the fact that reduced-unit radial distribution functions are isomorph invariant, we derive an expression for the shapes of isomorphs in the WU phase diagram of generalized Lennard-Jones systems of one or more types of particles. The isomorph shape depends only on the Lennard-Jones exponents; thus all isomorphs of standard Lennard-Jones systems (with exponents 12 and 6) can be scaled onto to a single curve. Two applications are given. One is testing the prediction that the solid-liquid coexistence curve follows an isomorph by comparing to recent simulations by Ahmed and Sadus [J. Chem. Phys. v131, 174504 (2009)]. Excellent agreement is found on the liquid side of the coexistence, whereas the agreement is worse on the solid side. A second application is the derivation of an approximate equation of state for generalized Lennard-Jones systems by combining the isomorph theory with the Rosenfeld-Tarazona expression for the temperature dependence of potential energy on isochores. It is shown that the new equation of state agrees well with simulations.Comment: 12 pages, 14 figures, Section on solid-liquid coexistence expande

    Estimating the density scaling exponent of viscous liquids from specific heat and bulk modulus data

    Full text link
    It was recently shown by computer simulations that a large class of liquids exhibits strong correlations in their thermal fluctuations of virial and potential energy [Pedersen et al., Phys. Rev. Lett. 100, 015701 (2008)]. Among organic liquids the class of strongly correlating liquids includes van der Waals liquids, but excludes ionic and hydrogen-bonding liquids. The present note focuses on the density scaling of strongly correlating liquids, i.e., the fact their relaxation time tau at different densities rho and temperatures T collapses to a master curve according to the expression tau propto F(rho^gamma/T) [Schroder et al., arXiv:0803.2199]. We here show how to calculate the exponent gamma from bulk modulus and specific heat data, either measured as functions of frequency in the metastable liquid or extrapolated from the glass and liquid phases to a common temperature (close to the glass transition temperature). Thus an exponent defined from the response to highly nonlinear parameter changes may be determined from linear response measurements
    • …
    corecore