Competition between glass transition and liquid-gas separation in attracting colloids

We present simulation results addressing the phenomena of colloidal gelation induced by attractive interactions. The liquid-gas transition is prevented by the glass arrest at high enough attraction strength, resulting in a colloidal gel. The dynamics of the system is controlled by the glass, with little effect of the liquid-gas transition. When the system separates in a liquid and vapor phases, even if the denser phase enters the non-ergodic region, the vapor phase enables the structural relaxation of the system as a whole.Comment: Proceedings of the glass conference in Pisa (September 06

Tagged-particle dynamics in a hard-sphere system: mode-coupling theory analysis

The predictions of the mode-coupling theory of the glass transition (MCT) for the tagged-particle density-correlation functions and the mean-squared displacement curves are compared quantitatively and in detail to results from Newtonian- and Brownian-dynamics simulations of a polydisperse quasi-hard-sphere system close to the glass transition. After correcting for a 17% error in the dynamical length scale and for a smaller error in the transition density, good agreement is found over a wide range of wave numbers and up to five orders of magnitude in time. Deviations are found at the highest densities studied, and for small wave vectors and the mean-squared displacement. Possible error sources not related to MCT are discussed in detail, thereby identifying more clearly the issues arising from the MCT approximation itself. The range of applicability of MCT for the different types of short-time dynamics is established through asymptotic analyses of the relaxation curves, examining the wave-number and density-dependent characteristic parameters. Approximations made in the description of the equilibrium static structure are shown to have a remarkable effect on the predicted numerical value for the glass-transition density. Effects of small polydispersity are also investigated, and shown to be negligible.Comment: 20 pages, 23 figure

Structural relaxation of polydisperse hard spheres: comparison of the mode-coupling theory to a Langevin dynamics simulation

We analyze the slow, glassy structural relaxation as measured through collective and tagged-particle density correlation functions obtained from Brownian dynamics simulations for a polydisperse system of quasi-hard spheres in the framework of the mode-coupling theory of the glass transition (MCT). Asymptotic analyses show good agreement for the collective dynamics when polydispersity effects are taken into account in a multi-component calculation, but qualitative disagreement at small $q$ when the system is treated as effectively monodisperse. The origin of the different small-$q$ behaviour is attributed to the interplay between interdiffusion processes and structural relaxation. Numerical solutions of the MCT equations are obtained taking properly binned partial static structure factors from the simulations as input. Accounting for a shift in the critical density, the collective density correlation functions are well described by the theory at all densities investigated in the simulations, with quantitative agreement best around the maxima of the static structure factor, and worst around its minima. A parameter-free comparison of the tagged-particle dynamics however reveals large quantiative errors for small wave numbers that are connected to the well-known decoupling of self-diffusion from structural relaxation and to dynamical heterogeneities. While deviations from MCT behaviour are clearly seen in the tagged-particle quantities for densities close to and on the liquid side of the MCT glass transition, no such deviations are seen in the collective dynamics.Comment: 23 pages, 26 figure

Independent [1,2]-number versus independent domination number

A [1, 2]-set S in a graph G is a vertex subset such that every vertex not in S has at least one and at most two neighbors in it. If the additional requirement that the set be independent is added, the existence of such sets is not guaranteed in every graph. In this paper we provide local conditions, depending on the degree of vertices, for the existence of independent [1, 2]-sets in caterpillars. We also study the relationship between independent [1, 2]-sets and independent dominating sets in this graph class, that allows us to obtain an upper bound for the associated parameter, the independent [1, 2]-number, in terms of the independent domination number.Peer ReviewedPostprint (published version

Bond formation and slow heterogeneous dynamics in adhesive spheres with long--ranged repulsion: Quantitative test of Mode Coupling Theory

A colloidal system of spheres interacting with both a deep and narrow attractive potential and a shallow long-ranged barrier exhibits a prepeak in the static structure factor. This peak can be related to an additional mesoscopic length scale of clusters and/or voids in the system. Simulation studies of this system have revealed that it vitrifies upon increasing the attraction into a gel-like solid at intermediate densities. The dynamics at the mesoscopic length scale corresponding to the prepeak represents the slowest mode in the system. Using mode coupling theory with all input directly taken from simulations, we reveal the mechanism for glassy arrest in the system at 40% packing fraction. The effects of the low-q peak and of polydispersity are considered in detail. We demonstrate that the local formation of physical bonds is the process whose slowing down causes arrest. It remains largely unaffected by the large-scale heterogeneities, and sets the clock for the slow cluster mode. Results from mode-coupling theory without adjustable parameters agree semi-quantitatively with the local density correlators but overestimate the lifetime of the mesoscopic structure (voids).Comment: 10 pages, 8 figure

Utilización de áridos procedentes de residuos de construcción y demolición en morteros activados alcalinamente

This study explores the technological feasibility of using construction and demolition waste (C&DW) as recycled aggregate in alkali activated mortars, ascertaining the mechanical and microstructural behavior. Shrinkage behavior of alkali activated slag mortars (AAS) and fire resistance of alkali activated fly ash (AAFA) incorporating recycled aggregates have been also tested Normalized siliceous sand and two types of recycled concrete aggregates were used in the mixes at different proportions. The findings showed that water demand was higher in mortars prepared with recycled aggregate. Partial replacement (20% - 80/20) of conventional aggregate with the recycled material was also observed to yield mortars with high mechanical strength, although total porosity also rose. Total replaclement, gave worse mechanical performance however. Fire resistance and shrinkage studies conducted indicated that alkaline cement mortars prepared with 80/20 recycled aggregated exhibit acceptable performance.Este estudio explora la viabilidad tecnológica del uso de residuos de construcción y demolición (C & DW) como árido reciclado en morteros activados alcalinamente, determinando el comportamiento mecánico y microestructural. Asimismo, se determinó la retracción sufrida por morteros de escoria activados alcalinamente (AAS) y la resistencia al fuego de morteros de cenizas volantes activadas alcalinamente (AAFA) con incorporación de estos áridos reciclado. Se utilizó arena silícea normalizada y dos tipos de áridos de hormigón reciclado en diferentes proporciones. Los resultados mostraron que la demanda de agua era más alta en los morteros preparados con árido reciclado. También se observó que el reemplazo parcial con un 20% (80/20) del árido convencional con el material reciclado producía morteros con alta resistencia mecánica, aunque la porosidad total también aumentaba. El reemplazo total, sin embargo, dio peor rendimiento mecánico. Los estudios de resistencia al fuego y retracción realizados, indicaron que los morteros de cementos activados alcalinamente preparados con áridos reciclados en proporción 80/20 presentan un rendimiento aceptable

HEPPA-II model-measurement intercomparison project: EPP indirect effects during the dynamically perturbed NH winter 2008-2009

Funke, B. et. al..--This work is distributed under the Creative Commons Attribution 3.0 License.We compare simulations from three high-top (with upper lid above 120 km) and five medium-top (with upper lid around 80 km) atmospheric models with observations of odd nitrogen (NOx D NO+NO2), temperature, and carbon monoxide from seven satellite instruments (ACE-FTS on SciSat, GOMOS, MIPAS, and SCIAMACHY on Envisat, MLS on Aura, SABER on TIMED, and SMR on Odin) during the Northern Hemisphere (NH) polar winter 2008/2009. The models included in the comparison are the 3-D chemistry transport model 3dCTM, the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, FinROSE, the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMO-NIA), the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the modelling tools for SOlar Climate Ozone Links studies (SOCOL and CAO-SOCOL), and the Whole Atmosphere Community Climate Model (WACCM4). The comparison focuses on the energetic particle precipitation (EPP) indirect effect, that is, the polar winter descent of NOx largely produced by EPP in the mesosphere and lower thermosphere. A particular emphasis is given to the impact of the sudden stratospheric warming (SSW) in January 2009 and the subsequent elevated stratopause (ES) event associated with enhanced descent of mesospheric air. The chemistry climate model simulations have been nudged toward reanalysis data in the troposphere and stratosphere while being unconstrained above. An odd nitrogen upper boundary condition obtained from MIPAS observations has further been applied to medium-top models. Most models provide a good representation of the mesospheric tracer descent in general, and the EPP indirect effect in particular, during the unperturbed (pre-SSW) period of the NH winter 2008/2009. The observed NOx descent into the lower mesosphere and stratosphere is generally reproduced within 20 %. Larger discrepancies of a few model simulations could be traced back either to the impact of the models' gravity wave drag scheme on the polar wintertime meridional circulation or to a combination of prescribed NOx mixing ratio at the uppermost model layer and low vertical resolution. In March-April, after the ES event, however, modelled mesospheric and stratospheric NOx distributions deviate significantly from the observations. The too-fast and early downward propagation of the NO x tongue, encountered in most simulations, coincides with a temperature high bias in the lower mesosphere (0.2-0.05 hPa), likely caused by an overestimation of descent velocities. In contrast, upper-mesospheric temperatures (at 0.05-0.001 hPa) are generally underestimated by the high-top models after the onset of the ES event, being indicative for too-slow descent and hence too-low NOx fluxes. As a consequence, the magnitude of the simulated NOx tongue is generally underestimated by these models. Descending NOx amounts simulated with mediumtop models are on average closer to the observations but show a large spread of up to several hundred percent. This is primarily attributed to the different vertical model domains in which the NOx upper boundary condition is applied. In general, the intercomparison demonstrates the ability of state-of- the-art atmospheric models to reproduce the EPP indirect effect in dynamically and geomagnetically quiescent NH winter conditions. The encountered differences between observed and simulated NOx, CO, and temperature distributions during the perturbed phase of the 2009 NH winter, however, emphasize the need for model improvements in the dynamical representation of elevated stratopause events in order to allow for a better description of the EPP indirect effect under these particular conditions.This work has been conducted in the frame of the WCRP/ SPARC SOLARIS-HEPPA activity. The IAA team was supported by the Spanish MCINN under grant ESP2014-54362-P and EC FEDER funds. The MPI-MET team was supported by the Max Planck Gesellschaft (MPG), and computational resources were made available by Deutsches Klimarechenzentrum (DKRZ) through support from Bundesministerium fur Bildung und Forschung (BMBF). The FMI team was supported by the Academy of Finland through the projects 276926 (SECTIC: Sun-Earth Connection Through Ion Chemistry), 258165, and 265005 (CLASP: Climate and Solar Particle Forcing). CAO team was supported by the Russian Science Foundation under grant 15-17-10024. SOCOL team was funded by Swiss National Science Foundation (SNSF) grants 200021-149182 (SILA), 200020-163206 (SIMA), and CRSII2-147659 (FUPSOL-II). S. Bender, M. Sinnhuber, and H. Nieder (all KIT) gratefully acknowledge funding by the Helmholtz Association of German Research Centres (HGF), grant VH-NG-624. NCAR is sponsored by the National Science Foundation (NSF). Computing resources for WACCM simulations were provided by the Climate Simulation Laboratory at NCAR's Computational and Information Systems Laboratory, sponsored by the NSF and other agencies. Work at the Jet Propulsion Laboratory, California Institute of Technology, was carried out under a contract with the National Aeronautics and Space Administration. The Atmospheric Chemistry Experiment (ACE), also known as SciSat, is a Canadian-led mission mainly supported by the Canadian Space Agency. Odin is a Swedish-led satellite project funded jointly by Sweden (SNSB), Canada (CSA), Finland (TEKES), and France (CNES) and is part of European Space Agency's (ESA) third-party mission program. We thank two anonymous reviewers for helpful suggestions that led to improvements in the quality of the present work.Peer reviewe

Active and Nonlinear Microrheology in Dense Colloidal Suspensions

We present a first-principles theory for the active nonlinear microrheology of colloidal model systems: for constant external force on a spherical probe particle embedded in a dense host dispersion, neglecting hydrodynamic interactions, we derive an exact expression for the friction. Within mode-coupling theory (MCT), we discuss the threshold external force needed to delocalize the probe from a host glass, and its relation to strong nonlinear velocity-force curves in a host fluid. Experimental microrheology data and simulations, which we performed, are explained with a simplified model

Virtual clinics in glaucoma care: face-to-face versus remote decision-making

BACKGROUND/AIMS: To examine the agreement in clinical decisions of glaucoma status made in a virtual glaucoma clinic with those made during a face-to-face consultation. METHODS: A trained nurse and technicians entered data prospectively for 204 patients into a proforma. A subsequent face-to-face clinical assessment was completed by either a glaucoma consultant or fellow. Proformas were reviewed remotely by one of two additional glaucoma consultants, and 12 months later, by the clinicians who had undertaken the original clinical examination. The interobserver and intraobserver decision-making agreements of virtual assessment versus standard care were calculated. RESULTS: We identified adverse disagreement between face-to-face and virtual review in 7/204 (3.4%, 95% CI 0.9% to 5.9%) patients, where virtual review failed to predict a need to accelerated follow-up identified in face-to-face review. Misclassification events were rare, occurring in 1.9% (95% CI 0.3% to 3.8%) of assessments. Interobserver κ (95% CI) showed only fair agreement (0.24 (0.04 to 0.43)); this improved to moderate agreement when only consultant decisions were compared against each other (κ=0.41 (0.16 to 0.65)). The intraobserver agreement κ (95% CI) for the consultant was 0.274 (0.073 to 0.476), and that for the fellow was 0.264 (0.031 to 0.497). CONCLUSIONS: The low rate of adverse misclassification, combined with the slowly progressive nature of most glaucoma, and the fact that patients will all be regularly reassessed, suggests that virtual clinics offer a safe, logistically viable option for selected patients with glaucoma