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

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

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

Theory and simulation of gelation, arrest and yielding in attracting colloids

We present some recent theory and simulation results addressing the phenomena of colloidal gelation at both high and low volume fractions, in the presence of short-range attractive interactions. We discuss the ability of mode-coupling theory and its adaptations to address situations with strong heterogeneity in density and/or dynamics. We include a discussion of the effect of attractions on the shear-thinning and yield behaviour under flow.Comment: 17 pages, 6 figure

Microrheology of isotropic and liquid-crystalline phases of hard rods by dynamic Monte Carlo simulations

Particle tracking in soft materials allows one to characterise the material’s local viscoelastic response, a technique referred to as microrheology (MR). In particular, MR can be especially powerful to ponder the impact of structural ordering on the tracer’s transport mechanism and thus disclose intriguing elements that cannot be observed in isotropic fluids. In this work, we perform Dynamic Monte Carlo simulations of isotropic and liquid-crystalline phases of rod-like particles and employ MR to characterise their linear viscoelastic response. By incorporating tracers of different diameters, we can assess the combined effect of size and ordering across the relevant time and length scales of the systems’ relaxation. While the dynamics of small tracers is dramatically determined by the background ordering, sufficiently large tracers have a reduced perception of the medium nanostructure and this difference directly influences the observed MR. Our results agree very well with the picture of a microviscosity increasing with the relevant system length scales, but also suggest the crucial relevance of long-ranged order as a key element governing the system’s viscoelastic response.The Royal Society (IES\R1\191066); Leverhulme Trust (RPG-2018-415); Spanish Ministerio de Ciencia, Innovación y Universidades (PGC2018-101555-B- I00); UAL/CECEU/FEDER (UAL18-FQM- B038-A); Consejería de Transformación Económica, Industria, Conocimiento y Universidades de la Junta de Andalucía/FEDER (P20-00816

Técnicas y métodos más adecuados para la identificación del cemento aluminoso y de cemento de base portland en hormigones

Instrumental techniques are indicated and the most adequated methodologies for determining the nature of the binder in concretes are explained. These methods are: a) Determination of the Silicic Moduli through chemical analysis of the sample. This test reveáis very different valúes between cement portland based concrete and high alumina cement based concretes. b) X-ray diffraction. It is considered as the best method. In the present paper the main diffraction Unes corresponding to the components of hydrated portland cement and high alumina cement, are given. As complementary methods having as limited use, DTA and TG are studied. The most significative changes of the hydrated phases of both cements when heated are shown. A valoration of the Oxine test is also done as well as a description of using the characteristic colour of the high alumina cement as a practical indicator for determining its presence in concretes. Finally, comments on the significance of the presence of different CaCO3 polymorhisms in portland cement concrete and high alumina cement concrétete are also done.Se indican las técnicas y se explican las metodologías más adecuadas para determinar la naturaleza del conglomerante aluminoso en hormigones. Estos métodos son: a) Determinación del Módulo Silícico, a través de Análisis Químico de la muestra problema, al considerar la acusada diferencia de aquel valor en hormigones de cemento portland, respecto a hormigones de cemento aluminoso. b) Difracción de Rayos X. Se considera la técnica idónea. Se dan en el presente trabajo las principales líneas de difracción de los compuestos característicos de los cementos hidratados, portland y aluminoso. Como métodos complementarios y de utilidad limitada se estudia el ATD y el TG. Se proponen las temperaturas más significativas de los compuestos hidratados de aquellos cementos, a través de lo cual se puede deducir la presencia de uno u otro tipo de cemento. Se hace una valoración del método de la "oxina", significando el alcance limitado del mismo. Igualmente se describe el color como indicador de gran interés práctico en la determinación del cemento aluminoso. Así como un comentario documentado sobre el significado de la presencia de diferentes polimorfías del CaCO3 en los hormigones de cemento portland y de cemento aluminoso