1,208 research outputs found
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 when the system is treated as
effectively monodisperse. The origin of the different small- 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
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 silÃcea 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 producÃa 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); ConsejerÃa de Transformación Económica, Industria, Conocimiento y Universidades de la Junta de AndalucÃa/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 metodologÃas más adecuadas para determinar la naturaleza del conglomerante aluminoso en hormigones. Estos métodos son:
a) Determinación del Módulo SilÃcico, a través de Análisis QuÃmico 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 lÃneas de difracción de los compuestos caracterÃsticos 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. Asà como un comentario documentado sobre el significado de la presencia de diferentes polimorfÃas del CaCO3 en los hormigones de cemento portland y de cemento aluminoso
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