39 research outputs found
Ley Extendida de Estados Correspondientes: Orígenes y Aplicaciones
La llamada “ley extendida de estados correspondientes” es usada actualmente para analizar y entender los diagramas de estado, dentro y fuera del equilibrio termodinámico, de una gran variedad de materiales auto-ensamblantes (proteínas, nano-coloides, partículas Janus, etc.). Esta ley provee una descripción compacta del papel que desempeña el potencial de interacción entre moléculas en la determinación de las propiedades termodinámicas y de transporte de los materiales antes mencionados. En esta plática hablaré de los orígenes de esta ley y sus aplicaciones en la determinación de fenómenos tales como el arresto dinámico de nano-partículas, la transición líquido-vapor en soluciones de proteínas y en los procesos de agregación coloidal
Evolutionary optimization of the Verlet closure relation for the hard-sphere and square-well fluids
The Ornstein-Zernike equation is solved for the hard-sphere and square-well
fluids using a diverse selection of closure relations; the attraction range of
the square-well is chosen to be In particular, for both fluids
we mainly focus on the solution based on a three-parameter version of the
Verlet closure relation [Mol. Phys. 42, 1291-1302 (1981)]. To find the free
parameters of the latter, an unconstrained optimization problem is defined as a
condition of thermodynamic consistency based on the compressibility and solved
using Evolutionary Algorithms. For the hard-sphere fluid, the results show good
agreement when compared with mean-field equations of state and accurate
computer simulation results; at high densities, i.e., close to the freezing
transition, expected (small) deviations are seen. In the case of the
square-well fluid, a good agreement is observed at low and high densities when
compared with event-driven molecular dynamics computer simulations. For
intermediate densities, the explored closure relations vary in terms of
accuracy. Our findings suggest that a modification of the optimization problem
to include, for example, additional thermodynamic consistency criteria could
improve the results for the type of fluids here explored.Comment: 12 pages, 6 figure
Covariant description of the colloidal dynamics on curved manifolds
Brownian motion is a universal characteristic of colloidal particles embedded in a host medium, and it is the fingerprint of molecular transport or diffusion, a generic feature of relevance not only in physics but also in several branches of science and engineering. Since its discovery, Brownian motion, also known as colloidal dynamics, has been important in elucidating the connection between the molecular details of the diffusing macromolecule and the macroscopic information on the host medium. However, colloidal dynamics is far from being completely understood. For instance, the diffusion of non-spherical colloids and the effects of the underlying geometry of the host medium on the dynamics of either passive or active particles are a few representative cases that are part of the current challenges in soft matter physics. In this contribution, we take a step forward to introduce a covariant description of the colloidal dynamics in curved spaces. Without the loss of generality, we consider the case where hydrodynamic interactions are neglected. This formalism will allow us to understand several phenomena, for instance, the curvature effects on the kinetics during spinodal decomposition and the thermodynamic properties of colloidal dispersion, to mention a few examples. This theoretical framework will also serve as the starting point to highlight the role of geometry on colloidal dynamics, an aspect that is of paramount importance to understanding more complex transport phenomena, such as the diffusive mechanisms of proteins embedded in cell membranes
Arrested dynamics of the dipolar hard-sphere model
We report the combined results of molecular dynamics simulations and
theoretical calculations concerning various dynamical arrest transitions in a
model system representing a dipolar fluid, namely, N (softcore) rigid spheres
interacting through a truncated dipole-dipole potential. By exploring different
regimes of concentration and temperature, we find three distinct scenarios for
the slowing down of the dynamics of the translational and orientational degrees
of freedom: At low ({} = 0.2) and intermediate ( = 0.4) volume
fractions, both dynamics are strongly coupled and become simultaneously
arrested upon cooling. At high concentrations ({} 0.6), the
translational dynamics shows the features of an ordinary glass transition,
either by compressing or cooling down the system, but with the orientations
remaining ergodic, thus indicating the existence of partially arrested states.
In this density regime, but at lower temperatures, the relaxation of the
orientational dynamics also freezes. The physical scenario provided by the
simulations is discussed and compared against results obtained with the
self-consistent generalized Langevin equation theory, and both provide a
consistent description of the dynamical arrest transitions in the system. Our
results are summarized in an arrested states diagram which qualitatively
organizes the simulation data and provides a generic picture of the glass
transitions of a dipolar fluid
Single-file dynamics of colloids in circular channels: Time scales, scaling laws and their universality
In colloidal systems, Brownian motion emerges from the massive separation of time and length scales associated with characteristic dynamics of the solute and solvent constituents. This separation of scales produces several temporal regimes in the colloidal dynamics when combined with the effects of the interaction between the particles, confinement conditions, and state variables, such as density and temperature. Some examples are the short- and long-time regimes in two- and three-dimensional open systems and the diffusive and subdiffusive regimes observed in the single-file (SF) dynamics along a straight line. In this paper, we address the way in which a confining geometry induces new time scales. We report on the dynamics of interacting colloidal particles moving along a circle by combining a heuristic theoretical analysis of the involved scales, Brownian dynamics computer simulations, and video-microscopy experiments with paramagnetic colloids confined to lithographic circular channels subjected to an external magnetic field. The systems display four temporal regimes in the following order: one-dimensional free diffusion, SF subdiffusion, free-cluster rotational diffusion, and the expected saturation due to the confinement. We also report analytical expressions for the mean-square angular displacement and crossover times obtained from scaling arguments, which accurately reproduce both experiments and simulations. Our generic approach can be used to predict the long-time dynamics of many other confined physical systems
Glassy dynamics in asymmetric binary mixtures of hard-spheres
The binary hard-sphere mixture is one of the simplest representations of a
many-body system with competing time and length scales. This model is relevant
to fundamentally understand both the structural and dynamical properties of
materials, such as metallic melts, colloids, polymers and bio-based composites.
It also allows us to study how different scales influence the physical behavior
of a multicomponent glass-forming liquid; a question that still awaits a
unified description. In this contribution, we report on distinct dynamical
arrest transitions in highly asymmetric binary colloidal mixtures, namely, a
single glass of big particles, in which the small species remains ergodic, and
a double glass with the simultaneous arrest of both components. When the
mixture approaches any glass transition, the relaxation of the collective
dynamics of both species becomes coupled. In the single glass domain, spatial
modulations occur due to the structure of the large spheres, a feature not
observed in the two-glass domain. The relaxation of the \emph{self} dynamics of
small and large particles, in contrast, become decoupled at the boundaries of
both transitions; the large species always displays dynamical arrest, whereas
the small ones appear arrested only in the double glass. Thus, in order to
obtain a complete picture of the distinct glassy states, one needs to take into
account the dynamics of both species
Hysteresis in Pressure-Driven DNA Denaturation
In the past, a great deal of attention has been drawn to thermal driven denaturation processes. In recent years, however, the discovery of stress-induced denaturation, observed at the one-molecule level, has revealed new insights into the complex phenomena involved in the thermo-mechanics of DNA function. Understanding the effect of local pressure variations in DNA stability is thus an appealing topic. Such processes as cellular stress, dehydration, and changes in the ionic strength of the medium could explain local pressure changes that will affect the molecular mechanics of DNA and hence its stability. In this work, a theory that accounts for hysteresis in pressure-driven DNA denaturation is proposed. We here combine an irreversible thermodynamic approach with an equation of state based on the Poisson-Boltzmann cell model. The latter one provides a good description of the osmotic pressure over a wide range of DNA concentrations. The resulting theoretical framework predicts, in general, the process of denaturation and, in particular, hysteresis curves for a DNA sequence in terms of system parameters such as salt concentration, density of DNA molecules and temperature in addition to structural and configurational states of DNA. Furthermore, this formalism can be naturally extended to more complex situations, for example, in cases where the host medium is made up of asymmetric salts or in the description of the (helical-like) charge distribution along the DNA molecule. Moreover, since this study incorporates the effect of pressure through a thermodynamic analysis, much of what is known from temperature-driven experiments will shed light on the pressure-induced melting issue