17 research outputs found
Density-Temperature-Softness Scaling of the Dynamics of Glass-forming Soft-sphere Liquids
The principle of dynamic equivalence between soft-sphere and hard-sphere
fluids [Phys. Rev. E \textbf{68}, 011405 (2003)] is employed to describe the
interplay of the effects of varying the density n, the temperature T, and the
softness (characterized by a softness parameter {\nu}^{-1}) on the dynamics of
glass-forming soft-sphere liquids in terms of simple scaling rules. The main
prediction is that the dynamic parameters of these systems, such as the
{\alpha}-relaxation time and the long-time self-diffusion coefficient, depend
on n, T, and {\nu} only through the reduced density n^\ast \equiv
n{\sigma}^{3}_{HS}(T, {\nu}),where the effective hard-sphere diameter
{\sigma}_{HS}(T, {\nu}) is determined, for example, by the
Andersen-Weeks-Chandler condition for soft-sphere-hard-sphere structural
equivalence. A number of scaling properties observed in recent simulations
involving glass-forming fluids with repulsive short range interactions are
found to be a direct manifestation of this general dynamic equivalence
principle. The self-consistent generalized Langevin equation (SCGLE) theory of
colloid dynamics is shown to accurately capture these scaling rule
Influence of hydrodynamics on many-particle diffusion in 2D colloidal suspensions
We study many-particle diffusion in 2D colloidal suspensions with full
hydrodynamic interactions through a novel mesoscopic simulation technique. We
focus on the behaviour of the effective scaled tracer and collective diffusion
coefficients and , where is the
single-particle diffusion coefficient, as a function of the density of the
colloids . At low Schmidt numbers , we find that
hydrodynamics has essentially no effect on the behaviour of . At
larger , is enhanced at all densities, although the
differences compared to the case without hydrodynamics are minor. The
collective diffusion coefficient, on the other hand, is much more strongly
coupled to hydrodynamical conservation laws and is distinctly different from
the purely dissipative case
Far-field approximation for hydrodynamic interactions in parallel-wall geometry
A complete analysis is presented for the far-field creeping flow produced by
a multipolar force distribution in a fluid confined between two parallel planar
walls. We show that at distances larger than several wall separations the flow
field assumes the Hele-Shaw form, i.e., it is parallel to the walls and varies
quadratically in the transverse direction. The associated pressure field is a
two-dimensional harmonic function that is characterized by the same multipolar
number m as the original force multipole. Using these results we derive
asymptotic expressions for the Green's matrix that represents Stokes flow in
the wall-bounded fluid in terms of a multipolar spherical basis. This Green's
matrix plays a central role in our recently proposed algorithm [Physica A xx,
{\bf xxx} (2005)] for evaluating many-body hydrodynamic interactions in a
suspension of spherical particles in the parallel-wall geometry. Implementation
of our asymptotic expressions in this algorithm increases its efficiency
substantially because the numerically expensive evaluation of the exact matrix
elements is needed only for the neighboring particles. Our asymptotic analysis
will also be useful in developing hydrodynamic algorithms for wall-bounded
periodic systems and implementing acceleration methods by using corresponding
results for the two-dimensional scalar potential.Comment: 28 pages 5 figure
Many-particle hydrodynamic interactions in parallel-wall geometry: Cartesian-representation method
This paper describes the results of our theoretical and numerical studies of
hydrodynamic interactions in a suspension of spherical particles confined
between two parallel planar walls, under creeping-flow conditions. We propose a
novel algorithm for accurate evaluation of the many-particle friction matrix in
this system--no such algorithm has been available so far.
Our approach involves expanding the fluid velocity field into spherical and
Cartesian fundamental sets of Stokes flows. The interaction of the fluid with
the particles is described using the spherical basis fields; the flow scattered
with the walls is expressed in terms of the Cartesian fundamental solutions. At
the core of our method are transformation relations between the spherical and
Cartesian basis sets. These transformations allow us to describe the flow field
in a system that involves both the walls and particles.
We used our accurate numerical results to test the single-wall superposition
approximation for the hydrodynamic friction matrix. The approximation yields
fair results for quantities dominated by single particle contributions, but it
fails to describe collective phenomena, such as a large transverse resistance
coefficient for linear arrays of spheres
Materiales ultrablandos: diagrama de fase de una suspensión coloidal de polímeros estrella
La determinación experimental y predicción teórica de diagrama de fases para las sustancias es un aspecto de la mayor importancia en la ciencia de materiales. La cristalización y fusión a pesar de ser fenómenos cotidianos y de estar dentro de la agenda de estudio de la mecánica estadística desde sus inicios, aún no provee una teoría que las explique de primeros principios. La dificultad principal se debe a que estas transiciones se presentan en sistemas concentrados convirtiéndose en un problema colectivo de muchos cuerpos. Es en este contexto que la prescripción de criterios fenomenológicos que permitan la localización de las líneas de transición es altamente valorada por la comunidad científica. Motivados por ello, en esta comunicación se presentan resultados obtenidos con el criterio de Löwen, mediante simulaciones computacionales, para una suspensión coloidal de partículas ultra suaves conformada por polímeros estrella
Síntesis y caracterización de nuevas nanopartículas de derivados de PBLG para su uso en nanomedicina
En este trabajo se describe la realización de la síntesis y caracterización de cinco derivados de poli-L-glutamato de gamma-bencilo (PBLG) por polimerización aniónica de apertura de anillo utilizando iniciadores de interés biomédico, así como la preparación y caracterización de nanopartículas poliméricas preparadas usando los polímeros sintetizados. Los polímeros fueron caracterizados por resonancia magnética nuclear (RMN 1H) y cromatografía de exclusión de tamaño (GPC/SEC). Las nanopartículas fueron preparadas por nanoprecipitación y caracterizadas por microscopía electrónica de transmisión (MET) y dispersión de luz (DLS/DDLS). Como se requiere para aplicaciones en nanomedicina, nanopartículas en el intervalo de 20 a 50 nm fueron obtenidas. Por otro lado, se presenta la técnica de dispersión de luz depolarizada (DDLS), como una técnica interesante para la caracterización morfológica y comportamiento dinámico de las nanopartículas poliméricas, en suspensión acuosa
Presión en medios granulares en silos: experimentos para un curso de fluidos
Se presentan las experiencias obtenidas en el análisis del comportamiento estático de un medio granular en un silo. Haciendo uso de un dispositivo experimental construido para tal fin, se explora el comportamiento de la presión ejercida sobre el fondo de un silo cuando sobre él descansa una columna de medio granular (maíz) sobrecargado y se compara con el de un líquido. Se utiliza el modelo teórico de Janssen para describir el comportamiento de la presión, obteniendo resultados satisfactorios. Este trabajo podrá servir de base en la elaboración de protocolos de prácticas para los laboratorios de los cursos de física clásica que se imparten en los programas de licenciatura en ciencias e ingeniería