83 research outputs found
Long time diffusion in suspensions of interacting charged colloids
A new expression is given for the long time diffusion coefficient DL(k) of charged interacting colloidal spheres in suspension, as a function of the wavenumber k, near k = km, where the static structure factor has a maximum. The expression is based on a physical analogy between a mode description of the behaviour of atomic fluids (as observed in neutron scattering) and of colloids (as observed in light scattering). Use of this expresssion in conjunction with a hard-sphere model yields good agreement with extant data on colloids
Phase behaviour of colloid + polymer mixtures
A new treatment of the phase behaviour of a colloid + nonadsorbing polymer mixture is described. The calculated phase diagrams show marked polymer partitioning between coexisting phases, an effect not considered in the usual effective-potential approaches to this problem. We also predict that under certain conditions an area of three-phase coexistence should appear in the phase diagram
Glasses in hard spheres with short-range attraction
We report a detailed experimental study of the structure and dynamics of
glassy states in hard spheres with short-range attraction. The system is a
suspension of nearly-hard-sphere colloidal particles and non-adsorbing linear
polymer which induces a depletion attraction between the particles. Observation
of crystallization reveals a re-entrant glass transition. Static light
scattering shows a continuous change in the static structure factors upon
increasing attraction. Dynamic light scattering results, which cover 11 orders
of magnitude in time, are consistent with the existence of two distinct kinds
of glasses, those dominated by inter-particle repulsion and caging, and those
dominated by attraction. Samples close to the `A3 point' predicted by mode
coupling theory for such systems show very slow, logarithmic dynamics.Comment: 22 pages, 18 figure
Evidence for Unusual Dynamical Arrest Scenario in Short Ranged Colloidal Systems
Extensive molecular dynamics simulation studies of particles interacting via
a short ranged attractive square-well (SW) potential are reported. The
calculated loci of constant diffusion coefficient in the
temperature-packing fraction plane show a re-entrant behavior, i.e. an increase
of diffusivity on cooling, confirming an important part of the high
volume-fraction dynamical-arrest scenario earlier predicted by theory for
particles with short ranged potentials. The more efficient localization
mechanism induced by the short range bonding provides, on average, additional
free volume as compared to the hard-sphere case and results in faster dynamics.Comment: 4 pages, 3 figure
Lattice-switch Monte Carlo
We present a Monte Carlo method for the direct evaluation of the difference
between the free energies of two crystal structures. The method is built on a
lattice-switch transformation that maps a configuration of one structure onto a
candidate configuration of the other by `switching' one set of lattice vectors
for the other, while keeping the displacements with respect to the lattice
sites constant. The sampling of the displacement configurations is biased,
multicanonically, to favor paths leading to `gateway' arrangements for which
the Monte Carlo switch to the candidate configuration will be accepted. The
configurations of both structures can then be efficiently sampled in a single
process, and the difference between their free energies evaluated from their
measured probabilities. We explore and exploit the method in the context of
extensive studies of systems of hard spheres. We show that the efficiency of
the method is controlled by the extent to which the switch conserves correlated
microstructure. We also show how, microscopically, the procedure works: the
system finds gateway arrangements which fulfill the sampling bias
intelligently. We establish, with high precision, the differences between the
free energies of the two close packed structures (fcc and hcp) in both the
constant density and the constant pressure ensembles.Comment: 34 pages, 9 figures, RevTeX. To appear in Phys. Rev.
Aspects of the dynamics of colloidal suspensions: Further results of the mode-coupling theory of structural relaxation
Results of the idealized mode-coupling theory for the structural relaxation
in suspensions of hard-sphere colloidal particles are presented and discussed
with regard to recent light scattering experiments. The structural relaxation
becomes non-diffusive for long times, contrary to the expectation based on the
de Gennes narrowing concept. A semi-quantitative connection of the wave vector
dependences of the relaxation times and amplitudes of the final
-relaxation explains the approximate scaling observed by Segr{\`e} and
Pusey [Phys. Rev. Lett. {\bf 77}, 771 (1996)]. Asymptotic expansions lead to a
qualitative understanding of density dependences in generalized Stokes-Einstein
relations. This relation is also generalized to non-zero frequencies thereby
yielding support for a reasoning by Mason and Weitz [Phys. Rev. Lett {\bf 74},
1250 (1995)]. The dynamics transient to the structural relaxation is discussed
with models incorporating short-time diffusion and hydrodynamic interactions
for short times.Comment: 11 pages, 9 figures; to be published in Phys. Rev.
Simple Fluids with Complex Phase Behavior
We find that a system of particles interacting through a simple isotropic
potential with a softened core is able to exhibit a rich phase behavior
including: a liquid-liquid phase transition in the supercooled phase, as has
been suggested for water; a gas-liquid-liquid triple point; a freezing line
with anomalous reentrant behavior. The essential ingredient leading to these
features resides in that the potential investigated gives origin to two
effective core radii.Comment: 7 pages including 3 eps figures + 1 jpeg figur
Brownian Dynamics Simulation of Polydisperse Hard Spheres
Standard algorithms for the numerical integration of the Langevin equation
require that interactions are slowly varying during to the integration
timestep. This in not the case for hard-body systems, where there is no
clearcut between the correlation time of the noise and the timescale of the
interactions. Starting from a short time approximation of the Smoluchowsky
equation, we introduce an algorithm for the simulation of the overdamped
Brownian dynamics of polydisperse hard-spheres in absence of hydrodynamics
interactions and briefly discuss the extension to the case of external drifts
Aging dynamics in a colloidal glass of Laponite
The aging dynamics of colloidal suspensions of Laponite, a synthetic clay, is
investigated using dynamic light stattering (DLS) and viscometry after a quench
into the glassy phase. DLS allows to follow the diffusion of Laponite particles
and reveals that there are two modes of relaxation. The fast mode corresponds
to a rapid diffusion of particles within "cages" formed by the neighboring
particles. The slow mode corresponds to escape from the cages: its average
relaxation time increases exponentially fast with the age of the glass. In
addition, the slow mode has a broad distribution of relaxation times, its
distribution becoming larger as the system ages. Measuring the concomitant
increase of viscosity as the system ages, we can relate the slowing down of the
particle dynamics to the viscosity.Comment: 9 pages, 8 Postscript figures, submitted to Phys. Rev.
Local influence of boundary conditions on a confined supercooled colloidal liquid
We study confined colloidal suspensions as a model system which approximates
the behavior of confined small molecule glass-formers. Dense colloidal
suspensions become glassier when confined between parallel glass plates. We use
confocal microscopy to study the motion of confined colloidal particles. In
particular, we examine the influence particles stuck to the glass plates have
on nearby free particles. Confinement appears to be the primary influence
slowing free particle motion, and proximity to stuck particles causes a
secondary reduction in the mobility of free particles. Overall, particle
mobility is fairly constant across the width of the sample chamber, but a
strong asymmetry in boundary conditions results in a slight gradient of
particle mobility.Comment: For conference proceedings, "Dynamics in Confinement", Grenoble,
March 201
- …