80 research outputs found
Monitoring spatially heterogeneous dynamics in a drying colloidal thin film
We report on a new type of experiment that enables us to monitor spatially
and temporally heterogeneous dynamic properties in complex fluids. Our approach
is based on the analysis of near-field speckles produced by light diffusely
reflected from the superficial volume of a strongly scattering medium. By
periodic modulation of an incident speckle beam we obtain pixel-wise ensemble
averages of the structure function coefficient, a measure of the dynamic
activity. To illustrate the application of our approach we follow the different
stages in the drying process of a colloidal thin film. We show that we can
access ensemble averaged dynamic properties on length scales as small as ten
micrometers over the full field of view.Comment: To appear in Soft Material
Surface tension of the isotropic-nematic interface
We present the first calculations of the pressure tensor profile in the
vicinity of the planar interface between isotropic liquid and nematic liquid
crystal, using Onsager's density functional theory and computer simulation.
When the liquid crystal director is aligned parallel to the interface, the
situation of lowest free energy, there is a large tension on the nematic side
of the interface and a small compressive region on the isotropic side. By
contrast, for perpendicular alignment, the tension is on the isotropic side.
There is excellent agreement between theory and simulation both in the forms of
the pressure tensor profiles, and the values of the surface tension.Comment: Minor changes; to appear in Phys. Rev.
Stochastic processes with finite correlation time: modeling and application to the generalized Langevin equation
The kangaroo process (KP) is characterized by various forms of the covariance
and can serve as a useful model of random noises. We discuss properties of that
process for the exponential, stretched exponential and algebraic (power-law)
covariances. Then we apply the KP as a model of noise in the generalized
Langevin equation and simulate solutions by a Monte Carlo method. Some results
appear to be incompatible with requirements of the fluctuation-dissipation
theorem because probability distributions change when the process is inserted
into the equation. We demonstrate how one can construct a model of noise free
of that difficulty. This form of the KP is especially suitable for physical
applications.Comment: 22 pages (RevTeX) and 4 figure
Defect structures and torque on an elongated colloidal particle immersed in a liquid crystal host
Combining molecular dynamics and Monte Carlo simulation we study defect
structures around an elongated colloidal particle embedded in a nematic liquid
crystal host. By studying nematic ordering near the particle and the
disclination core region we are able to examine the defect core structure and
the difference between two simulation techniques. In addition, we also study
the torque on a particle tilted with respect to the director, and modification
of this torque when the particle is close to the cell wall
Multicanonical Multigrid Monte Carlo
To further improve the performance of Monte Carlo simulations of first-order
phase transitions we propose to combine the multicanonical approach with
multigrid techniques. We report tests of this proposition for the
-dimensional field theory in two different situations. First, we
study quantum tunneling for in the continuum limit, and second, we
investigate first-order phase transitions for in the infinite volume
limit. Compared with standard multicanonical simulations we obtain improvement
factors of several resp. of about one order of magnitude.Comment: 12 pages LaTex, 1 PS figure appended. FU-Berlin preprint FUB-HEP 9/9
Orientational order in dipolar fluids consisting of nonspherical hard particles
We investigate fluids of dipolar hard particles by a certain variant of
density-functional theory. The proper treatment of the long range of the
dipolar interactions yields a contribution to the free energy which favors
ferromagnetic order. This corrects previous theoretical analyses. We determine
phase diagrams for dipolar ellipsoids and spherocylinders as a function of the
aspect ratio of the particles and their dipole moment. In the nonpolar limit
the results for the phase boundary between the isotropic and nematic phase
agree well with simulation data. Adding a longitudinal dipole moment favors the
nematic phase. For oblate or slightly elongated particles we find a
ferromagnetic liquid phase, which has also been detected in computer
simulations of fluids consisting of spherical dipolar particles. The detailed
structure of the phase diagram and its evolution upon changing the aspect ratio
are discussed in detail.Comment: 35 pages LaTeX with epsf style, 11 figures in eps format, submitted
to Phys. Rev.
On the nature of the so-called generic instabilities in dissipative relativistic hydrodynamics
It is shown that the so-called generic instabilities that appear in the
framework of relativistic linear irreversible thermodynamics, describing the
fluctuations of a simple fluid close to equilibrium, arise due to the coupling
of heat with hydrodynamic acceleration which appears in Eckart's formalism of
relativistic irreversible thermodynamics. Further, we emphasize that such
behavior should be interpreted as a contradiction to the postulates of linear
irreversible thermodynamics (LIT), namely a violation of Onsager's hypothesis
on the regression of fluctuations, and not as fluid instabilities. Such
contradictions can be avoided within a relativistic linear framework if a
Meixner-like approach to the phenomenological equations is employed.Comment: 13 pages, no figures. Accepted for publication in GR
Deconstructing classical water models at interfaces and in bulk
Using concepts from perturbation and local molecular field theories of
liquids we divide the potential of the SPC/E water model into short and long
ranged parts. The short ranged parts define a minimal reference network model
that captures very well the structure of the local hydrogen bond network in
bulk water while ignoring effects of the remaining long ranged interactions.
This deconstruction can provide insight into the different roles that the local
hydrogen bond network, dispersion forces, and long ranged dipolar interactions
play in determining a variety of properties of SPC/E and related classical
models of water. Here we focus on the anomalous behavior of the internal
pressure and the temperature dependence of the density of bulk water. We
further utilize these short ranged models along with local molecular field
theory to quantify the influence of these interactions on the structure of
hydrophobic interfaces and the crossover from small to large scale hydration
behavior. The implications of our findings for theories of hydrophobicity and
possible refinements of classical water models are also discussed
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