603 research outputs found
Swinging and Tumbling of Fluid Vesicles in Shear Flow
The dynamics of fluid vesicles in simple shear flow is studied using
mesoscale simulations of dynamically-triangulated surfaces, as well as a
theoretical approach based on two variables, a shape parameter and the
inclination angle, which has no adjustable parameters. We show that between the
well-known tank-treading and tumbling states, a new ``swinging'' state can
appear. We predict the dynamic phase diagram as a function of the shear rate,
the viscosities of the membrane and the internal fluid, and the reduced vesicle
volume. Our results agree well with recent experiments.Comment: 4 pages, 4 figure
Dynamic regimes of fluids simulated by multiparticle-collision dynamics
We investigate the hydrodynamic properties of a fluid simulated with a
mesoscopic solvent model. Two distinct regimes are identified, the `particle
regime' in which the dynamics is gas-like, and the `collective regime' where
the dynamics is fluid-like. This behavior can be characterized by the Schmidt
number, which measures the ratio between viscous and diffusive transport.
Analytical expressions for the tracer diffusion coefficient, which have been
derived on the basis of a molecular-chaos assumption, are found to describe the
simulation data very well in the particle regime, but important deviations are
found in the collective regime. These deviations are due to hydrodynamic
correlations. The model is then extended in order to investigate self-diffusion
in colloidal dispersions. We study first the transport properties of heavy
point-like particles in the mesoscopic solvent, as a function of their mass and
number density. Second, we introduce excluded-volume interactions among the
colloidal particles and determine the dependence of the diffusion coefficient
on the colloidal volume fraction for different solvent mean-free paths. In the
collective regime, the results are found to be in good agreement with previous
theoretical predictions based on Stokes hydrodynamics and the Smoluchowski
equation.Comment: 15 pages, 15 figure
Conformations, hydrodynamic interactions, and instabilities of sedimenting semiflexible filaments
The conformations and dynamics of semiflexible filaments subject to a
homogeneous external (gravitational) field, e.g., in a centrifuge, are studied
numerically and analytically. The competition between hydrodynamic drag and
bending elasticity generates new shapes and dynamical features. We show that
the shape of a semiflexible filament undergoes instabilities as the external
field increases. We identify two transitions that correspond to the excitation
of higher bending modes. In particular, for strong fields the filament
stabilizes in a non-planar shape, resulting in a sideways drift or in helical
trajectories. For two interacting filaments, we find the same transitions, with
the important consequence that the new non-planar shapes have an effective
hydrodynamic repulsion, in contrast to the planar shapes which attract
themselves even when their osculating planes are rotated with respect to each
other. For the case of planar filaments, we show analytically and numerically
that the relative velocity is not necessarily due to a different drag of the
individual filaments, but to the hydrodynamic interactions induced by their
shape asymmetry.Comment: 9 pages, 7 figures in Soft Matter (2015
Stability of bicontinuous cubic phases in ternary amphiphilic systems with spontaneous curvature
We study the phase behavior of ternary amphiphilic systems in the framework
of a curvature model with non-vanishing spontaneous curvature. The amphiphilic
monolayers can arrange in different ways to form micellar, hexagonal, lamellar
and various bicontinuous cubic phases. For the latter case we consider both
single structures (one monolayer) and double structures (two monolayers). Their
interfaces are modeled by the triply periodic surfaces of constant mean
curvature of the families G, D, P, C(P), I-WP and F-RD. The stability of the
different bicontinuous cubic phases can be explained by the way in which their
universal geometrical properties conspire with the concentration constraints.
For vanishing saddle-splay modulus , almost every phase considered
has some region of stability in the Gibbs triangle. Although bicontinuous cubic
phases are suppressed by sufficiently negative values of the saddle-splay
modulus , we find that they can exist for considerably lower
values than obtained previously. The most stable bicontinuous cubic phases with
decreasing are the single and double gyroid structures since
they combine favorable topological properties with extreme volume fractions.Comment: Revtex, 23 pages with 10 Postscript files included, to appear in J.
Chem. Phys. 112 (6) (February 2000
Fluctuating shells under pressure
Thermal fluctuations strongly modify the large length-scale elastic behavior
of crosslinked membranes, giving rise to scale-dependent elastic moduli. While
thermal effects in flat membranes are well understood, many natural and
artificial microstructures are modeled as thin elastic {\it shells}. Shells are
distinguished from flat membranes by their nonzero curvature, which provides a
size-dependent coupling between the in-plane stretching modes and the
out-of-plane undulations. In addition, a shell can support a pressure
difference between its interior and exterior. Little is known about the effect
of thermal fluctuations on the elastic properties of shells. Here, we study the
statistical mechanics of shape fluctuations in a pressurized spherical shell
using perturbation theory and Monte Carlo computer simulations, explicitly
including the effects of curvature and an inward pressure. We predict novel
properties of fluctuating thin shells under point indentations and
pressure-induced deformations. The contribution due to thermal fluctuations
increases with increasing ratio of shell radius to thickness, and dominates the
response when the product of this ratio and the thermal energy becomes large
compared to the bending rigidity of the shell. Thermal effects are enhanced
when a large uniform inward pressure acts on the shell, and diverge as this
pressure approaches the classical buckling transition of the shell. Our results
are relevant for the elasticity and osmotic collapse of microcapsules.Comment: To appear in PNAS; accepted version including Supplementary
Informatio
Defense mechanisms of empathetic players in the spatial ultimatum game
Experiments on the ultimatum game have revealed that humans are remarkably
fond of fair play. When asked to share an amount of money, unfair offers are
rare and their acceptance rate small. While empathy and spatiality may lead to
the evolution of fairness, thus far considered continuous strategies have
precluded the observation of solutions that would be driven by pattern
formation. Here we introduce a spatial ultimatum game with discrete strategies,
and we show that this simple alteration opens the gate to fascinatingly rich
dynamical behavior. Besides mixed stationary states, we report the occurrence
of traveling waves and cyclic dominance, where one strategy in the cycle can be
an alliance of two strategies. The highly webbed phase diagram, entailing
continuous and discontinuous phase transitions, reveals hidden complexity in
the pursuit of human fair play.Comment: 4 two-column pages, 5 figures; accepted for publication in Physical
Review Letter
Fluctuations of a long, semiflexible polymer in a narrow channel
We consider an inextensible, semiflexible polymer or worm-like chain, with
persistence length and contour length , fluctuating in a cylindrical
channel of diameter . In the regime , corresponding to a long,
tightly confined polymer, the average length of the channel
occupied by the polymer and the mean square deviation from the average vary as
and , respectively, where
and are dimensionless amplitudes. In earlier work
we determined and the analogous amplitude for a
channel with a rectangular cross section from simulations of very long chains.
In this paper we estimate and from the simulations.
The estimates are compared with exact analytical results for a semiflexible
polymer confined in the transverse direction by a parabolic potential instead
of a channel and with a recent experiment. For the parabolic confining
potential we also obtain a simple analytic result for the distribution of
or radial distribution function, which is asymptotically exact
for large and has the skewed shape seen experimentally.Comment: 21 pages, including 4 figure
The lamellar-to-isotropic transition in ternary amphiphilic systems
We study the dependence of the phase behavior of ternary amphiphilic systems
on composition and temperature. Our analysis is based on a curvature elastic
model of the surfactant film with sufficiently large spontaneous curvature and
sufficiently negative saddle-splay modulus that the stable phases are the
lamellar phase and a droplet microemulsion. In addition to the curvature
energy, we consider the contributions to the free energy of the long-ranged van
der Waals interaction and of the undulation modes. We find that for bending
rigidities of order k_B T, the lamellar phase extends further and further into
the water apex of the phase diagram as the phase inversion temperature is
approached, in good agreement with experimental results.Comment: LaTeX2e, 11 pages with references and 2 eps figures included,
submitted to Europhys. Let
Stress Tensors of Multiparticle Collision Dynamics Fluids
Stress tensors are derived for the multiparticle collision dynamics
algorithm, a particle-based mesoscale simulation method for fluctuating fluids,
resembling those of atomistic or molecular systems. Systems with periodic
boundary conditions as well as fluids confined in a slit are considered. For
every case, two equivalent expressions for the tensor are provided, the
internal stress tensor, which involves all degrees of freedom of a system, and
the external stress, which only includes the interactions with the confining
surfaces. In addition, stress tensors for a system with embedded particles are
determined. Based on the derived stress tensors, analytical expressions are
calculated for the shear viscosity. Simulations illustrate the difference in
fluctuations between the various derived expressions and yield very good
agreement between the numerical results and the analytically derived expression
for the viscosity
Solvent-free coarse-grained lipid model for large-scale simulations
A coarse-grained molecular model, which consists of a spherical particle and
an orientation vector, is proposed to simulate lipid membrane on a large length
scale. The solvent is implicitly represented by an effective attractive
interaction between particles. A bilayer structure is formed by
orientation-dependent (tilt and bending) potentials. In this model, the
membrane properties (bending rigidity, line tension of membrane edge, area
compression modulus, lateral diffusion coefficient, and flip-flop rate) can be
varied over broad ranges. The stability of the bilayer membrane is investigated
via droplet-vesicle transition. The rupture of the bilayer and worm-like
micelle formation can be induced by an increase in the spontaneous curvature of
the monolayer membrane.Comment: 13 pages, 19 figure
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