1,041 research outputs found

### Equilibrium Dynamics of Microemulsion and Sponge Phases

The dynamic structure factor $G({\bf k},\omega)$ is studied in a
time-dependent Ginzburg-Landau model for microemulsion and sponge phases in
thermal equilibrium by field-theoretic perturbation methods. In bulk contrast,
we find that for sufficiently small viscosity $\eta$, the structure factor
develops a peak at non-zero frequency $\omega$, for fixed wavenumber $k$ with
$k_0 < k {< \atop \sim} q$. Here, $2\pi/q$ is the typical domain size of oil-
and water-regions in a microemulsion, and $k_0 \sim \eta q^2$. This implies
that the intermediate scattering function, $G({\bf k}, t)$, {\it oscillates} in
time. We give a simple explanation, based on the Navier-Stokes equation, for
these temporal oscillations by considering the flow through a tube of radius $R
\simeq \pi/q$, with a radius-dependent tension.Comment: 24 pages, LaTex, 11 Figures on request; J. Phys. II France 4 (1994)
to be publishe

### Rheological properties of sheared vesicle and cell suspensions

Numerical simulations of vesicle suspensions are performed in two dimensions
to study their dynamical and rheological properties. An hybrid method is
adopted, which combines a mesoscopic approach for the solvent with a
curvature-elasticity model for the membrane. Shear flow is induced by two
counter-sliding parallel walls, which generate a linear flow profile. The flow
behavior is studied for various vesicle concentrations and viscosity ratios
between the internal and the external fluid. Both the intrinsic viscosity and
the thickness of depletion layers near the walls are found to increase with
increasing viscosity ratio.Comment: To be published in the DynaCaps 2014 Conference Proceedings (Procedia
IUTAM

### Dynamics and Rheology of Vesicle Suspensions in Wall-Bounded Shear Flow

The dynamics and rheology of suspensions of fluid vesicles or red blood cells
is investigated by a combination of molecular dynamics and mesoscale
hydrodynamics simulations in two dimensions. The vesicle suspension is confined
between two no-slip walls, which are driven externally to generate a shear flow
with shear rate $\dot\gamma$. The flow behavior is studied as a function of
$\dot\gamma$, the volume fraction of vesicles, and the viscosity contrast
between inside and outside fluids. Results are obtained for the encounter and
interactions of two vesicles, the intrinsic viscosity of the suspension, and
the cell-free layer near the walls.Comment: In press in EP

### Bending Frustration of Lipid-Water Mesophases Based on Cubic Minimal Surfaces

Inverse bicontinuous cubic phases are ubiquitous in lipid-water mixtures and
consist of a lipid bilayer forming a cubic minimal surface, thereby dividing
space into two cubic networks of water channels. For small hydrocarbon chain
lengths, the monolayers can be modeled as parallel surfaces to a minimal
midsurface. The bending energy of the cubic phases is determined by the
distribution of Gaussian curvature over the minimal midsurfaces which we
calculate for seven different structures (G, D, P, I-WP, C(P), S and F-RD). We
show that the free-energy densities of the structures G, D and P are
considerably lower than those of the other investigated structures due to their
narrow distribution of Gaussian curvature. The Bonnet transformation between G,
D, and P implies that these phases coexist along a triple line, which also
includes an excess water phase. Our model includes thermal membrane
undulations. Our qualitative predictions remain unchanged when higher order
terms in the curvature energy are included. Calculated phase diagrams agree
well with the experimental results for 2:1 lauric acid/dilauroyl
phosphatidylcholine and water.Comment: Revtex, 23 pages with 9 postscript figures included, to appear in
Langmui

### Analytic vortex solutions in an unusual Mexican hat potential

We introduce an unusual Mexican hat potential, a piecewise parabolic one, and
we show that its vortex solutions can be found analytically, in contrast to the
case of the standard Psi^4 field theory.Comment: 4 pages and 1 figure (missing in this version

### 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 $\bar \kappa$, 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 $\bar \kappa$, we find that they can exist for considerably lower
values than obtained previously. The most stable bicontinuous cubic phases with
decreasing $\bar \kappa < 0$ 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

### 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

### Swarm behavior of self-propelled rods and swimming flagella

Systems of self-propelled particles are known for their tendency to aggregate
and to display swarm behavior. We investigate two model systems, self-propelled
rods interacting via volume exclusion, and sinusoidally-beating flagella
embedded in a fluid with hydrodynamic interactions. In the flagella system,
beating frequencies are Gaussian distributed with a non-zero average. These
systems are studied by Brownian-dynamics simulations and by mesoscale
hydrodynamics simulations, respectively. The clustering behavior is analyzed as
the particle density and the environmental or internal noise are varied. By
distinguishing three types of cluster-size probability density functions, we
obtain a phase diagram of different swarm behaviors. The properties of
clusters, such as their configuration, lifetime and average size are analyzed.
We find that the swarm behavior of the two systems, characterized by several
effective power laws, is very similar. However, a more careful analysis reveals
several differences. Clusters of self-propelled rods form due to partially
blocked forward motion, and are therefore typically wedge-shaped. At higher rod
density and low noise, a giant mobile cluster appears, in which most rods are
mostly oriented towards the center. In contrast, flagella become
hydrodynamically synchronized and attract each other; their clusters are
therefore more elongated. Furthermore, the lifetime of flagella clusters decays
more quickly with cluster size than of rod clusters

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