6,927 research outputs found
Polyhedral vesicles
Polyhedral vesicles with a large bending modulus of the membrane such as the
gel phase lipid membrane were studied using a Brownian dynamics simulation. The
vesicles exhibit various polyhedral morphologies such as tetrahedron and cube
shapes. We clarified two types of line defects on the edges of the polyhedrons:
cracks of both monolayers at the spontaneous curvature of monolayer , and a crack of the inner monolayer at . Around the
latter defect, the inner monolayer curves positively. Our results suggested
that the polyhedral morphology is controlled by .Comment: 4 pages, 5 figure
Dynamics of Fluid Vesicles in Shear Flow: Effect of Membrane Viscosity and Thermal Fluctuations
The dynamical behavior of vesicles is investigated in simple shear flow. A simulation technique is presented that combines a three-dimensional particle-based mesoscopic model (multiparticle collision dynamics) for the solvent with a dynamically triangulated surface model for the membrane. In this model, thermal fluctuations of the solvent and of the membrane are consistently taken into account. The membrane viscosity can be varied by changing the bond-flip rate of the dynamically triangulated surface. Vesicles are found to transit from steady tank-treading to unsteady tumbling motion with increasing membrane viscosity. At small reduced volumes, the shear induces a transformation from a discocyte to a prolate shape at low membrane viscosity. On the other hand, at high membrane viscosity, the shear induces a transformation from prolate to discocyte, or tumbling motion accompanied by shape oscillations between these two states. Thermal fluctuations induce intermittent tumbling and smooth out the transitions. This effect can be understood from a simplified stochastic model
Microscopic Theory of Current-Spin Interaction in Ferromagnets
Interplay between magnetization dynamics and electric current in a conducting
ferromagnet is theoretically studied based on a microscopic model calculation.
First, the effects of the current on magnetization dynamics (spin torques) are
studied with special attention to the "dissipative" torques arising from
spin-relaxation processes of conduction electrons. Next, an analysis is given
of the "spin motive force", namely, a spin-dependent 'voltage' generation due
to magnetization dynamics, which is the reaction to spin torques. Finally, an
attempt is presented of a unified description of these effects.Comment: Written in December 2008, published in July 200
The Frequency of Active and Quiescent Galaxies with Companions: Implications for the Feeding of the Nucleus
We analyze the idea that nuclear activity, either AGN or star formation, can
be triggered by interactions, studying the percentage of active, HII and
quiescent galaxies with companions. Our sample was selected from the Palomar
survey, and avoids selection biases faced by previous studies. The comparison
between the local galaxy density distributions showed that in most cases there
is no statistically significant difference among galaxies of different activity
types. The comparison of the percentage of galaxies with nearby companions
showed that there is a higher percentage of LINERs, transition, and absorption
line galaxies with companions than Seyferts and HII galaxies. However, we find
that when we consider only galaxies of similar morphological types (ellipticals
or spirals), there is no difference in the percentage of galaxies with
companions among different activity types, indicating that the former result
was due to the morphology-density effect. Also, only small differences are
found when we consider galaxies with similar Halpha luminosities. The
comparison between HII galaxies of different Halpha luminosities shows that
there is a significantly higher percentage of galaxies with companions among
the higher luminosity HII galaxies, indicating that interactions increase the
amount of circumnuclear star formation, in agreement with previous results. The
fact that we find that galaxies of different activity types have the same
percentage of companions, suggests that interactions between galaxies is not a
necessary condition to trigger the nuclear activity in AGNs. We compare our
results with previous ones and discuss their implications. (abridged)Comment: 30 pages, including 6 figures and 3 tables. To appear in The
Astronomical Journal, November issu
Robustly Unstable Eigenmodes of the Magnetoshearing Instability in Accretion Disk
The stability of nonaxisymmetric perturbations in differentially rotating
astrophysical accretion disks is analyzed by fully incorporating the properties
of shear flows. We verify the presence of discrete unstable eigenmodes with
complex and pure imaginary eigenvalues, without any artificial disk edge
boundaries, unlike Ogilvie & Pringle(1996)'s claim. By developing the
mathematical theory of a non-self-adjoint system, we investigate the nonlocal
behavior of eigenmodes in the vicinity of Alfven singularities at
omega_D=omega_A, where omega_D is the Doppler-shifted wave frequency and
omega_A=k_// v_A is the Alfven frequency. The structure of the spectrum of
discrete eigenmodes is discussed and the magnetic field and wavenumber
dependence of the growth rate are obtained. Exponentially growing modes are
present even in a region where the local dispersion relation theory claims to
have stable eigenvalues. The velocity field created by an eigenmode is
obtained, which explains the anomalous angular momentum transport in the
nonlinear stage of this stability.Comment: 11pages, 11figures, to be published in ApJ. For associated eps files,
see http://dino.ph.utexas.edu/~knoguchi
Two-Dimensional Fluctuating Vesicles in Linear Shear Flow
The stochastic motion of a two-dimensional vesicle in linear shear flow is
studied at finite temperature. In the limit of small deformations from a
circle, Langevin-type equations of motion are derived, which are highly
nonlinear due to the constraint of constant perimeter length. These equations
are solved in the low temperature limit and using a mean field approach, in
which the length constraint is satisfied only on average. The constraint
imposes non-trivial correlations between the lowest deformation modes at low
temperature. We also simulate a vesicle in a hydrodynamic solvent by using the
multi-particle collision dynamics technique, both in the quasi-circular regime
and for larger deformations, and compare the stationary deformation correlation
functions and the time autocorrelation functions with theoretical predictions.
Good agreement between theory and simulations is obtained.Comment: 13 pages, 7 figure
Reversibility of Red blood Cell deformation
The ability of cells to undergo reversible shape changes is often crucial to
their survival. For Red Blood Cells (RBCs), irreversible alteration of the cell
shape and flexibility often causes anemia. Here we show theoretically that RBCs
may react irreversibly to mechanical perturbations because of tensile stress in
their cytoskeleton. The transient polymerization of protein fibers inside the
cell seen in sickle cell anemia or a transient external force can trigger the
formation of a cytoskeleton-free membrane protrusion of micrometer dimensions.
The complex relaxation kinetics of the cell shape is shown to be responsible
for selecting the final state once the perturbation is removed, thereby
controlling the reversibility of the deformation. In some case, tubular
protrusion are expected to relax via a peculiar "pearling instability".Comment: 4 pages, 3 figure
Ordering and arrangement of deformed red blood cells in flow through microcapillaries
The shapes and alignment of elastic vesicles similar to red blood cells (RBCs) in cylindrical capillary flow are investigated by mesoscopic hydrodynamic simulations. We study the collective flow behavior of many RBCs, where the capillary diameter is comparable to the diameter of the RBCs. Two essential control parameters are the RBC volume fraction (the tube hematocrit, H-T), and the suspension flow velocity. Depending on H-T, flow velocity and capillary radius, the RBC suspension exhibits a disordered phase and two distinct ordered phases, consisting of a single file of parachute-shaped cells and a zigzag arrangement of slipper-shaped cells, respectively. We argue that thermal fluctuations, included in the simulation method, coupled to hydrodynamic flows are important contributors to the RBC morphology. We examine the changes to the phase structures when the capillary diameter and the material properties (bending rigidity kappa and stretching modulus mu) of the model RBCs are varied, constructing phase diagrams for each case. We focus on capillary diameters, which range from about 1.0 to about 1.4 times the RBC long diameter. For the smallest capillary diameter, the single-file arrangement dominates; for the largest diameter, the ordered zigzag arrangement begins to loose its stability and alternates with an asymmetric structure with two lanes of differently oriented cells. In simulations with long capillaries, the coexistence of different phases can be observed
The Nuclear Activity of the Galaxies in the Hickson Compact Groups
In order to investigate the nuclear activity of galaxies residing in compact
groups of galaxies, we present results of our optical spectroscopic program
made at Okayama Astrophysical Observatory. We have performed optical
spectroscopy of 69 galaxies which belong to 31 Hickson Compact Groups (HCGs) of
Galaxies. Among them, three galaxies have discordant redshifts. Further,
spectral quality is too poor to classify other three galaxies. Therefore, we
describe our results for the remaining 63 galaxies.
Our main results are summarized below. (1) We have found in our sample; 28
AGN, 16 HII nuclei, and 19 normal galaxies which show no emission line. We used
this HCG sample for statistical analyses. (2) Comparing the frequency
distributions of activity types between the HCGs and the field galaxies whose
data are taken from Ho, Filippenko, & Sargent (382 field galaxies), we find
that the frequency of HII nuclei in the HCGs is significantly less than that in
the field. However, this difference may be due to selection bias that our HCG
sample contains more early-type galaxies than the field, because it is known
that HII nuclei are rarer in early-type galaxies than in later ones. (3)
Applying correction this morphological bias to the HCG sample, we find that
there is no statistically significant difference in the frequency of occurrence
of emission-line galaxies between the HCGs and the field. This implies that the
dense galaxy environment in the HCGs does not affect triggering both the AGN
activity and the nuclear starburst. We discuss some implications on the nuclear
activity in the HCG galaxies.Comment: 33 pages (3 aasms4 LaTeX files), 5 figures (5 Postscript files:
excluded Figure 1), Accepted for publication in The Astronomical Journa
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