7,671 research outputs found
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
Dynamical regimes and hydrodynamic lift of viscous vesicles under shear
The dynamics of two-dimensional viscous vesicles in shear flow, with
different fluid viscosities and inside and
outside, respectively, is studied using mesoscale simulation techniques.
Besides the well-known tank-treading and tumbling motions, an oscillatory
swinging motion is observed in the simulations for large shear rate. The
existence of this swinging motion requires the excitation of higher-order
undulation modes (beyond elliptical deformations) in two dimensions.
Keller-Skalak theory is extended to deformable two-dimensional vesicles, such
that a dynamical phase diagram can be predicted for the reduced shear rate and
the viscosity contrast . The simulation results
are found to be in good agreement with the theoretical predictions, when
thermal fluctuations are incorporated in the theory. Moreover, the hydrodynamic
lift force, acting on vesicles under shear close to a wall, is determined from
simulations for various viscosity contrasts. For comparison, the lift force is
calculated numerically in the absence of thermal fluctuations using the
boundary-integral method for equal inside and outside viscosities. Both methods
show that the dependence of the lift force on the distance of
the vesicle center of mass from the wall is well described by an effective
power law for intermediate distances with vesicle radius .
The boundary-integral calculation indicates that the lift force decays
asymptotically as far from the wall.Comment: 13 pages, 13 figure
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
Dynamic Modes of Microcapsules in Steady Shear Flow: Effects of Bending and Shear Elasticities
The dynamics of microcapsules in steady shear flow was studied using a
theoretical approach based on three variables: The Taylor deformation parameter
, the inclination angle , and the phase angle of
the membrane rotation. It is found that the dynamic phase diagram shows a
remarkable change with an increase in the ratio of the membrane shear and
bending elasticities. A fluid vesicle (no shear elasticity) exhibits three
dynamic modes: (i) Tank-treading (TT) at low viscosity of
internal fluid ( and relaxes to constant values), (ii)
Tumbling (TB) at high ( rotates), and (iii) Swinging
(SW) at middle and high shear rate (
oscillates). All of three modes are accompanied by a membrane ()
rotation. For microcapsules with low shear elasticity, the TB phase with no
rotation and the coexistence phase of SW and TB motions are induced by
the energy barrier of rotation. Synchronization of rotation with
TB rotation or SW oscillation occurs with integer ratios of rotational
frequencies. At high shear elasticity, where a saddle point in the energy
potential disappears, intermediate phases vanish, and either or
rotation occurs. This phase behavior agrees with recent simulation results of
microcapsules with low bending elasticity.Comment: 11 pages, 14 figure
A Constant Bar Fraction out to Redshift z~1 in the Advanced Camera for Surveys Field of the Tadpole Galaxy
Bar-like structures were investigated in a sample of 186 disk galaxies larger
than 0.5 arcsec that are in the I-band image of the Tadpole galaxy taken with
the HST ACS. We found 22 clear cases of barred galaxies, 21 galaxies with small
bars that appear primarily as isophotal twists in a contour plot, and 11 cases
of peculiar bars in clump-cluster galaxies, which are face-on versions of chain
galaxies. The latter bars are probably young, as the galaxies contain only weak
interclump emission. Four of the clearly barred galaxies at z~0.8-1.2 have
grand design spirals. The bar fraction was determined as a function of galaxy
inclination and compared with the analogous distribution in the local Universe.
The bar fraction was also determined as a function of galaxy angular size.
These distributions suggest that inclination and resolution effects obscure
nearly half of the bars in our sample. The bar fraction was also determined as
a function of redshift. We found a nearly constant bar fraction of 0.23+-0.03
from z~0 to z=1.1. When corrected for inclination and size effects, this
fraction is comparable to the bar fraction in the local Universe, ~0.4, as
tabulated for all bar and Hubble types in the Third Reference Catalogue of
Galaxies. The average major axis of a barred galaxy in our sample is ~10 kpc
after correcting for redshift with a LambdaCDM cosmology. Galaxy bars were
present in normal abundance at least ~8 Gy ago (z~1); bar dissolution cannot be
common during a Hubble time unless the bar formation rate is comparable to the
dissolution rate.Comment: to appear in ApJ, Sept 1, 2004, Vol 612, 18 pg, 12 figure
The Advantage of Increased Resolution in the Study of Quasar Absorption Systems
We compare a new R = 120,000 spectrum of PG1634+706 (z_QSO = 1.337,m_V =
14.9) obtained with the HDS instrument on Subaru to a R = 45, 000 spectrum
obtained previously with HIRES/Keck. In the strong MgII system at z = 0.9902
and the multiple cloud, weak MgII system at z = 1.0414, we find that at the
higher resolution, additional components are resolved in a blended profile. We
find that two single-cloud weak MgII absorbers were already resolved at R =
45,000, to have b = 2 - 4 km/s. The narrowest line that we measure in the R =
120, 000 spectrum is a component of the Galactic NaI absorption, with b =
0.90+/-0.20 km/s. We discuss expectations of similarly narrow lines in various
applications, including studies of DLAs, the MgI phases of strong MgII
absorbers, and high velocity clouds. By applying Voigt profile fitting to
synthetic lines, we compare the consistency with which line profile parameters
can be accurately recovered at R = 45,000 and R = 120,000. We estimate the
improvement gained from superhigh resolution in resolving narrowly separated
velocity components in absorption profiles. We also explore the influence of
isotope line shifts and hyperfine splitting in measurements of line profile
parameters, and the spectral resolution needed to identify these effects. Super
high resolution spectra of quasars, which will be routinely possible with
20-meter class telescopes, will lead to greater sensitivity for absorption line
surveys, and to determination of more accurate physical conditions for cold
phases of gas in various environments.Comment: To appear in AJ. Paper with better resolution images available at
http://www.astro.psu.edu/users/anand/superhigh.AJ.pd
Kinematic Effects of Tidal Interaction on Galaxy Rotation Curves
We use self-consistent N-body models, in conjunction with models of test
particles moving in galaxy potentials, to explore the initial effects of
interactions on the rotation curves of spiral galaxies. Using nearly
self-consistent disk/bulge/halo galaxy models (Kuijken & Dubinski 1995), we
simulate the first pass of galaxies on nearly parabolic orbits; we vary orbit
inclinations, galaxy halo masses and impact parameters. For each simulation, we
mimic observed rotation curves of the model galaxies. Transient
interaction-induced features of the curves include distinctly rising or falling
profiles at large radii and pronounced bumps in the central regions. Remarkably
similar features occur in our statistical sample of optical emission-line
rotation curves of spiral galaxies in tight pairs and n-tuples.Comment: 9 pages, 2 figures, accepted for publication in ApJ Letter
Dynamics of Fluid Vesicles in Oscillatory Shear Flow
The dynamics of fluid vesicles in oscillatory shear flow was studied using
differential equations of two variables: the Taylor deformation parameter and
inclination angle . In a steady shear flow with a low viscosity
of internal fluid, the vesicles exhibit steady tank-treading
motion with a constant inclination angle . In the oscillatory flow
with a low shear frequency, oscillates between or
around for zero or finite mean shear rate ,
respectively. As shear frequency increases, the vesicle
oscillation becomes delayed with respect to the shear oscillation, and the
oscillation amplitude decreases. At high with , another limit-cycle oscillation between and
is found to appear. In the steady flow, periodically rotates
(tumbling) at high , and and the vesicle shape
oscillate (swinging) at middle and high shear rate. In the
oscillatory flow, the coexistence of two or more limit-cycle oscillations can
occur for low in these phases. For the vesicle with a fixed shape,
the angle rotates back to the original position after an oscillation
period. However, it is found that a preferred angle can be induced by small
thermal fluctuations.Comment: 11 pages, 13 figure
The regeneration of stellar bars by tidal interactions. Numerical simulations of fly-by encounters
We study the regeneration of stellar bars triggered by a tidal interaction,
using numerical simulations of either purely stellar or stellar+gas disc
galaxies. We find that interactions which are sufficiently strong to regenerate
the bar in the purely stellar models do not lead to a regeneration in the
dissipative models, owing to the induced gas inflow in those models. In models
in which the bar can be regenerated, we find a tight correlation between the
strength and the pattern speed of the induced bar. This relation can be
explained by a significant radial redistribution of angular momentum in the
disc due to the interaction, similar to the processes and correlations found
for isolated barred spirals. We furthermore show that the regenerated bars show
the same dynamical properties as their isolated counterparts.Comment: 18 pages, 26 figures, accepted for publication in MNRA
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