285 research outputs found
Tissue fusion over non-adhering surfaces
Tissue fusion eliminates physical voids in a tissue to form a continuous
structure and is central to many processes in development and repair. Fusion
events in vivo, particularly in embryonic development, often involve the
purse-string contraction of a pluricellular actomyosin cable at the free edge.
However in vitro, adhesion of the cells to their substrate favors a closure
mechanism mediated by lamellipodial protrusions, which has prevented a
systematic study of the purse-string mechanism. Here, we show that monolayers
can cover well-controlled mesoscopic non-adherent areas much larger than a cell
size by purse-string closure and that active epithelial fluctuations are
required for this process. We have formulated a simple stochastic model that
includes purse-string contractility, tissue fluctuations and effective friction
to qualitatively and quantitatively account for the dynamics of closure. Our
data suggest that, in vivo, tissue fusion adapts to the local environment by
coordinating lamellipodial protrusions and purse-string contractions
An theorem for Boltzmann's equation for the Yard-Sale Model of asset exchange
In recent work, Boltzmann and Fokker-Planck equations were derived for the
"Yard-Sale Model" of asset exchange. For the version of the model without
redistribution, it was conjectured, based on numerical evidence, that the
time-asymptotic state of the model was oligarchy -- complete concentration of
wealth by a single individual. In this work, we prove that conjecture by
demonstrating that the Gini coefficient, a measure of inequality commonly used
by economists, is an function of both the Boltzmann and Fokker-Planck
equations for the model.Comment: 10 pages, 1 figure, submitted to proceedings of the 23rd conference
on the Discrete Simulation of Fluid Dynamics (DSFD 2014
Co-transport-induced instability of membrane voltage in tip-growing cells
A salient feature of stationary patterns in tip-growing cells is the key role
played by the symports and antiports, membrane proteins that translocate two
ionic species at the same time. It is shown that these co-transporters
destabilize generically the membrane voltage if the two translocated ions
diffuse differently and carry a charge of opposite (same) sign for symports
(antiports). Orders of magnitude obtained for the time and lengthscale are in
agreement with experiments. A weakly nonlinear analysis characterizes the
bifurcation
Magnetic order in the Ising model with parallel dynamics
It is discussed how the equilibrium properties of the Ising model are
described by an Hamiltonian with an antiferromagnetic low temperature behavior
if only an heat bath dynamics, with the characteristics of a Probabilistic
Cellular Automaton, is assumed to determine the temporal evolution of the
system.Comment: 9 pages, 3 figure
Gyromagnetic ratio of rapidly rotating compact stars in general relativity
We numerically calculate equilibrium configurations of uniformly rotating and
charged neutron stars, in the case of insulating material and neglecting the
electromagnetic forces acting on the equilibrium of the fluid. This allows us
to study the behaviour of the gyromagnetic ratio for those objects, when
varying rotation rate and equation of state for the matter. Under the
assumption of low charge and incompressible fluid, we find that the
gyromagnetic ratio is directly proportional to the compaction parameter M/R of
the star, and very little dependent on its angular velocity. Nevertheless, it
seems impossible to have g=2 for these models with low charge-to-mass ratio,
where matter consists of a perfect fluid and where the collapse limit is never
reached.Comment: 11 pages, 6 figures, accepted for publication in Classical and
Quantum Gravit
Escape of the martian protoatmosphere and initial water inventory
Latest research in planet formation indicate that Mars formed within a few
million years (Myr) and remained a planetary embryo that never grew to a more
massive planet. It can also be expected from dynamical models, that most of
Mars' building blocks consisted of material that formed in orbital locations
just beyond the ice line which could have contained ~0.1-0.2 wt. % of H2O. By
using these constraints, we estimate the nebula-captured and catastrophically
outgassed volatile contents during the solidification of Mars' magma ocean and
apply a hydrodynamic upper atmosphere model for the study of the soft X-ray and
extreme ultraviolet (XUV) driven thermal escape of the martian protoatmosphere
during the early active epoch of the young Sun. The amount of gas that has been
captured from the protoplanetary disk into the planetary atmosphere is
calculated by solving the hydrostatic structure equations in the protoplanetary
nebula. Depending on nebular properties such as the dust grain depletion
factor, planetesimal accretion rates and luminosities, hydrogen envelopes with
masses >=3x10^{19} g to <=6.5x10^{22} g could have been captured from the
nebula around early Mars. Depending of the before mentioned parameters, due to
the planets low gravity and a solar XUV flux that was ~100 times stronger
compared to the present value, our results indicate that early Mars would have
lost its nebular captured hydrogen envelope after the nebula gas evaporated,
during a fast period of ~0.1-7.5 Myr. After the solidification of early Mars'
magma ocean, catastrophically outgassed volatiles with the amount of ~50-250
bar H2O and ~10-55 bar CO2 could have been lost during ~0.4-12 Myr, if the
impact related energy flux of large planetesimals and small embryos to the
planet's surface lasted long enough, that the steam atmosphere could have been
prevented from condensing. If this was not the case... (continued)Comment: 47 pages, 10 figures, 3 tables, submitted to PS
Nouvelle stratĂ©gie antibiofilm par dĂ©pĂŽt LBL dâun polyĂ©lectrolyte cationique sur la membrane de dialyse anionique AN69
Cette Ă©tude prĂ©sente une stratĂ©gie antibiofilm appliquĂ©e Ă une membrane de dialyse, lâAN69; il sâagit de rĂ©aliser une modification initiale de la surface de la membrane par un polyĂ©lectrolyte cationique, le poly(diallyldimĂ©thyl-ammonium), notĂ© PDADMA, selon un protocole de type « layer-by-layer ». Les caractĂ©ristiques physico-chimiques des deux membranes, lâAN69 et lâAN69 modifiĂ©e par le PDADMA, sont suivies par la dĂ©termination de la modification de la charge de la membrane en mettant en oeuvre des mesures de potentiels dâĂ©coulement et de nombres de transports de Li+, de permĂ©abilitĂ© hydraulique et des analyses morphologiques et topographiques effectuĂ©es par les microscopies Ă©lectroniques Ă balayage et Ă force atomique, respectivement avant et aprĂšs exposition Ă E. coli et Ă un biofilm marin. Nos rĂ©sultats montrent tout dâabord un rĂŽle majeur jouĂ© par lâattraction Ă©lectrostatique entre les microorganismes et lâAN69 modifiĂ©e par le PDADMA Ă lâorigine dâune adhĂ©sion forte des bactĂ©ries.Par ailleurs, nous avons mis au point un protocole original dâĂ©limination du biofilm marin. Ce protocole consiste Ă immerger la membrane modifiĂ©e par le PDADMA et encrassĂ©e, dans une solution de chlorure de sodium 2M afin dâĂ©cranter les charges Ă©lectrostatiques Ă lâorigine de lâaccroche du polyĂ©lectrolyte et permettre le dĂ©crochage du PDADMA qui entraĂźne avec lui lâencrassement (constituĂ© de bactĂ©ries et autres rĂ©sidus de biofilm). La recharge dâun film frais de PDADMA permet alors une rĂ©utilisation Ă lâinfini de la membrane AN69. La simplicitĂ© de ce protocole « de rĂ©gĂ©nĂ©ration » ouvre la possibilitĂ© dâune modification non permanente des membranes de dialyse, dans le but de limiter les problĂšmes rĂ©currents de biocolmatage et dâaugmenter les durĂ©es de vie des membranes en milieu marin.The harmful effects generated by biofilms have an extremely high cost. In this study, we were interested in the formation of marine biofilms forming on the wellknown anionic dialysis membrane AN69 used in the reduction of seawater salinity for the preparation of body-washing solutions. We focused our study on two membranes: AN69 and AN69 modified (denoted AN69 mod) by the sorption of poly(diallyldimethyl-ammonium), a cationic polyelectrolyte denoted by PDADMA. Physico-chemical characteristics (membrane charge from Li+ transport and streaming potential measurements, water flux from hydraulic permeability measurements and morphological/topographical measurements using SEM and AFM, respectively) were monitered during the exposure of both membranes to E. coli and natural marine biofouling solutions. Our results showed that the modification of the AN69 membrane by PDADMA its charge and increased its electrostatic affinity for bacteria. Electrostatics forces are, therefore, the main forces responsible for membrane biofouling. Furthermore we developed a simple antibiofilm protocol based on a one-step dipping operation applied to the fouled AN69 membrane which eliminates the PDADMA and associated bacteria. For the first time, all deposited marine bacteria were easily eliminated allowing for the regeneration and sustainable use of the AN69 membrane
Contraction of cross-linked actomyosin bundles
Cross-linked actomyosin bundles retract when severed in vivo by laser
ablation, or when isolated from the cell and micromanipulated in vitro in the
presence of ATP. We identify the time scale for contraction as a viscoelastic
time tau, where the viscosity is due to (internal) protein friction. We obtain
an estimate of the order of magnitude of the contraction time tau ~ 10-100 s,
consistent with available experimental data for circumferential microfilament
bundles and stress fibers. Our results are supported by an exactly solvable,
hydrodynamic model of a retracting bundle as a cylinder of isotropic, active
matter, from which the order of magnitude of the active stress is estimated.Comment: To be published in Physical Biolog
Nonlinear oscillator with parametric colored noise: some analytical results
The asymptotic behavior of a nonlinear oscillator subject to a multiplicative
Ornstein-Uhlenbeck noise is investigated. When the dynamics is expressed in
terms of energy-angle coordinates, it is observed that the angle is a fast
variable as compared to the energy. Thus, an effective stochastic dynamics for
the energy can be derived if the angular variable is averaged out. However, the
standard elimination procedure, performed earlier for a Gaussian white noise,
fails when the noise is colored because of correlations between the noise and
the fast angular variable. We develop here a specific averaging scheme that
retains these correlations. This allows us to calculate the probability
distribution function (P.D.F.) of the system and to derive the behavior of
physical observables in the long time limit
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