212 research outputs found
A database for the industrial trawl fishery of Cote d'Ivoire
Fishery statistics for the industrial trawl fishery of Cote d'Ivoire have been well documented since 1968. However, data processing has changed significantly with time and some of the data files have been lost. In 1997, the Centre de Recherches Oceanologiques d'Abidjan decided to retrieve and process all trawl data available from different sources. This paper gives an overview of the database covering the period 1968 to 1997 and describes its coverage, format, structure and use. The database was developed using MS ACCESS and is a powerful tool for storing information about this fishery, and for analysis of its dynamics over a period of 30 years
Active-to-absorbing state phase transition in the presence of fluctuating environments: Weak and strong dynamic scaling
We investigate the scaling properties of phase transitions between survival
and extinction (active-to-absorbing state phase transition, AAPT) in a model,
that by itself belongs to the directed percolation (DP) universality class,
interacting with a spatio-temporally fluctuating environment having its own
non-trivial dynamics. We model the environment by (i) a randomly stirred fluid,
governed by the Navier-Stokes (NS) equation, and (ii) a fluctuating surface,
described either by the Kardar-Parisi-Zhang (KPZ) or the Edward-Wilkinson (EW)
equations. We show, by using a one-loop perturbative field theoretic set up,
that depending upon the spatial scaling of the variance of the external forces
that drive the environment (i.e., the NS, KPZ or EW equations), the system may
show {\em weak} or {\em strong dynamic scaling} at the critical point of active
to absorbing state phase transitions. In the former case AAPT displays scaling
belonging to the DP universality class, whereas in the latter case the
universal behavior is different.Comment: 17 pages, 2 figures, accepted in PR
Deviations from the mean field predictions for the phase behaviour of random copolymers melts
We investigate the phase behaviour of random copolymers melts via large scale
Monte Carlo simulations. We observe macrophase separation into A and B--rich
phases as predicted by mean field theory only for systems with a very large
correlation lambda of blocks along the polymer chains, far away from the
Lifshitz point. For smaller values of lambda, we find that a locally
segregated, disordered microemulsion--like structure gradually forms as the
temperature decreases. As we increase the number of blocks in the polymers, the
region of macrophase separation further shrinks. The results of our Monte Carlo
simulation are in agreement with a Ginzburg criterium, which suggests that mean
field theory becomes worse as the number of blocks in polymers increases.Comment: 6 pages, 4 figures, Late
Post-Tanner spreading of nematic droplets
The quasistationary spreading of a circular liquid drop on a solid substrate
typically obeys the so-called Tanner law, with the instantaneous base radius
R(t) growing with time as R ~ t^{1/10} -- an effect of the dominant role of
capillary forces for a small-sized droplet. However, for droplets of nematic
liquid crystals, a faster spreading law sets in at long times, so that R ~
t^alpha with alpha significantly larger than the Tanner exponent 1/10. In the
framework of the thin film model (or lubrication approximation), we describe
this "acceleration" as a transition to a qualitatively different spreading
regime driven by a strong substrate-liquid interaction specific to nematics
(antagonistic anchoring at the interfaces). The numerical solution of the thin
film equation agrees well with the available experimental data for nematics,
even though the non-Newtonian rheology has yet to be taken into account. Thus
we complement the theory of spreading with a post-Tanner stage, noting that the
spreading process can be expected to cross over from the usual
capillarity-dominated stage to a regime where the whole reservoir becomes a
diffusive film in the sense of Derjaguin.Comment: 15 pages, 4 figures, accepted in JPCM special issu
Soft Listeria: actin-based propulsion of liquid drops
We study the motion of oil drops propelled by actin polymerization in cell
extracts. Drops deform and acquire a pear-like shape under the action of the
elastic stresses exerted by the actin comet. We solve this free boundary
problem and calculate the drop shape taking into account the elasticity of the
actin gel and the variation of the polymerization velocity with normal stress.
The pressure balance on the liquid drop imposes a zero propulsive force if
gradients in surface tension or internal pressure are not taken into account.
Quantitative parameters of actin polymerization are obtained by fitting theory
to experiment.Comment: 5 pages, 4 figure
Fission of a multiphase membrane tube
A common mechanism for intracellular transport is the use of controlled
deformations of the membrane to create spherical or tubular buds. While the
basic physical properties of homogeneous membranes are relatively well-known,
the effects of inhomogeneities within membranes are very much an active field
of study. Membrane domains enriched in certain lipids in particular are
attracting much attention, and in this Letter we investigate the effect of such
domains on the shape and fate of membrane tubes. Recent experiments have
demonstrated that forced lipid phase separation can trigger tube fission, and
we demonstrate how this can be understood purely from the difference in elastic
constants between the domains. Moreover, the proposed model predicts timescales
for fission that agree well with experimental findings
A model of inversion of DNA charge by a positive polymer: fractionization of the polymer charge
Charge inversion of a DNA double helix by an oppositely charged flexible
polyelectrolyte (PE) is considered. We assume that, in the neutral state of the
DNA-PE complex, each of the DNA charges is locally compensated by a PE charge.
When an additional PE molecule is adsorbed by DNA, its charge gets fractionized
into monomer charges of defects (tails and arches) on the background of the
perfectly neutralized DNA. These charges spread all over the DNA eliminating
the self-energy of PE. This fractionization mechanism leads to a substantial
inversion of the DNA charge, a phenomenon which is widely used for gene
delivery.Comment: 4 pages, 2 figures. Improved figures and various corrections to tex
Pinning of a solid--liquid--vapour interface by stripes of obstacles
We use a macroscopic Hamiltonian approach to study the pinning of a
solid--liquid--vapour contact line on an array of equidistant stripes of
obstacles perpendicular to the liquid. We propose an estimate of the density of
pinning stripes for which collective pinning of the contact line happens. This
estimate is shown to be in good agreement with Langevin equation simulation of
the macroscopic Hamiltonian. Finally we introduce a 2--dimensional mean field
theory which for small strength of the pinning stripes and for small capillary
length gives an excellent description of the averaged height of the contact
line.Comment: Plain tex, 12 pages, 3 figures available upon reques
Molecular Weight Dependence of Spreading Rates of Ultrathin Polymeric Films
We study experimentally the molecular weight dependence of spreading
rates of molecularly thin precursor films, growing at the bottom of droplets of
polymer liquids. In accord with previous observations, we find that the radial
extension R(t) of the film grows with time as R(t) = (D_{exp} t)^{1/2}. Our
data substantiate the M-dependence of D_{exp}; we show that it follows D_{exp}
\sim M^{-\gamma}, where the exponent \gamma is dependent on the chemical
composition of the solid surface, determining its frictional properties with
respect to the molecular transport. In the specific case of hydrophilic
substrates, the frictional properties can be modified by the change of the
relative humidity (RH). We find that \gamma \approx 1 at low RH and tends to
zero when RH gets progressively increased. We propose simple theoretical
arguments which explain the observed behavior in the limits of low and high RH.Comment: 4 pages, 2 figures, to appear in PR
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