332 research outputs found
Solid like friction of a polymer chain
We propose a simple friction model for isolated polymer chains on a solid
substrate. The chains are pulled at constant velocity by one end, the other end
can be trapped on the solid substrate on localised sites. We focus on the
energy dissipation due to the traps. This simple model leads to non trivial
friction laws, depending on the velocity and the distance between traps. Some
refinements of the model such as the effect of thermal fluctuation are also
reported.Comment: 16 pages, 4 eps figures, accepted for publuication in Eur. Phys. J. E
New version of 20/07/2000 minor modifications to figure
Compression of finite size polymer brushes
We consider edge effects in grafted polymer layers under compression. For a
semi-infinite brush, the penetration depth of edge effects is larger than the natural height and the actual height
. For a brush of finite lateral size (width of a stripe or radius of a
disk), the lateral extension of the border chains follows the scaling law
. The scaling function is estimated within
the framework of a local Flory theory for stripe-shaped grafting surfaces. For
small , decays as a power law in agreement with simple arguments.
The effective line tension and the variation with compression height of the
force applied on the brush are also calculated.Comment: 6 pages, 7 figures, submitted to PCC
Stretching necklaces
Polyelectrolytes in poor solvents show a necklace structure where collapsed
polymer pearls are linked to stretched strings. In the present paper the
elasticity of such chains is studied in detail. Different deformation regimes
are addressed. The first is the continuous regime, where many pearls are
present. A continuous force extension relation ship is calculated. The main
contribution comes from the tension balance and the electrostatic repulsion of
consecutive pearls. The main correction term stems from the finite size of the
pearls, which monitors their surface energy. For a finite amount of pearls
discontinuous stretching is predicted. Finally counterion effects are discussed
qualitatively.Comment: to appear in European Phys. Journal E (soft matter
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
Surface-mediated attraction between colloids
We investigate the equilibrium properties of a colloidal solution in contact
with a soft interface. As a result of symmetry breaking, surface effects are
generally prevailing in confined colloidal systems. In this Letter, particular
emphasis is given to surface fluctuations and their consequences on the local
(re)organization of the suspension. It is shown that particles experience a
significant effective interaction in the vicinity of the interface. This
potential of mean force is always attractive, with range controlled by the
surface correlation length. We suggest that, under some circumstances,
surface-induced attraction may have a strong influence on the local particle
distribution
Dissipation in Dynamics of a Moving Contact Line
The dynamics of the deformations of a moving contact line is studied assuming
two different dissipation mechanisms. It is shown that the characteristic
relaxation time for a deformation of wavelength of a contact line
moving with velocity is given as . The velocity
dependence of is shown to drastically depend on the dissipation
mechanism: we find for the case when the dynamics is governed
by microscopic jumps of single molecules at the tip (Blake mechanism), and
when viscous hydrodynamic losses inside the moving
liquid wedge dominate (de Gennes mechanism). We thus suggest that the debated
dominant dissipation mechanism can be experimentally determined using
relaxation measurements similar to the Ondarcuhu-Veyssie experiment [T.
Ondarcuhu and M. Veyssie, Nature {\bf 352}, 418 (1991)].Comment: REVTEX 8 pages, 9 PS figure
Generic phase diagram of active polar films
We study theoretically the phase diagram of compressible active polar gels
such as the actin network of eukaryotic cells. Using generalized hydrodynamics
equations, we perform a linear stability analysis of the uniform states in the
case of an infinite bidimensional active gel to obtain the dynamic phase
diagram of active polar films. We predict in particular modulated flowing
phases, and a macroscopic phase separation at high activity. This qualitatively
accounts for experimental observations of various active systems, such as
acto-myosin gels, microtubules and kinesins in vitro solutions, or swimming
bacterial colonies.Comment: 4 pages, 1 figur
Fluctuations of a driven membrane in an electrolyte
We develop a model for a driven cell- or artificial membrane in an
electrolyte. The system is kept far from equilibrium by the application of a DC
electric field or by concentration gradients, which causes ions to flow through
specific ion-conducting units (representing pumps, channels or natural pores).
We consider the case of planar geometry and Debye-H\"{u}ckel regime, and obtain
the membrane equation of motion within Stokes hydrodynamics. At steady state,
the applied field causes an accumulation of charges close to the membrane,
which, similarly to the equilibrium case, can be described with renormalized
membrane tension and bending modulus. However, as opposed to the equilibrium
situation, we find new terms in the membrane equation of motion, which arise
specifically in the out-of-equilibrium case. We show that these terms lead in
certain conditions to instabilities.Comment: 7 pages, 2 figures. submitted to Europhys. Let
Width distribution of contact lines on a disordered substrate
We have studied the roughness of a contact line of a liquid meniscus on a
disordered substrate by measuring its width distribution. The comparison
between the measured width distribution and the width distribution calculated
in previous works, extended here to the case of open boundary conditions,
confirms that the Joanny-de Gennes model is not sufficient to describe the
dynamics of contact lines at the depinning threshold. This conclusion is in
agreement with recent measurements which determine the roughness exponent by
extrapolation to large system sizes.Comment: 4 pages, 3 figure
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