1,655 research outputs found
Complexation between oppositely charged polyelectrolytes: beyond the Random Phase Approximation
We consider the phase behavior of polymeric systems by calculating the
structure factors beyond the Random Phase Approximation. The effect of this
correction to the mean-field RPA structure factor is shown to be important in
the case of coulombic systems. Two examples are given: simple electrolytes and
mixtures of incompatible oppositely charged polyelectrolytes. In this last
case, all former studies predicted an enhancement of compatibility for
increasing charge densities; we also describe the complexation transition
between the polyelectrolytes. We determine a phase diagram of the
polyelectrolyte mixture that includes both complexation and incompatibility.Comment: 18 pages, 4 figures. Submitted to EPJ-
Spontaneous creation of macroscopic flow and metachronal waves in an array of cilia
Cells or bacteria carrying cilia on their surface show many striking features
: alignment of cilia in an array, two-phase asymmetric beating for each cilium,
coordination between cilia and existence of metachronal waves with a constant
phase difference between two adjacent cilia. We give simple theoretical
arguments based on hydrodynamic coupling and an internal mechanism of the
cilium derived from the behavior of a collection of molecular motors, to
account qualitatively for these cooperative features. Hydrodynamic interactions
can lead to the alignment of an array of cilia. We study the effect of a
transverse external flow and obtain a two-phase asymmetrical beating, faster
along the flow and slower against the flow, proceeding around an average curved
position. We show that an aligned array of cilia is able to spontaneously break
the left-right symmetry and to create a global average flow. Metachronism
arises as a local minimum of the beating threshold and leads to a rather
constant flow
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
Phase separation and nucleation in mixtures of particles with different temperatures
Differences in activities in colloidal particles are sufficient to drive
phase separation between active and passive (or less active) particles, even if
they have only excluded volume interactions. In this paper, we study the phase
separation kinetics and propose a theory of phase separation of colloidal
mixtures in the diffusive limit. Our model considers a mixture of diffusing
particles coupled to different thermostats, it thus has a non-equilibrium
nature due to the temperature differences. However, we show that indeed the
system recovers an effective equilibrium thermodynamics in the dilute limit. We
obtain phase diagrams showing the asymmetry in concentrations due to activity
differences. By using a more general approach, we show the equivalence of phase
separation kinetics with the well known Cahn-Hilliard theory. On the other
hand, higher order expansions in concentration indicate the emergence of
non-equilibrium effects leading to a breakdown of the equilibrium analogy. We
lay out the general theory in terms of accessible parameters which we
demonstrate by several applications. In this simple formalism, we capture a
positive surface tension for hard spheres}, and interesting scaling laws for
interfacial properties, droplet growth dynamics, and phase segregation
conditions. \rev{Several of our results are in agreement with existing
numerical simulations while we also propose testable predictions.Comment: Published version, 19 pages (main text+appendix), 4 figure
Bistability, oscillations and bidirectional motion of ensemble of hydrodynamically-coupled molecular motors
We analyze the collective behavior of hydrodynamically coupled molecular
motors. We show that the local fluxes induced by motors displacement can induce
the experimentally observed bidirectional motion of cargoes and vesicles. By
means of a mean--field approach we show that sustained oscillations as well as
bistable collective motor motion arise even for very large collection of
motors, when thermal noise is irrelevant. The analysis clarifies the physical
mechanisms responsible for such dynamics by identifying the relevant coupling
parameter and its dependence on the geometry of the hydrodynamic coupling as
well as on system size. We quantify the phase diagram for the different phases
that characterize the collective motion of hydrodynamically coupled motors and
show that sustained oscillations can be reached for biologically relevant
parameters, hence demonstrating the relevance of hydrodynamic interactions in
intracellular transport
Charge Distribution on Annealed Polyelectrolytes
We investigate the equilibrium charge distribution along a single annealed
polyelectrolyte chain under different conditions. The coupling between the
conformation of the chain and the local charge distribution is described for
various solvent qualities and salt concentration. In salt free solution, we
find a slight charge depletion in the central part of the chain: the charges
accumulate at the ends. The effect is less important if salt is added to the
solution since the charge inhomogeneity is localized close to the chain ends
over a distance of order of the Debye length. In the case of poor solvent
conditions we find a different charging of beads and strings in the framework
of the necklace model. This inhomogeneity leads to a charge instability and a
first order transition between spherical globules and elongated chains.Comment: 20 pages, 4 figure
Motion of an Adhesive Gel in a Swelling Gradient: a Mechanism for Cell Locomotion
Motivated by the motion of nematode sperm cells, we present a model for the
motion of an adhesive gel on a solid substrate. The gel polymerizes at the
leading edge and depolymerizes at the rear. The motion results from a
competition between a self-generated swelling gradient and the adhesion on the
substrate. The resulting stress provokes the rupture of the adhesion points and
allows for the motion. The model predicts an unusual force-velocity relation
which depends in significant ways on the point of application of the force.Comment: 4 pages, 1 figur
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
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