467 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
Adsorption of polyampholytes on charged surfaces
We have studied the adsorption of neutral polyampholytes on model charged
surfaces that have been characterized by contact angle and streaming current
measurements. The loop size distributions of adsorbed polymer chains have been
obtained using atomic force microscopy (AFM) and compared to recent theoretical
predictions. We find a qualitative agreement with theory; the higher the
surface charge, the smaller the number of monomers in the adsorbed layer, in
agreement with theory. We propose an original scenario for the adsorption of
polyampholytes on surfaces covered with both neutral long-chain and charged
short-chain thiols.Comment: 11 pages, 17 figures, accepted for publication in EPJ
Casimir stresses in active nematic films
We calculate the Casimir stresses in a thin layer of active fluid with nematic order. By using a stochastic hydrodynamic approach for an active fluid layer of finite thickness L, we generalize the Casimir stress for nematic liquid crystals in thermal equilibrium to active systems. We show that the active Casimir stress differs significantly from its equilibrium counterpart. For contractile activity, the active Casimir stress, although attractive like its equilibrium counterpart, diverges logarithmically as L approaches a threshold of the spontaneous flow instability from below. In contrast, for small extensile activity, it is repulsive, has no divergence at any L and has a scaling with L different from its equilibrium counterpart
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
Influence of sequence correlations on the adsorption of random copolymers onto homogeneous planar surfaces
Using a reference system approach, we develop an analytical theory for the
adsorption of random heteropolymers with exponentially decaying and/or
oscillating sequence correlations on planar homogeneous surfaces. We obtain a
simple equation for the adsorption-desorption transition line. This result as
well as the validity of the reference system approach is tested by a comparison
with numerical lattice calculations
Polymer adsorption onto random planar surfaces: Interplay of polymer and surface correlation
We study the adsorption of homogeneous or heterogeneous polymers onto
heterogeneous planar surfaces with exponentially decaying site-site
correlations, using a variational reference system approach. As a main result,
we derive simple equations for the adsorption-desorption transition line. We
show that the adsorption threshold is the same for systems with quenched and
annealed disorder. The results are discussed with respect to their implications
for the physics of molecular recognition
Collective dynamics of molecular motors pulling on fluid membranes
The collective dynamics of weakly coupled processive molecular motors are
considered theoretically. We show, using a discrete lattice model, that the
velocity-force curves strongly depend on the effective dynamic interactions
between motors and differ significantly from a simple mean field prediction.
They become essentially independent of if it is large enough. For strongly
biased motors such as kinesin this occurs if . The study of a
two-state model shows that the existence of internal states can induce
effective interactions.Comment: 5 pages, 5 figure
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