1,794 research outputs found
Two-loop Functional Renormalization Group of the Random Field and Random Anisotropy O(N) Models
We study by the perturbative Functional Renormalization Group (FRG) the
Random Field and Random Anisotropy O(N) models near , the lower critical
dimension of ferromagnetism. The long-distance physics is controlled by
zero-temperature fixed points at which the renormalized effective action is
nonanalytic. We obtain the beta functions at 2-loop order, showing that despite
the nonanalytic character of the renormalized effective action, the theory is
perturbatively renormalizable at this order. The physical results obtained at
2-loop level, most notably concerning the breakdown of dimensional reduction at
the critical point and the stability of quasi-long range order in , are
shown to fit into the picture predicted by our recent non-perturbative FRG
approach.Comment: 19 pages, 20 figures. Minor correction
DNA electrophoresis in designed channels
We present a simple description on the electrophoretic dynamics of
polyelectrolytes going through designed channels with narrow constrictions of
slit geometry. By analyzing rheological behaviours of the stuck chain, which is
coupled to the effect of solvent flow, three critical electric fields
(permeation field , deformation field and injection field , with polymerization
index) are clarified. Between and , the chain migration
is dictated by the driven activation process. In particular, at ,
the stuck chain at the slit entrance is strongly deformed, which enhances the
rate of the permeation. From these observations, electrophoretic mobility at a
given electric field is deduced, which shows non-monotonic dependence on .
For long enough chains, mobility increases with , in good agreement with
experiments. An abrupt change in the electrophoretic flow at a threshold
electric field is formally regarded as a nonequilibrium phase transition.Comment: 11 pages, 8 figure
Interfacial layering in a three-component polymer system
We study theoretically the temporal evolution and the spatial structure of
the interface between two polymer melts involving three different species (A,
A* and B). The first melt is composed of two different polymer species A and A*
which are fairly indifferent to one another (Flory parameter chi_AA* ~ 0). The
second melt is made of a pure polymer B which is strongly attracted to species
A (chi_AB 0). We then show
that, due to these contradictory tendencies, interesting properties arise
during the evolution of the interface after the melts are put into contact: as
diffusion proceeds, the interface structures into several adjacent
"compartments", or layers, of differing chemical compositions, and in addition,
the central mixing layer grows in a very asymmetric fashion. Such unusual
behaviour might lead to interesting mechanical properties, and demonstrates on
a specific case the potential richness of multi-component polymer interfaces
(as compared to conventional two-component interfaces) for various
applications.Comment: Revised version, to appear in Macromolecule
Straightening of Thermal Fluctuations in Semi-Flexible Polymers by Applied Tension
We investigate the propagation of a suddenly applied tension along a
thermally excited semi-flexible polymer using analytical approximations,
scaling arguments and numerical simulation. This problem is inherently
non-linear. We find sub-diffusive propagation with a dynamical exponent of 1/4.
By generalizing the internal elasticity, we show that tense strings exhibit
qualitatively different tension profiles and propagation with an exponent of
1/2.Comment: Latex file; with three postscript figures; .ps available at
http://dept.physics.upenn.edu/~nelson/pull.p
Monomer dynamics of a wormlike chain
We derive the stochastic equations of motion for a tracer that is tightly
attached to a semiflexible polymer and confined or agitated by an externally
controlled potential. The generalised Langevin equation, the power spectrum,
and the mean-square displacement for the tracer dynamics are explicitly
constructed from the microscopic equations of motion for a weakly bending
wormlike chain by a systematic coarse-graining procedure. Our accurate
analytical expressions should provide a convenient starting point for further
theoretical developments and for the analysis of various single-molecule
experiments and of protein shape fluctuations.Comment: 6 pages, 4 figure
Behavior of a polymer chain in a critical binary solvent
We present a field-theoretic renormalization group analysis of a polymer
chain immersed in a binary good solvent close to its critical demixing point.
We first show that this problem can be mapped on a bicritical field theory,
i.e. a -model with a mass anisotropy. This implies that the
end-to-end distance of the polymer is now controlled by a new critical exponent
related to the quadratic mass anisotropy operator . To show this
we solve the RG equation and calculate explicitly the exponents and the mean
end-to-end length of the chain.Comment: 6 pages, accepted in Europhys. Let
Probing structural relaxation in complex fluids by critical fluctuations
Complex fluids, such as polymer solutions and blends, colloids and gels, are
of growing interest in fundamental and applied soft-condensed-matter science. A
common feature of all such systems is the presence of a mesoscopic structural
length scale intermediate between atomic and macroscopic scales. This
mesoscopic structure of complex fluids is often fragile and sensitive to
external perturbations. Complex fluids are frequently viscoelastic (showing a
combination of viscous and elastic behaviour) with their dynamic response
depending on the time and length scales. Recently, non-invasive methods to
infer the rheological response of complex fluids have gained popularity through
the technique of microrheology, where the diffusion of probe spheres in a
viscoelastic fluid is monitored with the aid of light scattering or microscopy.
Here we propose an alternative to traditional microrheology that does not
require doping of probe particles in the fluid (which can sometimes drastically
alter the molecular environment). Instead, our proposed method makes use of the
phenomenon of "avoided crossing" between modes associated with the structural
relaxation and critical fluctuations that are spontaneously generated in the
system.Comment: 4 pages, 4 figure
One- and two-particle microrheology
We study the dynamics of rigid spheres embedded in viscoelastic media and
address two questions of importance to microrheology. First we calculate the
complete response to an external force of a single bead in a homogeneous
elastic network viscously coupled to an incompressible fluid. From this
response function we find the frequency range where the standard assumptions of
microrheology are valid. Second we study fluctuations when embedded spheres
perturb the media around them and show that mutual fluctuations of two
separated spheres provide a more accurate determination of the complex shear
modulus than do the fluctuations of a single sphere.Comment: 4 pages, 1 figur
Viscoelastic Effect on Hydrodynamic Relaxation in Polymer Solutions
The viscoelastic effect on the hydrodynamic relaxation in semidilute polymer
solutions is investigated. From the linearized two-fluid model equations, we
predict that the dynamical asymmetry coupling between the velocity fluctuations
and the viscoelastic stress influences on the hydrodynamic relaxation process,
resulting in a wave-number-dependent shear viscosity.Comment: 7pages; To be published in Journal of the Physical Society of
Japan,Vol 72,No2,(2003
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