5,004 research outputs found
Dewetting dynamics of stressed viscoelastic thin polymer films
Ultrathin polymer films that are produced e.g. by spin-coating are believed
to be stressed since polymers are 'frozen in' into out-of-equilibrium
configurations during this process. In the framework of a viscoelastic thin
film model, we study the effects of lateral residual stresses on the dewetting
dynamics of the film. The temporal evolution of the height profiles and the
velocity profiles inside the film as well as the dissipation mechanisms are
investigated in detail. Both the shape of the profiles and the importance of
frictional dissipation vs. viscous dissipation inside the film are found to
change in the course of dewetting. The interplay of the non-stationary
profiles, the relaxing initial stress and changes in the dominance of the two
dissipation mechanisms caused by nonlinear friction with the substrate is
responsible for the rich behavior of the system. In particular, our analysis
sheds new light on the occurrence of the unexpected maximum in the rim width
obtained recently in experiments on PS-PDMS systems.Comment: 11 pages, 10 figure
Pseudo-Casimir force in confined nematic polymers
We investigate the pseudo-Casimir force in a slab of material composed of
nematically ordered long polymers. We write the total mesoscopic energy
together with the constraint connecting the local density and director
fluctuations and evaluate the corresponding fluctuation free energy by standard
methods. It leads to a pseudo-Casimir force of a different type than in the
case of standard, short molecule nematic. We investigate its separation
dependence and its magnitude and explicitly derive the relevant limiting cases.Comment: 7 pages, 2 figure
Thermodynamic approach to the dewetting instability in ultrathin films
The fluid dynamics of the classical dewetting instability in ultrathin films
is a non-linear process. However, the physical manifestation of the instability
in terms of characteristic length and time scales can be described by a
linearized form of the initial conditions of the films's dynamics. Alternately,
the thermodynamic approach based on equating the rate of free energy decrease
to the viscous dissipation [de Gennes, C. R. Acad. Paris.v298, 1984] can give
similar information. Here we have evaluated dewetting in the presence of
thermocapillary forces arising from a film-thickness (h) dependent temperature.
Such a situation can be found during pulsed laser melting of ultrathin metal
films where nanoscale effects lead to a local h-dependent temperature. The
thermodynamic approach provides an analytical description of this
thermocapillary dewetting. The results of this approach agree with those from
linear theory and experimental observations provided the minimum value of
viscous dissipation is equated to the rate of free energy decrease. The flow
boundary condition that produces this minimum viscous dissipation is when the
film-substrate tangential stress is zero. The physical implication of this
finding is that the spontaneous dewetting instability follows the path of
minimum rate of energy loss.Comment: 8 pages, 3 figures. Under revie
Solution of a model of SAW's with multiple monomers per site on the Husimi lattice
We solve a model of self-avoiding walks which allows for a site to be visited
up to two times by the walk on the Husimi lattice. This model is inspired in
the Domb-Joyce model and was proposed to describe the collapse transition of
polymers with one-site interactions only. We consider the version in which
immediate self-reversals of the walk are forbidden (RF model). The phase
diagram we obtain for the grand-canonical version of the model is similar to
the one found in the solution of the Bethe lattice, with two distinct
polymerized phases, a tricritical point and a critical endpoint.Comment: 16 pages, including 6 figure
Topological Constraints at the Theta Point: Closed Loops at Two Loops
We map the problem of self-avoiding random walks in a Theta solvent with a
chemical potential for writhe to the three-dimensional symmetric
U(N)-Chern-Simons theory as N goes to 0. We find a new scaling regime of
topologically constrained polymers, with critical exponents that depend on the
chemical potential for writhe, which gives way to a fluctuation-induced
first-order transition.Comment: 5 pages, RevTeX, typo
The QCD string tension curve, the ferromagnetic magnetization, and the quark-antiquark confining potential at finite Temperature
We study the string tension as a function of temperature, fitting the SU(3)
lattice QCD finite temperature free energy potentials computed by the Bielefeld
group. We compare the string tension points with order parameter curves of
ferromagnets, superconductors or string models, all related to confinement. We
also compare the SU(3) string tension with the one of SU(2) Lattice QCD. With
the curve providing the best fit to the finite temperature string tensions, the
spontaneous magnetization curve, we then show how to include finite
temperature, in the state of the art confining and chiral invariant quark
models.Comment: 9 pages, 12 figure
Stereo-selective swelling of imprinted cholesteric networks
Molecular chirality, and the chiral symmetry breaking of resulting
macroscopic phases, can be topologically imprinted and manipulated by
crosslinking and swelling of polymer networks. We present a new experimental
approach to stereo-specific separation of chiral isomers by using a cholesteric
elastomer in which a helical director distribution has been topological
imprinted by crosslinking. This makes the material unusual in that is has a
strong phase chirality, but no molecular chirality at all; we study the nature
and parameters controlling the twist-untwist transition. Adding a racemic
mixture to the imprinted network results in selective swelling by only the
component of ``correct'' handedness. We investigate the capacity of demixing in
a racemic environment, which depends on network parameters and the underlying
nematic order
Untwisting of a cholesteric elastomer by a mechanical field
A mechanical strain field applied to a monodomain cholesteric elastomer will
unwind the helical director distribution. There is an analogy with the
classical problem of an electric field applied to a cholesteric liquid crystal,
but with important differences. Frank elasticity is of minor importance unless
the gel is very weak. The interplay is between director anchoring to the rubber
elastic matrix and the external mechanical field. Stretching perpendicular to
the helix axis induces the uniform unwound state via the elimination of sharp,
pinned twist walls above a critical strain. Unwinding through conical director
states occurs when the elastomer is stretched along the helical axis.Comment: 4 pages, RevTeX 3 style, 3 EPS figure
Chirality transfer and stereo-selectivity of imprinted cholesteric networks
Imprinting of cholesteric textures in a polymer network is a method of
preserving a macroscopically chiral phase in a system with no molecular
chirality. By modifying the elastics properties of the network, the resulting
stored helical twist can be manipulated within a wide range since the
imprinting efficiency depends on the balance between the elastics constants and
twisting power at network formation. One spectacular property of phase
chirality imprinting is the created ability of the network to adsorb
preferentially one stereo-component from a racemic mixture. In this paper we
explore this property of chirality transfer from a macroscopic to the molecular
scale. In particular, we focus on the competition between the phase chirality
and the local nematic order. We demonstrate that it is possible to control the
subsequent release of chiral solvent component from the imprinting network and
the reversibility of the stereo-selective swelling by racemic solvents
Anomalous scaling and gapless fermions of d-wave superconductors in magnetic field
The problem of d-wave quasiparticles in a weakly disordered Abrikosov vortex
lattice is studied. Starting with a periodic lattice, the topological structure
of the magnetic crystal momenta of gapless fermions is found for the
particle-hole symmetric case. If in addition the site centered inversion
symmetry is present, both the location and the number of the gapless fermions
can be determined using an index theorem. In the case of spatially aperiodic
vortex array, Simon and Lee scaling is found to be violated due to a quantum
anomaly. The electronic density of states is found to scale with the
root-mean-square vortex displacement as \sqrt{H}f(\bu_{rms}^2/\xi^2), while
thermal conductivity is H-independent, but {\it different} from the H=0 case.Comment: 4 page
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