7,238 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
Pacman percolation: a model for enzyme gel degradation
We study a model for the gel degradation by an enzyme, where the gel is
schematized as a cubic lattice, and the enzyme as a random walker, that cuts
the bonds over which it passes. The model undergoes a (reverse) percolation
transition, which for low density of enzymes falls in a universality class
different from random percolation. In particular we have measured a gel
fraction critical exponent beta=1.0+-0.1, in excellent agreement with
experiments made on the real system.Comment: 4 pages, 7 eps 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
Ginzburg-Landau theory of dirty two band superconductors
In this paper we study the effect of non-magnetic impurities on two-band
superconductors by deriving the corresponding Ginzburg-Landau (GL)
equation. Depending on the strength of (impurity-induced) inter-band scattering
we find that there are two distinctive regions where the superconductors behave
very differently. In the strong impurity induced inter-band scattering regime
, where mean-life time an electron stays in one
band the two-band superconductor behaves as an effective one-band dirty
superconductor. In the other limit , the dirty two-band
superconductor is described by a network of frustrated two-band superconductor
grains connected by Josepshon tunnelling junctions. We argue that most pnictide
superconductors are in the later regime.Comment: 4 pages, 1 figur
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
Long-range Casimir interactions between impurities in nematic liquid crystals and the collapse of polymer chains in such solvents
The elastic interactions between objects embedded in a nematic liquid crystal
are usually caused by the average distorsion-rather than by the fluctuations-of
the nematic orientational field. We argue that for sufficiently small
particles, the nematic-mediated interaction originates purely from the
fluctuations of the nematic director. This Casimir interaction decays as
d^(-6), d being the distance between the particles, and it dominates van der
Waals interactions close to the isotropic-to-nematic transition. Considering
the nematic as a polymer solvent, we show that the onset of this Casimir
interaction at the isotropic-to-nematic transition can discontinuously induce
the collapse of a flexible polymer chain from the swollen state to the globular
state, without crossing the Theta-point.Comment: 6 pages, 1 figur
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
First-order scaling near a second-order phase transition: Tricritical polymer collapse
The coil-globule transition of an isolated polymer has been well established
to be a second-order phase transition described by a standard tricritical O(0)
field theory. We provide compelling evidence from Monte Carlo simulations in
four dimensions, where mean-field theory should apply, that the approach to
this (tri)critical point is dominated by the build-up of first-order-like
singularities masking the second-order nature of the coil-globule transition:
the distribution of the internal energy having two clear peaks that become more
distinct and sharp as the tricritical point is approached. However, the
distance between the peaks slowly decays to zero. The evidence shows that the
position of this (pseudo) first-order transition is shifted by an amount from
the tricritical point that is asymptotically much larger than the width of the
transition region. We suggest an explanation for the apparently contradictory
scaling predictions in the literature.Comment: 4 pages, 2 figures included in tex
First study of the three-gluon static potential in Lattice QCD
We estimate the potential energy for a system of three static gluons in
Lattice QCD. This is relevant for the different models of three-body glueballs
have been proposed in the literature, either for gluons with a constituent
mass, or for massless ones. A Wilson loop adequate to the static hybrid
three-body system is developed. We study different spacial geometries, to
compare the starfish model with the triangle model, for the three-gluon
potential. We also study two different colour structures, symmetric and
antisymmetric, and compare the respective static potentials. A first simulation
is performed in a periodic Lattice, with and fm.Comment: 8 pages, 10 figure
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