7,668 research outputs found

### Phase transitions of nematic rubbers

Single crystal nematic elastomers undergo a transition from a strongly
ordered phase N to an "isotropic" phase I. We show that: (a) samples produced
under tension by the Finkelmann procedure are intrinsically anisotropic and
should show a small (temperature dependent) birefringence in the high
temperature I phase. (b) for the I->Ntransition via cooling there is a spinodal
limit but for the N->I transition via heating there is no soft mode at the
standard spinodal temperature. (c) the N->I transition is reminiscent of a
martensitic transformation: nucleation of the I phase should occur in the form
of platelets, making a well defined angle with the director.Comment: 7 pages, 3 figures (To appear in Europhys. Lett.

### 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

### Nucleation of the crystalline phase of proteins in the presence of semidilute non-adsorbing polymer

Starting from a protein solution which is metastable with respect to the
crystalline phase, the effect of adding semidilute non-adsorbing polymer is
considered. It is found to increase the chemical potential of the protein by a
few tenths of kT, which may be enough to lower the barrier to nucleation of the
crystalline phase by enough to allow crystallisation. It is also shown that
assuming that the polymer induces a pairwise additive attraction leads to
qualitatively incorrect results.Comment: 5 pages, 1 figur

### Scaling theory for the free-energy barrier to homogeneous nucleation of a non-critical phase near a critical point

Homogeneous nucleation of a new phase near an Ising-like critical point of
another phase transition is studied. A scaling analysis shows that the free
energy barrier to nucleation contains a singular term with the same scaling as
the order parameter associated with the critical point. The total magnetisation
of the nucleus scales as the response function and so it diverges.
Vapour-liquid critical points are in the Ising universality class and so our
results imply that near such a critical point the number of molecules in a
nucleus of a another phase, such as a crystalline phase, diverges as the
isothermal compressibility. The case where symmetry prevents coupling between
the nucleus and the order parameter is also considered.Comment: 7 pages including 2 figures (revision adds consideration of nuclei
which do not couple to the order parameter and some dynamic scaling

### 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

### 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

### Ginzburg-Landau theory of dirty two band $s_{\pm}$ superconductors

In this paper we study the effect of non-magnetic impurities on two-band
$s_{\pm}$ 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
$T_c<<\tau^{-1}_t$, where $\tau_t\sim$ 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 $T_c\geq\tau^{-1}_t$, 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

### Bulk and surface biaxiality in nematic liquid crystals

Nematic liquid crystals possess three different phases: isotropic, uniaxial,
and biaxial. The ground state of most nematics is either isotropic or uniaxial,
depending on the external temperature. Nevertheless, biaxial domains have been
frequently identified, especially close to defects or external surfaces. In
this paper we show that any spatially-varying director pattern may be a source
of biaxiality. We prove that biaxiality arises naturally whenever the symmetric
tensor \Sb=(\grad \nn)(\grad \nn)^T possesses two distinct nonzero
eigenvalues. The eigenvalue difference may be used as a measure of the expected
biaxiality. Furthermore, the corresponding eigenvectors indicate the directions
in which the order tensor \QQ is induced to break the uniaxial symmetry about
the director \nn. We apply our general considerations to some examples. In
particular we show that, when we enforce homeotropic anchoring on a curved
surface, the order tensor become biaxial along the principal directions of the
surface. The effect is triggered by the difference in surface principal
curvatures

### 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

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