267 research outputs found
Exponential Decay of Correlations in a Model for Strongly Disordered 2D Nematic Elastomers
Lattice Monte-Carlo simulations were performed to study the equilibrium
ordering in a two-dimensional nematic system with quenched random disorder.
When the disordering field, which competes against the aligning effect of the
Frank elasticity, is sufficiently strong, the long-range correlation of the
director orientation is found to decay as a simple exponential, Exp[-r/x]. The
correlation length {x} itself also decays exponentially with increasing
strength of the disordering field. This result represents a new type of
behavior, distinct from the Gaussian and power-law decays predicted by some
theories.Comment: Latex file (4 pages) + 2 EPS figure
Glycidyl-methacrylate-based electrospun mats and catalytic silver nanoparticles
P(AN-GMA) and PGMA fibers coated with monodisperse silver nanoparticles have been prepared by a combination of electrospinning and electroless plating. The morphology of the electrospun fibers remains unchanged after surface hydrazination. Oxidation of hydrazine in an ammoniacal solution of AgNO3 reduces and deposits silver atoms along the fiber surface, which then coalesce to Ag particles. The size of the silver nanoparticles is varied between 20-60 nm. Since the density of the active sites for silver reduction is lower in P(AN-GAAA), a smaller particle size could be obtained. The catalytic activity of the silver nanoparticles has been confirmed
Saturation in the quasiadiabatical limit: a time-dependent projection operator formalism approach
Quasi-long range order in the random anisotropy Heisenberg model
The large distance behaviors of the random field and random anisotropy
Heisenberg models are studied with the functional renormalization group in
dimensions. The random anisotropy model is found to have a phase
with the infinite correlation radius at low temperatures and weak disorder. The
correlation function of the magnetization obeys a power law . The
magnetic susceptibility diverges at low fields as . In the random field model the correlation radius is found
to be finite at the arbitrarily weak disorder.Comment: 4 pages, REVTe
Nonlinear Elasticity, Fluctuations and Heterogeneity of Nematic Elastomers
Liquid crystal elastomers realize a fascinating new form of soft matter that
is a composite of a conventional crosslinked polymer gel (rubber) and a liquid
crystal. These {\em solid} liquid crystal amalgams, quite similarly to their
(conventional, fluid) liquid crystal counterparts, can spontaneously partially
break translational and/or orientational symmetries, accompanied by novel soft
Goldstone modes. As a consequence, these materials can exhibit unconventional
elasticity characterized by symmetry-enforced vanishing of some elastic moduli.
Thus, a proper description of such solids requires an essential modification of
the classical elasticity theory. In this work, we develop a {\em rotationally
invariant}, {\em nonlinear} theory of elasticity for the nematic phase of ideal
liquid crystal elastomers. We show that it is characterized by soft modes,
corresponding to a combination of long wavelength shear deformations of the
solid network and rotations of the nematic director field. We study thermal
fluctuations of these soft modes in the presence of network heterogeneities and
show that they lead to a large variety of anomalous elastic properties, such as
singular length-scale dependent shear elastic moduli, a divergent elastic
constant for splay distortion of the nematic director, long-scale
incompressibility, universal Poisson ratios and a nonlinear stress-strain
relation fo arbitrary small strains. These long-scale elastic properties are
{\em universal}, controlled by a nontrivial zero-temperature fixed point and
constitute a qualitative breakdown of the classical elasticity theory in
nematic elastomers. Thus, nematic elastomers realize a stable ``critical
phase'', characterized by universal power-law correlations, akin to a critical
point of a continuous phase transition, but extending over an entire phase.Comment: 61 pages, 24 eps pages, submitted to Annals of Physic
Slow stress relaxation in randomly disordered nematic elastomers and gels
Randomly disordered (polydomain) liquid crystalline elastomers align under
stress. We study the dynamics of stress relaxation before, during and after the
Polydomain-Monodomain transition. The results for different materials show the
universal ultra-slow logarithmic behaviour, especially pronounced in the region
of the transition. The data is approximated very well by an equation Sigma(t) ~
Sigma_{eq} + A/(1+ Alpha Log[t]). We propose a theoretical model based on the
concept of cooperative mechanical resistance for the re-orientation of each
domain, attempting to follow the soft-deformation pathway. The exact model
solution can be approximated by compact analytical expressions valid at short
and at long times of relaxation, with two model parameters determined from the
data.Comment: 4 pages (two-column), 5 EPS figures (included via epsfig
Electro-Mechanical Fredericks Effects in Nematic Gels
The solid nematic equivalent of the Fredericks transition is found to depend
on a critical field rather than a critical voltage as in the classical case.
This arises because director anchoring is principally to the solid rubbery
matrix of the nematic gel rather than to the sample surfaces. Moreover, above
the threshold field, we find a competition between quartic (soft) and
conventional harmonic elasticity which dictates the director response. By
including a small degree of initial director misorientation, the calculated
field variation of optical anisotropy agrees well with the conoscopy
measurements of Chang et al (Phys.Rev.E56, 595, 1997) of the electro-optical
response of nematic gels.Comment: Latex (revtex style), 5 EPS figures, submitted to PRE, corrections to
discussion of fig.3, cosmetic change
Soft and non-soft structural transitions in disordered nematic networks
Properties of disordered nematic elastomers and gels are theoretically
investigated with emphasis on the roles of non-local elastic interactions and
crosslinking conditions. Networks originally crosslinked in the isotropic phase
lose their long-range orientational order by the action of quenched random
stresses, which we incorporate into the affine-deformation model of nematic
rubber elasticity. We present a detailed picture of mechanical quasi-Goldstone
modes, which accounts for an almost completely soft polydomain-monodomain (P-M)
transition under strain as well as a ``four-leaf clover'' pattern in
depolarized light scattering intensity. Dynamical relaxation of the domain
structure is studied using a simple model. The peak wavenumber of the structure
factor obeys a power-law-type slow kinetics and goes to zero in true mechanical
equilibrium. The effect of quenched disorder on director fluctuation in the
monodomain state is analyzed. The random frozen contribution to the fluctuation
amplitude dominates the thermal one, at long wavelengths and near the P-M
transition threshold. We also study networks obtained by crosslinking
polydomain nematic polymer melts. The memory of initial director configuration
acts as correlated and strong quenched disorder, which renders the P-M
transition non-soft. The spatial distribution of the elastic free energy is
strongly dehomogenized by external strain, in contrast to the case of
isotropically crosslinked networks.Comment: 19 pages, 15 EPS figure
Quasi-long-range order in nematics confined in random porous media
We study the effect of random porous matrices on the ordering in nematic
liquid crystals. The randomness destroys orientational lang-range order and
drives the liquid crystal into a glass state. We predict two glass phases one
of which possesses quasi-long-range order. In this state the correlation length
is infinite and the correlation function of the order parameter obeys a power
dependence on the distance. The small-angle light-scattering amplitude diverges
but slower than in the bulk nematic. In the uniaxially strained porous matrices
two new phases emerge. One type of strain induces an anisotropic
quasi-long-range-ordered state while the other stabilizes nematic long-range
order.Comment: 4 pages, Revte
Symmetries and Elasticity of Nematic Gels
A nematic liquid-crystal gel is a macroscopically homogeneous elastic medium
with the rotational symmetry of a nematic liquid crystal. In this paper, we
develop a general approach to the study of these gels that incorporates all
underlying symmetries. After reviewing traditional elasticity and clarifying
the role of broken rotational symmetries in both the reference space of points
in the undistorted medium and the target space into which these points are
mapped, we explore the unusual properties of nematic gels from a number of
perspectives. We show how symmetries of nematic gels formed via spontaneous
symmetry breaking from an isotropic gel enforce soft elastic response
characterized by the vanishing of a shear modulus and the vanishing of stress
up to a critical value of strain along certain directions. We also study the
phase transition from isotropic to nematic gels. In addition to being fully
consistent with approaches to nematic gels based on rubber elasticity, our
description has the important advantages of being independent of a microscopic
model, of emphasizing and clarifying the role of broken symmetries in
determining elastic response, and of permitting easy incorporation of spatial
variations, thermal fluctuations, and gel heterogeneity, thereby allowing a
full statistical-mechanical treatment of these novel materials.Comment: 21 pages, 4 eps figure
- …