180 research outputs found
Light scattering by optically anisotropic scatterers II: T--matrix computations for radially and uniformly anisotropic droplets
This is the second paper in a series on light scattering from optically
anisotropic scatterers embedded in an isotropic medium. The apparently complex
T-matrix theory involving mixing of angular momentum components turns out to be
an efficient approach to calculating scattering in these systems. We present
preliminary results of numerical calculations of the scattering by spherical
droplets in some simple cases. The droplets contain optically anisotropic
material with local radial or uniform anisotropy. We concentrate on cases in
which the scattering is due only to the local optical anisotropy within the
scatterer. For radial anisotropy we find non-monotonic dependence of the
scattering cross-section on the degree of anisotropy can occur in a regime for
which both the Rayleigh and semi-classical theories are inapplicable. For
uniform anisotropy the cross-section is strongly dependent on the angle between
the incident light and the optical axis, and for larger droplets this
dependence is non-monotonic.Comment: 14 pages, 6 figures, uses RevTex
Modeling planar degenerate wetting and anchoring in nematic liquid crystals
We propose a simple surface potential favoring the planar degenerate
anchoring of nematic liquid crystals, i.e., the tendency of the molecules to
align parallel to one another along any direction parallel to the surface. We
show that, at lowest order in the tensorial Landau-de Gennes order-parameter,
fourth-order terms must be included. We analyze the anchoring and wetting
properties of this surface potential. In the nematic phase, we find the desired
degenerate planar anchoring, with positive scalar order-parameter and some
surface biaxiality. In the isotropic phase, we find, in agreement with
experiments, that the wetting layer may exhibit a uniaxial ordering with
negative scalar order-parameter. For large enough anchoring strength, this
negative ordering transits towards the planar degenerate state
Hysteresis in Two-Dimensional Liquid Crystal Models
We make a numerical study of hysteresis loop shapes within a generalized two-dimensional Random Anisotropy Nematic (RAN) model at zero temperature. The hysteresis loops appear on cycling a static external ordering field. Ordering in these systems is history dependent and involves interplay between the internal coupling constant J, the anisotropy random field D, and the ordering external field H. Here the external field is represented by a traceless tensor, analogous to extension-type fields in continuum mechanics. The calculations use both a mean field approach and full lattice simulations. Our analysis suggests the existence of two qualitatively different solutions, which we denote as symmetric and symmetry breaking. For the set of parameters explored, only the symmetric solutions are stable. Both approaches yield qualitatively similar hysteresis curves, which are manifested either by single or double loops. But the quantitative differences indicate that mean field estimates are only of limited predictive value
Influence of Homeotropic Anchoring Walls upon Nematic and Smectic Phases
McMillan liquid crystal model sandwiched between strong homeotropic anchoring
walls is studied. Phase transitions between isotropic, nematic, and smectic A
phases are investigated for wide ranges of an interaction parameter and of the
system thickness. It is confirmed that the anchoring walls induce an increase
in transition temperatures, dissappearance of phase transitions, and an
appearance of non-spontaneous nematic phase. The similarity between influence
of anchoring walls and that of external fields is discussed.Comment: 5 pages, 6 figure
Liquid crystal anchoring transitions on aligning substrates processed by plasma beam
We observe a sequence of the anchoring transitions in nematic liquid crystals
(NLC) sandwiched between the hydrophobic polyimide substrates treated with the
plasma beam. There is a pronounced continuous transition from homeotropic to
low tilted (nearly planar) alignment with the easy axis parallel to the
incidence plane of the plasma beam (the zenithal transition) that takes place
as the exposure dose increases. In NLC with positive dielectric anisotropy, a
further increase in the exposure dose results in in-plane reorientation of the
easy axis by 90 degrees (the azimuthal transition). This transition occurs
through the two-fold degenerated alignment characteristic for the second order
anchoring transitions. In contrast to critical behavior of anchoring, the
contact angle of NLC and water on the treated substrates monotonically declines
with the exposure dose. It follows that the surface concentration of
hydrophobic chains decreases continuously. The anchoring transitions under
consideration are qualitatively interpreted by using a simple phenomenological
model of competing easy axes which is studied by analyzing anchoring diagrams
of the generalized polar and non-polar anchoring models.Comment: revtex4, 18 pages, 10 figure
Surface alignment and anchoring transitions in nematic lyotropic chromonic liquid crystal
The surface alignment of lyotropic chromonic liquid crystals (LCLCs) can be
not only planar (tangential) but also homeotropic, with self-assembled
aggregates perpendicular to the substrate, as demonstrated by mapping optical
retardation and by three-dimensional imaging of the director field. With time,
the homeotropic nematic undergoes a transition into a tangential state. The
anchoring transition is discontinuous and can be described by a double-well
anchoring potential with two minima corresponding to tangential and homeotropic
orientation.Comment: Accepted for publication in Phys. Rev. Lett. (Accepted Wednesday Jun
02, 2010
Binary separation in very thin nematic films: thickness and phase coexistence
The behavior as a function of temperature of very thin films (10 to 200 nm)
of pentylcyanobiphenyl (5CB) on silicon substrates is reported. In the vicinity
of the nematic/isotropic transition we observe a coexistence of two regions of
different thicknesses: thick regions are in the nematic state while thin ones
are in the isotropic state. Moreover, the transition temperature is shifted
downward following a 1/h^2 law (h is the film thickness). Microscope
observations and small angle X-ray scattering allowed us to draw a phase
diagram which is explained in terms of a binary first order phase transition
where thickness plays the role of an order parameter.Comment: 5 pages, 3 figures, submitted to PRL on the 26th of Apri
Annihilation of edge dislocations in smectic A liquid crystals
This paper presents a theoretical study of the annihilation of edge dislocations in the same smectic plane in a bulk smectic-A phase. We use a time-dependent Landau-Ginzburg approach where the smectic ordering is described by the complex order parameter psi( r--> ,t) =eta e(iphi) . This quantity allows both the degree of layering and the position of the layers to be monitored. We are able to follow both precollision and postcollision regimes, and distinguish different early and late behaviors within these regimes. The early precollision regime is driven by changes in the phi ( r--> ) configuration. The relative velocity of the defects is approximately inversely proportional to the interdefect separation distance. In the late precollision regime the symmetry changes within the cores of defects also become influential. Following the defect collision, in the early postcollision stage, bulk layer order is approached exponentially in time. At very late times, however, there seems to be a long-time power-law tail in the order parameter fluctuation relaxation
- …