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We study stable equilibria of liquid crystals in the flow being at rest and the stable dynamic states for nematic liquid crystals under weak shear flow for the Doi model [M. Doi, S.F. Edwards, The Theory of Polymer Dynamics, Oxford University Press, 1986]. It is first theoretically proven that there is a hysteresis phenomenon in the flow being at rest when the non-dimensional potential intensity among particles changes. Furthermore, in the weak shear flow, we show that there exist many stable dynamic states: flow aligning, tumbing, log-rolling and kayaking, which depend on the initial concentrated orientation of liquid crystal particles. The results are consistent with those of numerical simulation [M.G. Forest, Q. Wang, R. Zhou, The weak shear kinetic phase diagram for nematic polymers, Rheol. Acta 43 (2004) 17-37; M.G. Forest, R. Zhou, Q.Wang, Full-tensor alignment criteria for sheared nematic polymers, J. Rheol. 47 (2003) 105-127] and experimental discoveries [W.R. Burghardt, Molecular orientation and rheology in sheared lyotropic liquid crystalline polymers, Macromol. Chem. Phys. 199 (1998) 471-488; Ch. Gähwiller, Temperature dependence of flow alignment in nematic liquid crystals, Phys. Rev. Lett. 28 (1972) 1554-1556]. Theoretical analysis is reported the first time that the Kayaking state does not circulate around a fixed direction but the asymmetric axis will periodically change. 相似文献
3.
In this work we demonstrate the properties of spatial solitary waves in chiral nematic liquid crystals with an external electric field. Such self-trapped beams, called nematicons, were created due to the optical reorientation nonlinearity for a light power of a few tens of milliwatts. We show that the direction of propagation of such nematicons can be changed by applying an external electric field. Additionally, this effect can be modified by changing the input polarization of the light beam. The experimental results were obtained in four independent guiding layers created by a chiral nematic structure. 相似文献
4.
The paper reports new nematic liquid crystals that are paramagnetic. 相似文献
5.
Different modes of bistable switching in liquid crystals with frequency inversion of the dielectric anisotropy sign are discussed.
The study is performed by numerical simulation and experimentally. It is shown that dual frequency driving can be effectively
used to control switching between topologically equivalent and non-equivalent director field distributions. The experimental
results on temperature performance of the dual-frequency switching and possible driving methods for energy consumption and
expanding the temperature range are presented. 相似文献
6.
在数值和实验上研究了向列相液晶中空间光孤子的相互作用.向列相液晶中的响应函数为指数衰减函数,这不同于Snyder-Mitchell模型.数值模拟发现向列相液晶中的孤子相互作用仍然满足交叉点位置与输入功率的平方根成反比,强非局域下的Snyder-Mitchell模型交叉点位置与两束光的间距无关,而在向列相液晶中交叉点位置与两束光的间距有关;在实验上发现,向列相液晶中两束光交叉点的位置与输入功率平方根成反比,还和形成孤子的临界功率以及两束光之间的间距有关.
关键词:
强非局域非线性
向列相液晶
孤子相互作用
交叉点 相似文献
7.
《Phase Transitions》2012,85(4):345-352
Excitation of solitary waves and their propagation in surface-stabilized ferroelectric liquid crystal cells under alternating external electric field is investigated both theoretically and experimentally. The effect of solitary waves on electro-optic response spectra is analyzed for different amplitudes of applied fields, temperatures, and sample thicknesses. It is shown that solitons can only be excited within narrow ranges of frequencies of the sufficiently strong electric fields. The minimal frequency, at which soliton waves appear in ferroelectric smectic liquid crystals, is found to be related to the material constants of these systems. It is proved that measuring this threshold frequency gives the possibility to determine one of the material parameters, if the others are known. In this way, the intra-smectic-layer elastic constant is found for systems with the chevron geometry. 相似文献
8.
K. A. Brzdąkiewicz U. A. Laudyn M. A. Karpierz T. R. Woliński J. Wójcik 《Opto-Electronics Review》2006,14(4):287-292
We investigate linear and nonlinear light propagation in the photonic crystal fibers infiltrated with nematic liquid crystals.
Such a photonic structure, with periodic modulation of refractive index, which could be additionally controlled by the temperature
and by the optical power, allows for the study of discrete optical phenomena. Our theoretical investigations, carried out
with the near infrared wavelength of 830 nm, for both focusing and defocusing Kerr-type nonlinearity, show the possibility
of the transverse light localization, which can result in the discrete soliton generation. In addition, we present the preliminary
experimental results on the linear light propagation in the photonic crystal fiber with the glycerin-water solution and 6CHBT
nematics, as the guest materials. 相似文献
9.
We consider multiple light scattering in a nematic liquid crystal. Using the Monte Carlo method, we calculate for the first
time the effect of a magnetic field on the shape of the peak of coherent backscattering taking into account the long-range
action of fluctuations of the orientational order and anisotropy of the scattering length. For a small number of initial and
final scattering events, we take into account the ordinary mode of light, which is weakly scattered in a nematic liquid crystal
(NLC), whereas a strongly scattered extraordinary mode is taken into account for all scattering events. For simplicity, we
use a single-constant approximation of the NLC elastic moduli. We show that the angular shape of the peak of coherent backscattering
remains nearly unchanged, whereas the magnetic field and the scattering phase function vary by several orders of magnitude. 相似文献
10.
Michael P. Allen 《Molecular physics》2016,114(18):2574-2579
ABSTRACTIn this note, the density functional theory for chiral nematic liquid crystals is expressed in terms of rotational invariant expansion coefficients of the pair-excluded area at fixed separation. This modification from the standard approach, without making any additional assumptions, yields a more efficient procedure for minimising the free energy, as well as providing insight into the origins of the effect away from the long-pitch limit. 相似文献
11.
The different approximations that have been used in applying Bethe’s cluster model to the nematic-isotropic phase transition
are examined. It is shown that the introduction of a higher order term in the mean field potential of an outer molecule of
the cluster improves the consistency of the theory considerably. In particular, the importance of satisfying Chang’s relation
is emphasized. Calculations are presented of the long and short range order parameters, heat of transition and specific heat
for different values ofz, the number of nearest neighbours around any given molecule, for both nonpolar and antiparallel near neighbour correlations.
Even the new mean field potential appears to be inadequate forz=3. 相似文献
12.
A crystalline substrate such as mica or gypsum can orient a thin layer of nematic liquid crystal spread on top. By adsorbing one or more other kinds of molecules onto the surface, we have observed transitions in the orientation of the molecules on the surface. These “anchoring transitions” are analyzed via Landau-Ginzburg theory and may be first or second-order. The second-order anchoring transition is the first two-dimensional system where critical fluctuations may be observed optically. 相似文献
13.
N. V. Madhusudana 《Phase Transitions》2013,86(1-3):177-191
Electroconvective instabilities in nematic liquid crystals are caused by the electric force acting on charge densities generated in the medium due to a distortion in the orientation field of the anisotropic medium. The conventional view is that anisotropic conductivity provides the mechanism for the charge accumulation. Several experimental results however can be accounted for only by invoking an additional mechanism involving flexoelectric polarization for the charge accumulation. In this article we discuss the recent experimental and theoretical developments in this field. 相似文献
14.
Connecting two columnar phase forming discotic mesogens via a short rigid spacer leads to the formation of a π-conjugated
discotic dimer snowing discotic nematic (N
D) phase. Attaching branched-alkyl chains directly to the core in hexaalkynylbenzene resulted in the stabilisation of ND phase at ambient temperature. Pentalkynylbenzene derivatives possessing a combination of normal- and branched-alkoxy chains
display a very broad ND, phase which is stable well below and above the room temperature. 相似文献
15.
R. B. Alaverdyan A. M. Grigoryan Yu. S. Chilingaryan 《Journal of Contemporary Physics (Armenian Academy of Sciences)》2008,43(1):19-22
Laser-induced convective motion in thin layers (less than a millimeter) of nematic liquid crystals is investigated experimentally. It is shown that convection arising in the presence of the vertical gradient of temperature is toroidal. Velocities of convective motions in the liquid crystal are measured for two different diameters of the incident laser beam. Comparison is made of the threshold of the temperature gradient of convective motions obtained by theoretical estimations and with use of experimental parameters. 相似文献
16.
We study numerically the effect of an external magnetic or electric field on the director profiles of a nematic liquid crystal
around a spherical particle. We pay particular attention to the stability of a hyperbolic hedgehog defect accompanying the
particle, which transforms into a Saturn-ring defect encircling the particle under a sufficiently strong external field. We
focus on the particle size dependence of the two important threshold field strengths: the “thermodynamic-transition” field
strength H1 at which the hedgehog and the Saturn-ring configurations have the equal free energy, and the critical field strength H2 at which the hedgehog loses its (meta)stability. Our numerical results demonstrate that while H1 is non-monotonically dependent on the particle radius R0, H2 monotonically increases with R0 and the dependence of H2 is weak for large R0. The non-monotonic dependence of H1 on R0 can be explained by comparing the energies of the two configurations and assuming the dependence of those energies on a rescaled
field. A crude argument of the energetics of a hyperbolic hedgehog defect under an external field shows that for an asymptotically
large R0 the critical field strength is independent of R0, which agrees with our numerical finding. 相似文献
17.
The influence of controllable parameters like temperature and wavelength on the trajectories of light in a nematic liquid crystal with topological defects is studied through a geometric model. The model incorporates phenomenological details as how the refractive indices depend on such parameters. The deflection of light by the topological defect is then shown to be greater at lower temperatures and shorter wavelengths. 相似文献
18.
The effect of magnetic fields on out-of-plane orientations of liquid crystalline polymers (LCPs) under simple shear flows is numerically analyzed using the Doi–Hess equation. The evolution equation for the probability distribution function of the LCP molecules is directly solved without any approximation closure. The initial director is parallel to the vorticity direction. Two cases of the magnetic fields are considered (1) the magnetic field parallel to the flow direction, and (2) the magnetic field parallel to the velocity gradient direction. For both cases a log-rolling orientation state is detected at low shear rates. However, the director is quickly aligned along the direction of magnetic fields because of the deformation of molecules. The field affects on the scalar order parameter rather than the major orientation direction for the magnetic field parallel to the flow direction. On the other hand regarding the magnetic field along the vorticity gradient direction, the effect of the magnetic field is more remarkable on the major orientation in comparison with the effect on the scalar order parameter. Also it is be found that the order parameter is increased obviously with increasing the magnetic fields. It is an efficient way to improve the performance of LCP materials. 相似文献
19.
G. Barbero L.R. Evangelista N.V Madhusudana 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):327-331
We analyse the influence of adsorbed ions and the resulting surface electric field and its gradient on the anchoring properties
of nematics with ionic conductivity. We take into account two physical mechanisms for the coupling of the nematic director
with the surface electric field: (i) the dielectric anisotropy and (ii) the coupling of the quadrupolar component of the flexoelectric
coefficient with the field gradient. It is shown that for sufficiently large fields near saturated coverage of the adsorbed
ions, there can be a spontaneous curvature distortion in the cell even when the anchoring energy is infinitely strong. We
also discuss the director distortion when the anchoring energy of the surface is finite.
Received: 29 September 1997 / Received in final from: 10 November 1997 / Accepted: 18 November 1997 相似文献
20.
é. V. Gevorkyan 《Russian Physics Journal》1981,24(4):343-346
The behavior of nematic liquid crystals in a magnetic field having a static and a rotating component is investigated. Steady-state solutions of the problem (synchronous regime) are obtained, and their stability is analyzed. The influence of the boundary conditions is discussed, along with possible experimental applications. It is shown that the static component of the magnetic field extends the stability range of the synchronous regime.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 57–60, April, 1981. 相似文献