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1.
We describe a new nematic liquid-crystal colloid system which is characterized by both charge stabilization of the particles and an interaction force. We estimate the effective charge of the particles by electrophoretic measurements and find that in such systems the director anchoring energy W is very low and the particles have little director distortion around them. The interaction force is created by producing a radial distribution of the nematic order parameter around a locally isotropic region created by ir laser heating. We theoretically describe this as being due to the induced flexoelectric polarization, the quadrupolar symmetry of which provides the required long-range force acting on charged particles. 相似文献
2.
To account for azimuthal surface anchoring of a nematic liquid crystal, Berreman [Phys. Rev. Lett. 28, 1683 (1972)10.1103/PhysRevLett.28.1683] proposed a simple model attributing the surface anchoring to the elastic distortion of the liquid crystal induced by the grooves of a surface. He showed that the surface anchoring energy is proportional to sin(2)varphi, with varphi being the angle between the director at infinity and the direction of the surface grooves. We argue that his assumption of negligibly small azimuthal distortion of the nematic is not valid. Proper treatment of the azimuthal distortion reveals that the Berreman's model should yield a surface anchoring energy proportional to sin(4)varphi. This implies that surface grooves alone cannot contribute to the surface anchoring coefficient in the usual Rapini-Papoular sense. 相似文献
3.
《Physics Reports》2001,351(6):387-474
This article reviews the physics of colloidal dispersions in nematic liquid crystals as a novel challenging type of soft matter. We first investigate the nematic environment of one particle with a radial anchoring of the director at its surface. Three possible structures are identified and discussed in detail; the dipole, the Saturn-ring and the surface-ring configuration. Secondly, we address dipolar and quadrupolar two-particle interactions with the help of a phenomenological theory. Thirdly, we calculate the anisotropic Stokes drag of a particle in a nematic environment which determines the Brownian motion of particles via the Stokes–Einstein relation. We then turn our interest towards colloidal dispersions in complex geometries where we identify the dipolar configuration and study its formation. Finally, we demonstrate that surface-induced nematic order above the nematic-isotropic phase transition results in a strongly attractive but short-range two-particle interaction. Its strength can be controlled by temperature and thereby induce flocculation in an otherwise stabilized dispersion. 相似文献
4.
S. B. Chernyshuk B. I. Lev H. Yokoyama 《Journal of Experimental and Theoretical Physics》2001,93(4):760-770
We studied the collective elastic interaction in a system of many macroparticles embedded in a nematic liquid crystal. A theoretical approach to the interaction of macroparticles via deformation of the director field [1] is developed. It is found that the director field distortion induced by many particles leads to the screening of the elastic pair interaction potential. This screening strongly depends on the shape of the embedded particles: it exists for anisotropic particles and is absent for spherical ones. Our results are valid for the homeotropic and the planar anchoring on the particle surface and for different Frank constants. We apply our results to cylindrical particles in a nematic liquid crystal. In a system of magnetic cylindrical grains suspended in a nematic liquid crystal, the external magnetic field perpendicular to the grain orientation results in inclining the grains to the director and induces an elastic Yukawa-law attraction between the grains. The appearance of this elastic attraction can explain the cellular texture in magnetically doped liquid crystals in the presence of the magnetic field [2]. 相似文献
5.
Smalyukh II Lavrentovich OD Kuzmin AN Kachynski AV Prasad PN 《Physical review letters》2005,95(15):157801
Using laser tweezers, we study colloidal interactions of solid microspheres in the nematic bulk caused by elastic distortions around the particles with tangential surface anchoring. The interactions overcome the Brownian motion when the interparticle separation r-->p is less than 3 particle diameters. The particles attract when the angle theta between r-->p and the uniform far-field director n0 is between 0 degrees and approximately 70 degrees and repel when 75 degrees 相似文献
6.
7.
以修正后的Rapini-Papoular锚定能公式为基础, 用理论分析和数值计算相结合的方法, 详细研究表面极化对液晶盒饱和特性的影响. 得出了求解指向矢分布的数学方程, 推导出了计算正常二级转变饱和电压的解析式, 同时结合最新报道, 给出了异常一级转变饱和电压的数值计算方法. 此外, 本文引入了反映指向矢分布特征的参量, 讨论了表面极化对此参量的影响. 结果表明, 指向矢最大倾角的位置随表面极化的增大将远离中央平面向基板移动. 一级转变饱和电压随弱锚定基板表面极化的增大而减小, 随强锚定基板表面极化的增大而增大. 而对二级转变, 饱和电压随表面极化的变化与一级转变恰恰相反. 本文所得结论对液晶表面物理的理解及液晶实际应用都具有重要的指导意义. 相似文献
8.
L. M. Blinov D. Z. Radzhabov S. V. Yablonsky S. S. Yakovenko 《Il Nuovo Cimento D》1990,12(9):1353-1361
Summary The flexo-electric polarization of hybrid nematic cells filled with 5 CB was measured as a function of cell thickness. To
this aim the pyroelectric response of a nematic layer with small amount of light absorbing dye dissolved was detected in a
nematic temperature range. From the experimental data the angle of the director deviation at the homeotropic boundary was
calculated as a function of cell thickness. This dependence allows the qualitative determination of the shape of the potential
well for the director deviation which is inconsistent with Rapini’s sinesquared form. The ?apparent? Rapini’s anchoring energy
varies with a surface director angle (at the homeotropic boundary) from 5·10−3 to 3.5·10−2 erg·cm−2.
Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988. 相似文献
9.
10.
Peter Kop?anský Martina Koneracká Vlasta Závišová Katalin Fodor-Csorba Anikó Vajda Eric Beaugnon 《Journal of magnetism and magnetic materials》2010,322(22):3696-3700
The studied ferronematic is a nematic liquid crystal (ZLI1695) of low negative anisotropy of the diamagnetic susceptibility (χa<0) doped with the magnetic particles Fe3O4. Structural instabilities are interpreted within Burylov and Raikher's theory. The high magnetic fields were oriented perpendicular (Freedericksz transition) or parallel to the initial director. Using capacitance measurements the Freedericksz threshold magnetic field of the ferronematic BFN, and the critical magnetic field Bmax, at which the initial parallel orientation between the director and the magnetic moment of magnetic particles breaks down, have been determined. The values of these quantities have been used to estimate the surface density of the anchoring energy W of liquid crystal molecules on the surface of the magnetic particles. The obtained values indicate a soft anchoring of the liquid crystal on the magnetic particles with a preferred parallel orientation of the magnetic moment of magnetic particles and the director. 相似文献
11.
The equilibrium tilt angle profile in a cell limited by two concentric cylinders filled with nematic liquid crystals is determined for strong homeotropic anchoring at the surfaces. The anchoring condition is such that the nematic director is perpendicular to the cylinder axes and a radial nonuniform electric field is applied to investigate a Fréedericksz transition. The distortions induced by the field remain in the plane perpendicular to the cylinder axes, and a threshold field is analytically determined indicating a transition from a pure splay to a splay-bend conformation of the director. It is shown that this transition can be induced by the thickness of the region between the two cylinders, and can be detected even in the absence of an external field. If the maximum value of the tilt angle is assumed as an order parameter, its behavior near to the transition can be used to obtain the critical exponent, which is the same as the one obtained in the mean field approximation. These results are indications that nontrivial consequences may occur when complex fluids are subject to non-planar geometries. 相似文献
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13.
A planar nematic liquid crystal cell (NLC) doped with spherical ferroelectric nanoparticles is considered. Polarisation of the nanoparticles are assumed to be along the NLC molecules parallel and antiparallel to the director with equal probability. The NLC molecules anchoring to the cell walls are considered to be strong, while soft anchoring at the nanoparticles surface is supposed. Behaviour of the NLC molecules and nanoparticles in the presence of a perpendicular electric field to the NLC cell is theoretically investigated. The electric field of the nanoparticles is taken into account in the calculations. Freedericksz transition (FT) threshold field in the presence of nanoparticles is found. Then, the director and particles reorientations for the electric fields larger than the threshold field are studied. Measuring the onset of the nanoparticles reorientation is proposed as a new method for the FT threshold measurement. 相似文献
14.
van der Beek D Reich H van der Schoot P Dijkstra M Schilling T Vink R Schmidt M van Roij R Lekkerkerker H 《Physical review letters》2006,97(8):087801
We study interfacial phenomena in a colloidal dispersion of sterically stabilized gibbsite platelets, exhibiting coexisting isotropic and nematic phases separated by a sharp horizontal interface. The nematic phase wets a vertical glass wall and polarized light micrographs reveal homeotropic surface anchoring both at the free isotropic-nematic interface and at the wall. On the basis of complete wetting of the wall by the nematic phase, as found in our density functional calculations and computer simulations, we analyze the balance between Frank elasticity and surface anchoring near the contact line. Because of weak surface anchoring, the director field in the capillary rise region is uniform. From the measured rise (6 microm) of the meniscus at the wall we determine the isotropic-nematic surface tension to be 3 nN/m, in quantitative agreement with our theoretical and simulation results. 相似文献
15.
Flows around a ball used in baseball games are calculated using third-order upwinddifference method with various seam positions determined by two rotation angles. Those are four-seam rotation with an angle: a and two-seam rotation with an angle: b. The computed results of the four-seam rotation are compared with experimental data measured in a wind tunnel and computed drag coefficients qualitatively agree well with experiments. However, lift coefficients do not agree well. The computed results and geometrical symmetry suggest that a supporting rod in the wind tunnel would have strong influence on the accuracy of the measurement. Flow changes in two-seam rotation are also simulated. It is found that the lowest drag force is observed atb=90 and that the value is less than half of the largest drag force ata=30 and 60 degrees. The largest lift force is observed atb=20 degree. In this case, a projection of the seam line on the top causes a large separation while smooth surface without the seam at the bottom dose not separate the flow. A pair of longitudinal vortices are found in the wake, which make wake slant and generate large lift force. 相似文献
16.
Threshold and saturation properties of the field-induced twist cell with two parameters weak anchoring boundaries
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On the basis of two-parameter formula of weak surface coupling anchoring
energy of nematic liquid crystals proposed by Zhao et al recently, the general
torque equilibrium equation and boundary conditions of the director have
been obtained and the threshold field as well as the saturation field of the
field-induced twist cell have been analysed for three kinds of
configurations, i.e. homogeneous, splay and Pi cells formed by
different rubbing conditions and pretilt angles. The results
indicated that the polar anchoring has no effect on the threshold field. It
is determined only by the twist anchoring and pretilt angle. But, the polar
anchoring and twist anchoring are coupling with each other and have a great
influence on the saturation field. The formulae for calculating the
threshold field and saturation field are given. These results will be very
useful in understanding surface physics and the design of liquid crystal
cells. 相似文献
17.
The interaction between particle-like sources of the
nematic director distortions (e.g., colloids, point
defects, macromolecules in nematic emulsions) allows for a useful
analogy with the electrostatic multipole interaction between
charged bodies. In this paper we develop this analogy to the level
corresponding to the charge density and consider the general
status of the pairwise approach to the nematic emulsions with
finite-size colloids. It is shown that the elastic analog of the
surface electric charge density is represented by the two
transverse director components on the surface imposing the
director distortions. The elastic multipoles of a particle are
expressed as integrals over the charge density distribution on
this surface. Because of the difference between the scalar
electrostatics and vector nematostatics, the number of elastic
multipoles of each order is doubled compared to that in the
electrostatics: there are two elastic charges, two vectors of
dipole moments, two quadrupolar tensors, and so on. The
two-component elastic charge is expressed via the vector of
external mechanical torque applied on the particle. As a result,
the elastic Coulomb-like coupling between two particles is found
to be proportional to the scalar product of the two external
torques and does not directly depend on the particles' form and
anchoring. The real-space Green function method is used to develop
the pairwise approach to nematic emulsions and determine its form
and restrictions. The pairwise potentials are obtained in the
familiar form, but, in contrast to the electrostatics, they
describe the interaction between pairs (dyads) of the elastic
multipole moments. The multipole moments are shown to be uniquely
determined by the single-particle director field, unperturbed by
other particles. The pairwise approximation is applicable only in
the leading order in the small ratio particle
size-to-interparticle distance as the next order contains
irreducible three-body terms. 相似文献
18.
S. Faetti M. Nobili I. Raggi 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(3):445-453
In this paper we report an experimental investigation on the dynamics of the azimuthal director reorientation at a nematic-solid
interface. Three qualitatively different kinds of substrates have been investigated: I) intrinsically anisotropic SiO-substrates
(-evaporation), II) isotropic SiO-substrates (-evaporation) and III) rubbed PVA-substrates. In the case II), an in-plane anisotropy was induced cooling slowly the thermotropic
nematic liquid crystal (NLC) from the isotropic phase in the presence of a 0.75 T magnetic field. The reorientation dynamic
of the surface azimuthal director angle at the switching-on and off of a magnetic (or electric) field has been investigated.
All the substrates show comparable azimuthal anchoring energies and two dynamic regimes: a fast dynamic response, driven by
the bulk director reorientation and an extremely slow reorientation. The slow dynamics is explained in terms of anisotropic
adsorption of NLC molecules on the solid substrate and is well represented by a stretched exponential.
Received 7 December 1998 相似文献
19.
We study by simulation the physics of two colloidal particles in a cholesteric liquid crystal with tangential order parameter alignment at the particle surface. The effective force between the pair is attractive at short range and favors assembly of colloid dimers at specific orientations relative to the local director field. When pulled through the fluid by a constant force along the helical axis, we find that such a dimer rotates, either continuously or stepwise with phase-slip events. These cases are separated by a sharp dynamical transition and lead, respectively, to a constant or an ever-increasing phase lag between the dimer orientation and the local nematic director. 相似文献