Nematic Liquid Crystal Mode Research Articles

Surface transitions in nematic liquid crystals oriented with Langmuir-Blodgett films

In this paper two types of surface transitions in nematic liquid crystals are theoretically analyzed with a simple model in which the anchoring energy is the result of both steric and electric interactions. The physical model is proper to explain both the transition temperature experimentally observed in homeotropically aligned nematic liquid crystals, and the change in the mean molecular direction due to the variation of the number of surfactant films, piled on the boundary surfaces with the Langmuir-Blodgett technique.

Flexoelectricity and alignment phase transitions in nematic liquid crystals

The influence of the flexoelectric effect on the phase diagram relevant to the order transition induced by an electric field on a nematic liquid crystal is considered. The analysis shows that this influence can be important. It is found that the order phase transition between the initially undistorted and distorted configurations always takes place. In contrast, the phase transition between the distorted and the saturated configurations is possible only if the dielectric anisotropy is large enough. The stability of the phases is analysed. The existence of a tricritical point is predicted. The dependence of the tricritical point on the flexoelectric coefficient is discussed too. The limits of our calculations and the performed simplifying hypotheses are critically analysed

Light-scattering study of Fréedericksz transitions in nematic liquid crystals

We introduce a novel technique to study the static distortion of a nematic liquid crystal in the neighbourhood of a Freedericksz transition, based on the analysis of the power spectral density of quasi-elastic light-scattering. Under suitable conditions the symmetry breaking of the director profile due to the Freedericksz transition induces a similar symmetry breaking in the angular dependence of the power spectral density, thus allowing a precise determination of the critical field. Experimental results are shown for the case of splay (twist) transition induced by an electric (magnetic) field. The correlation of the scattered fields at different angles is analyzed

A time-resolved Raman microprobe for studying the splay Freedericksz transition in nematic liquid crystals

The authors describe in detail a Raman microprobe that is capable of acquiring high quality Raman spectra from thin films of nematic liquid crystal, and of recording the transient response to applied electric fields, of the signal associated with particular Raman active bands. The apparatus has been used to acquire very low noise flat baseline spectra from a 14.85 mu m thick sample of planar aligned nematic 4-pentyl-4 '-cyanobiphenyl (5CB). From the transient data in the limit of small deformations, the authors have used the analysis of Pieranski et al (1973) to determine effective viscosity coefficients over a range of temperatures for the major Raman bands associated with the rigid core and flexible alkyl chain sections of the molecules. The derived viscosity coefficients bracket those otherwise determined by classical light scattering and permittivity techniques. It is suggested that these data reflect a bending of the molecular director field even in the small distortions limit.

TRICRITICAL POINT IN ORDER-DISORDER PHASE TRANSITIONS FOR NEMATICS

A complete analysis of the order-disorder phase transition in nematic liquid crystals induced by an external field is presented. The case of nearly compensated nematics in weak anchoring situation is discussed. The investigation shows that the parameter controlling the order of the phase transition is a decreasing function of the anchoring strength. The important limiting cases of very weak and very strong anchoring are considered too.

Theory of nonlinear transient patterns in the splay Fréedericksz transition.

A nonlinear theory of transient patterns in the electrically induced splay Fr\'eedericksz transition in nematic liquid crystals is presented. Growth conditions for periodic perturbations with a wave vector perpendicular to the initial director alignment, superposed on the homogeneous realignment process, have ben investigated. The decisive effect leading to this type of pattern formation arises from the field distortion resulting from the electric anisotropies. The nonlinear analysis with rigid boundary conditions delivers the onset of the pattern-forming instability at a critical value of the control parameter ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{c}}$, which appears to be too large.

High Speed Nematic Liquid Crystal Modulators

Abstract Nematic liquid crystal modulators with fast response time, high contrast ratio and low operation voltage are demonstrated. The transient nematic effect, together with reflective mode operation, is responsible for the observed fast response times. The use of a tunable liquid crystal phase compensator results in excellent contrast at a reasonably low operation voltage.

Field-induced first-order phase transition and spinodal point in nematic liquid crystals.

We report the discovery of a novel, magnetic-field-driven first-order phase transition in nematic liquid crystals. The transition is between a spatially periodic distortion, stable at intermediate field strengths, and a uniform distortion which is the ground state at high fields. From studies of dynamics we show that the transition is first order. We also find a spinodal point, below which the transition from the uniform to the periodic state occurs continuously. We discuss the utility of thickness gradient samples and determine the equilibrium wavelength of the periodic phase as a function of magnetic-field strength.

Surface order transition in nematic liquid crystals

Surface order transition in nematic liquid crystals

Non-Equilibrium Phase Transitions in Nematic Liquid Crystals under Shear Flow†

Abstract We discuss the impact of shear flow on the isotropic-nematic transition in crystalline liquids, paying particular attention to the notion of a non-equilibrium coexistence line. We establish a non-equilibrium phase diagram spanned by temperature and externally applied shear stress, by interpreting thc locally stable solutions to the equations of motion for the entire nematic order parameter and fluid velocity, in the case of planar Couette flow. This system displays several characteristics reminiscent of homogeneous equilibrium phenomena: a region in the phase diagram in which two states are locally stable; a critical point at which this two-state region vanishes; and singularities associated with this non- equilibrium critical point. By analyzing inhomogeneous solutions of the equations of motion we address the issue of whether a particular state is selected within the two state region. We find state selection throughout the two-state region, except on a coexistence line interpolating between the...

Anchoring transitions of nematic liquid crystals

Abstract 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.

Surface order transition in nematic liquid crystals induced by the flexoelectric polarization

The influence of the flexoelectric polarization on the nematic surface energy is analyzed. It is shown that if the dielectric constant of the substrate is lower than a critical value the undistorted orientation is no more the one corresponding to lowest energy. The presence of a conducting substrate is also considered. In this case it is shown that by changing the thickness of the dielectric material deposited over the conducting material, a surface order transition is expected. Furthermore, the influence of a finite anchoring energy between nematic and substrate on the critical value of the dielectric constant is discussed.

A quantum fluids approach to frustrated Heisenberg models

Emphasising the close analogies between antiferromagnetism and neutral superfluidity, the authors develop a gauge-invariant quantum fluids description of non-bipartite Heisenberg systems. The antiferromagnet is treated as a spin superfluid with a rotational gauge invariance associated with the continuity of spin flow. They show how an extended Schwinger boson approach naturally incorporates the Onsager reaction fields generated by spin fluctuations, and correctly reproduces the semiclassical behaviour of spin wave theory in the large-S limit. The important short-wavelength physics of fluctuation-stabilised order is also captured by this description. For two-dimensional helimagnets at small S, the method predicts that the twist will survive the loss of sublattice magnetisation, closely analogous to the biaxial-uniaxial transition of nematic liquid crystals.

Nematic liquid crystal modulator with response time less than 100 μs at room temperature

A high-speed nematic liquid crystal modulator with response time less than 100 μs at room temperature was demonstrated. The effects that contribute to the observed high speed are the transient nematic effect, a new liquid crystal mixture, and a holding voltage technique.

Critical behavior in anchoring transitions of nematic liquid crystals

The adsorption of water onto a crystal surface can result in a first-order transition in the orientation of the director of a nematic liquid crystal spread on top of the surface. Here, we show that coadsorption of two or three chemicals can lead to a phase diagram having first-order transition boundaries, critical lines, and critical points. A simple Landau theory accounts for most of the phenomena we observe. This experiment gives the first direct optical visualization of a two-dimensional system near its critical point.

Systematic studies of the anchoring transition in nematic liquid crystals.

When a nematic liquid crystal is spread on a crystalline substrate, the latter can orient the nematic molecules on top. Recently, it was shown that if water vapor is adsorbed onto the nematic-substrate interface, a discontinuous jump in the orientation can be observed. In this article we report a systematic study of the conditions under which this anchoring transition takes place. We find that, in addition to water, alcohols and diols cause a transition. Further, the nematic orientation depends only on relative humidity and not directly on temperature.

Electric Field Control Bistability and Local Fredericks Transitions in Nematic Liquid Crystals

Abstract The possibilities of obtaining different metastable orientational states in nematic layers and control by the electric field of local Fredericks transition kinetics are shown. The theoretical substantiation of the obtained results are given.

Freedericksz transitions in nematic liquid crystals: The effects of an in-plane electric field.

We have studied the effects of perpendicular electric and magnetic fields on a liquid-crystal sample aligned between glass plates. One field is applied to stabilize the initial alignment, and the other is applied in order to drive the transition; the field and initial alignment directions define six possible geometries. We find that, in each case in which an electric field is present parallel to the glass plates, the Freedericksz transition from a uniform to an elastically deformed state can be first order irrespective of whether the electric field drives or stabilizes the transition. Using material parameters characteristic of the liquid-crystal 4-cyano-4'-n-pentylbiphenyl (5CB), we show that two of the transitions should be first order for this material in all values of stabilizing fields and that two should exhibit a tricritical point as the stabilizing field is increased.

Electric-field-induced twist and bend Freedericksz transitions in nematic liquid crystals.

By considering the Frank free energy, we show that the electric-field-induced twist and bend Freedericksz transitions can be first order. This agrees with the prediction of Arakelyan, Karayan, and Chilingaryan [Sov. Phys.--Dokl. 29, 202 (1984)] from Landau theory for the bend geometry. We have carried out dielectric measurements on the liquid-crystal 5CB (4-cyano-4'-n-pentylbiphenyl) that provide the first experimental evidence that these transitions are first order. The effects of a competing magnetic field on the threshold voltage and on the width of the transition region are compared with theory. In the presence of a competing magnetic field, the bend transition in 5CB is to a modulated rather than a uniform phase.

Implications of considering the Brochard-Leger wall as a solitary wave.

The approximations used by Wang in explaining the dynamic behavior of walls related to the Fr\'eedericksz transition in nematic liquid crystals (Brochard-Leger walls) as solitary waves [Phys. Lett. 112A, 402 (1985); Phys. Rev. A 32, 3126 (1985); 34, 5179 (1986)] are so severe as to make the results dubious.