Modelling a nematic liquid crystal

The bulk phase liquid crystalline behaviour of a cyclic siloxane with a pentamethylcyclosiloxane core and biphenyl-4-allyloxybenzoate mesogens (BCS) was studied using molecular dynamics (MD) and wide angle X-ray analysis. This material exhibits partial crystallinity at room temperature and liquid crystalline behaviour above 120° C. For the MD simulations an ensemble of 27 molecules with 135 mesogenic units was simulated and a molecular mechanics force field was used to model the structural anisotropy of the siloxane molecules. Simulations were carried out both at room temperature and at an elevated temperature (425 K). Room temperature simulations showed that, contrary to our initial assumptions, the low energy molecular conformations were not cylindrical but splayed in shape. During the simulation a smectic-like, tilted layer structure was found to evolve for the cluster when full atom potentials were used, while no such development was observed when electrostatic interactions were neglected. The presence of a tilted layered structure was also suggested by the X-ray data. These results indicate that long range electrostatic interactions are significant for the molecular system under study. In order to calculate the orientational order parameter, the orientation of the molecular axis had to be determined. This was achieved by describing the mesogen shapes to be ellipsoidal and defining the principal axis of the ellipsoids to be the molecular directors. By sampling over 200 ps of simulation at 425 K, the time averaged order parameter (S) was calculated. The calculated S of 0.36 was comparable to the value of 0.4-0.45 found from the experimental data. Apart from providing insight into the relative importance of the various competing forces in the formation of the liquid crystalline phase, these simulations are also expected to be useful in predicting the mesophase behaviour of liquid crystalline systems.     

Dynamics of the field-induced twist deformation in weakly anchored nematic layers

The dynamics of the field-induced one-dimensional twist deformation in weakly anchored nematic layers was investigated by means of numerical simulation. The rise time constants and the decay time constants, characteristic for the onset and for the decay of the deformation, respectively, were determined. The time of delay, which had passed before the decay started, was revealed in the case of finite anchoring. For weaker anchoring, the time of delay and the decay time constant were longer. Surprisingly, the onset of the deformation was slower when the anchoring was weaker, provided that the ratio of the actual field strength to the threshold field strength was fixed.     

Mass transport in a chiral nematic/nematic liquid crystal system

The optical microscopic mass transport technique has been used to study diffusion phenomenon in a chiral nematic/nematic solute/solvent mixture. Analysis of the concentration-distance, concentration-time and distance-time of the diffusion profile gave the diffusion coefficient of the system as a function of time, distance and concentration, respectively. The mutual diffusion coefficient of the system was independent of the distance and time, showing an average value of 2.65 × 10^-7 cm² s^-1. In non-steady state diffusion, the diffusion coefficient was dependent on both distance and time. The diffusion coefficient exhibited an inverse relation with the local concentration of the chiral solute. The self-diffusion coefficient of the nematic solvent gave a value of 3.4 × 10^-7 cm² s^-1 via extrapolation to zero concentration of the solute.     

NMR polarization echoes in a nematic liquid crystal

We have modified the polarization echo (PE) sequence through the incorporation of Lee-Goldburg cross polarization steps to quench the 1H-1H dipolar dynamics. In this way, the 13C becomes an ideal local probe to inject and detect polarization in the proton system. This improvement made possible the observation of the local polarization P(00)(t) and polarization echoes in the interphenyl proton of the liquid crystal N-(4-methoxybenzylidene)-4-butylaniline. The decay of P(00)(t) was well fitted to an exponential law with a characteristic time tau(C) approximately 310 micros. The hierarchy of the intramolecular dipolar couplings determines a dynamical bottleneck that justifies the use of the Fermi Golden Rule to obtain a spectral density consistent with the structural parameters. The time evolution of P(00)(t) was reversed by the PE sequence generating echoes at the time expected by the scaling of the dipolar Hamiltonian. This indicates that the reversible 1H-1H dipolar interaction is the main contribution to the local polarization decrease and that the exponential decay for P(00)(t) does not imply irreversibility. The attenuation of the echoes follows a Gaussian law with a characteristic time tau(phi) approximately 527 micros. The shape and magnitude of the characteristic time of the PE decay suggest that it is dominated by the unperturbed homonuclear dipolar Hamiltonian. This means that tau(phi) is an intrinsic property of the dipolar coupled network and not of other degrees of freedom. In this case, one cannot unambiguously identify the mechanism that produces the decoherence of the dipolar order. This is because even weak interactions are able to break the fragile multiple coherences originated on the dipolar evolution, hindering its reversal. Other schemes to investigate these underlying mechanisms are proposed.     

Ising wall instability in a nematic liquid crystal

A new instability for a splay-bend Ising wall was found in a 5CB nematic liquid crystal layer. This instability, which occurs in the presence of an external horizontal magnetic field, is driven by the elastic anisotropy of the liquid crystal material. Depending on the homogeneity of the magnetic field, the unstable straight interface evolves towards a new steady state or undergoes a spinodal decomposition into facets. Energy arguments are given in order to explain these physical phenomena.     

Ising wall instability in a nematic liquid crystal

A new instability for a splay-bend Ising wall was found in a 5CB nematic liquid crystal layer. This instability, which occurs in the presence of an external horizontal magnetic field, is driven by the elastic anisotropy of the liquid crystal material. Depending on the homogeneity of the magnetic field, the unstable straight interface evolves towards a new steady state or undergoes a spinodal decomposition into facets. Energy arguments are given in order to explain these physical phenomena.     

Sub-twisted nematic liquid crystal display

A new reflective liquid crystal display, the sub-twisted nematic LCD (SBTN), is presented. The SBTN, specifically designed for its twist angle, the product of cell thickness and birefringence, the direction of its only polarizer, etc. exhibits good electro-optic performance. Brightness, contrast, colour dispersion and multiplexing ability, etc. were theoretically and experimentally examined. The prototype showed high brightness, little dispersion, good mutliplexing ability, good viewing angle and response behaviour. It also offers good manufacturing tolerance and low production cost.     

Sub-twisted nematic liquid crystal display

A new reflective liquid crystal display, the sub-twisted nematic LCD (SBTN), is presented. The SBTN, specifically designed for its twist angle, the product of cell thickness and birefringence, the direction of its only polarizer, etc. exhibits good electro-optic performance. Brightness, contrast, colour dispersion and multiplexing ability, etc. were theoretically and experimentally examined. The prototype showed high brightness, little dispersion, good mutliplexing ability, good viewing angle and response behaviour. It also offers good manufacturing tolerance and low production cost.     

Dynamics of a nanowire in twisted nematic liquid crystal

The dynamics of a nanowire immersed in a nematic liquid crystal sandwiched between two parallel plates are studied by applying an equivalent capacitance approach used in electrostatics. A lower cut-off eigenfrequency for the oscillation of the nanowire is found and the lighter the mass, the smaller the critical cell separation at which the cut-off frequency occurs is needed. A simulation of the dynamical process of the metal wires shows that the relaxation time is proportional to η/m in the small mass region.     

Flexoelectro-optic effect in a hybrid nematic liquid crystal cell

W e analyse the influence of charged impurities and flexoelectric polarization on the optical transmission of a hybrid aligned nematic liquid crystal cell. The theoretical results obtained within the framework of the Poisson-Boltzmann equation and Frank elastic theory are compared with the observed optical response [N. V. Madhusudana and G. Durand,J. Phys.Lett. 46, L-195 (1985)]. We show that impurities can be very important for the behaviour of the system in the low field regime where the flexoelectric effect is relevant, and we determine the flexoelectric coefficient, the anchoring strength, and the concentration of impurities in the sample previously studied by Madhusudana and Durand.     

Ionic impurities in nematic liquid crystal displays

This paper shows that the dielectric anisotropy of conductivity in cells composed of nematic liquid crystal E7 (NLC-E7) is related to the fact that the diffusion constant (D) is greater in a cell with homeotropic alignment than in one with homogeneous alignment (D _∥ > D _⊥). This behaviour can be understood by the study of the dielectric properties of the NLC based on the ionic hopping behaviour and on the analysis of the electrical conductivity in relation to the voltage applied.     

Ionic relaxation in nematic liquid crystal cells

We investigate the dependence of the relaxation time of the current flowing in a nematic cell submitted to an external dc voltage on the physical properties of the substrate. We show that previously presented analyses of the same problem are not very useful for practical applications. We compare our theoretical predictions with experimental data, and show that the agreement is rather good. The influence of the adsorption-desorption phenomenon on the relaxation time is also discussed.     

Instabilities of nematic liquid crystal films

We discuss instabilities exhibited by free surface nematic liquid crystal (NLC) films of nanoscale thickness deposited on solid substrates, with a focus on surface instabilities that lead to dewetting. Such instabilities have been discussed extensively; however, there is still no consensus regarding the interpretation of experimental results, appropriate modeling approaches, or instability mechanisms. Instabilities of thin NLC free surface films are related to a wider class of problems involving dewetting of non-Newtonian fluids. For nanoscale films, the substrate–film interaction, often modeled by a suitable disjoining pressure, becomes relevant. For NLCs, one can extend the formulation to include the elastic energy of the NLC film, leading to an ‘effective’ disjoining pressure, playing an important role in instability development. Focusing on thin film modeling within the framework of the long-wave asymptotic model, we discuss various instability mechanisms and outline problems where new research is needed.     

Crystallisation of nematic liquid crystal mixtures

In this paper, we propose a method to accelerate the crystallisation of nematic liquid crystal mixtures based on crystallisation theory. This method is to hold a nematic liquid crystal sample at a temperature suitable for crystal growth after aging it at a temperature suitable for nucleation. After we specified these temperatures of a nematic liquid crystal mixture using differential scanning calorimetry, we demonstrate that the two-temperature aging method is effective for the crystallisation of other nematic liquid crystal mixtures in which the crystal-liquid crystal transition temperature has so far been undetectable.     

Photopolymerization induced orientation transition in a nematic liquid crystal cell

We report the observation of a liquid crystal tilt transition from homeotropic to planar orientation induced by photopolymerization of the alignment layer in the absence of liquid crystal. The alignment agent is a unique, polymerizable lecithin (DC₂₃PC), which induces homeotropic alignment before UV exposure. After non-polarized UV exposure, a tilted orientation is obtained. Moreover, further buffing of the UV treated substrate yields a homogeneous alignment. We believe that the conformation change in the lecithin array caused by solid state polymerization is primarily responsible for the transition. These results help to explain the mechanism of liquid crystal alignment and will lead to several potential applications.     

Twist molecular orientation transition in a nematic liquid crystal cell

We theoretically demonstrate that the application of a transverse next-to-static electric field to a planar nematic liquid crystal cell - obtained by strong planar anchoring of the liquid crystal on the first surface and negligible interaction with the second surface - causes an electro-reorientational transition of the Freedericksz kind. We show that, above the transition the cell behaves as a twisted nematic liquid crystal cell with total twist depending on the applied voltage. Preliminary experimental results confirming the predicted effect are presented.     

Propagative patterns in the convection of a nematic liquid crystal

We show here that the convective instability of a nematic liquid crystal subjected to an A.C. electric field (the conduction regime), is never stationary, contrary to the widely accepted picture. Indeed, we have found that the roll structure translates uniformly along the wavevector direction. In the low frequency part of the conduction regime, the structure is rather homogeneous in space and travels at a very low velocity, while inside the high frequency part, the rolls are localized inside stable domains and the propagation velocity is higher by three orders of magnitude. Closer to the cut-off frequency, we have also found a novel time dependent state, where the amplitude of the rolls oscillates periodically in time.     

Dynamics of surface anchoring breaking in a nematic liquid crystal

We present a simple dynamical model which describes the time dependence of the threshold electric field for breaking the surface anchoring of nematics. This model includes a surface friction, resulting from volume hydrodynamic dissipation. Experimental data with AC and DC pulsed fields are explained by this model. For DC, the threshold difference versus the field polarity is well-described by the flexo-electric effects in a uniform field for short times and in a non-uniform field for long times.     

Dynamics of surface anchoring breaking in a nematic liquid crystal

Abstract

We present a simple dynamical model which describes the time dependence of the threshold electric field for breaking the surface anchoring of nematics. This model includes a surface friction, resulting from volume hydrodynamic dissipation. Experimental data with AC and DC pulsed fields are explained by this model. For DC, the threshold difference versus the field polarity is well-described by the flexo-electric effects in a uniform field for short times and in a non-uniform field for long times.     

Linear dichroism of perylene derivatives in a nematic liquid crystal

The polarized absorption spectra of derivatives of 3,4,9-perylenetricarboxylic acid and of 3,4,9,10-perylenetetracarboxylic acid dissolved in the nematic liquid crystal mixture ZLI 1695 have been recorded as a function of temperature. On the basis of these spectra, the directions of the absorption transition moments with respect to the principle axes of the order tensor, and the order parameters S * and D * for perylene derivatives have been determined assuming a C s or C 2 point symmetry group for the dyes molecules. From a generalized equation describing properties of anisotropic phases, the conditions necessary to determine a complete set of the order parameters and the pieces of possible information that can be obtained from the polarized optical spectroscopy are discussed.