Nematic Liquid Crystal MBBA Research Articles

Motion of suspended particles in a nematic liquid crystal

The motion of suspended particles in a nematic liquid crystal MBBA has been observed and measured under an optical microscope and using an automatic image processing system. In the nematic phase the motion of the suspended particles seems to be “frozen”, but in the isotropic state the Brownian motion of the particles can be clearly observed and satisfies Einstein's formula. The experiment implies that the “freeze” phenomenon may be associated with the local directional order of molecules in the nematic phase.

Phenomenological Approach to the Description of the Dimerization Kinetics in Nematic Mesophase

Abstract The expression has been obtained, which determines the observed reaction rate constant in the nematic mesophase as a function of scalar liquid crystal order parameter. The expression includes the experimentally obtained parameters. It has been used for the estimation of the rate constants for 2-methyl-2-nitrosopropane at selected temperatures in nematic liquid crystal MBBA.

Observation of optical bistability by prism excitation of a nonlinear film-guided wave

The prism excitation of a film waveguide with an optically nonlinear substrate consisting of the nematic liquid crystal MBBA has been investigated experimentally. We observed bistable behavior with respect to the incident optical power and the coupling angle as well.

Selective dipolar order and cross relaxation in proton NMR: application to liquid crystals

Methods to exploit the large dipole-dipole interactions among protons in magneto-aligned liquid crystals are described. These allow simultaneous determination of the spin-lattice relaxation rates of the Zeeman and dipolar reservoirs. In particular, a selected group of spins whose dipolar splitting is significantly different from those of the other spin groups may be selectively excited to monitor its relaxation rates. It is demonstrated that cross relaxation effects exist among different spin subgroups in the nematic liquid crystal MBBA.

Interferometric measurement of large thermal nonlinear index in MBBA

A Twyman-Green interferometer is used to study a large optical nonlinearity, believed to be thermal in origin, in the nematic liquid crystal MBBA. Fringe shifts arising from laser-induced heating of the nematic layer are measured and compared to a solution of the heat equation. There is reasonable agreement between experiment and theory in the dependence of fringe shift on beam intensity and cell temperature.

Saturation Studies of Spin Probes Dissolved in a Glass Forming Nematic Liquid Crystal MBBA

Abstract We have performed saturation studies of spin probes dissolved in the glass forming nematic liquid crystal MBBA that are similar to our previously reported studies of probes dissolved in an isotropic liquid. We have also repeated the measurements using a technique that eliminates a systematic error and find a similar functional form for the spin-lattice relaxation time (T 1) vs. temperature even though the absolute values are somewhat different. Both sets of data show evidence of the previously reported main glass and chain melting transitions.

Theory of second-harmonic generation in nematic liquid crystals.

In recent years the observation of optical second-harmonic generation (SHG) in nematic liquid crystal (NLC) MBBA [N-(4-methoxybenzylidene)-4'-n-butylaniline] raised again the question of whether NLC's possess centrosymmetry. Lyakhov et al. [Izv. Akad. Nauk SSSR 45, 917 (1981)] suggested that these observations can only be explained by considering the NLC as noncentrosymmetric and optically biaxial. In this paper it is shown phenomenologically that even with ${D}_{\ensuremath{\infty}h}$ symmetry, SHG arising from ee\ensuremath{\rightarrow}e, oo\ensuremath{\rightarrow}e, and oe\ensuremath{\rightarrow}o interactions can occur in an aligned NLC. With curvature strains SHG arising from ee\ensuremath{\rightarrow}o, ee\ensuremath{\rightarrow}e, oe\ensuremath{\rightarrow}o, oe\ensuremath{\rightarrow}e, oo\ensuremath{\rightarrow}o, and oo\ensuremath{\rightarrow}e interactions are all possible. Several experimental observations are satisfactorily explained by the present calculation. The order of magnitude of the effective susceptibilities estimated from a quadrupole-moment mechanism agrees with the values obtained by Arakelyan et al. [Mol. Cryst. Liq. Cryst. 71, 137 (1981)] and by Barnik et al. [Mol. Cryst. Liq. Cryst. 98, 1 (1983)]. One may conclude that it is possible to explain the occurrence of SHG in NLC's without having to invoke noncentrosymmetry of the system.

Polarization analysis of induced gratings in nematic liquid crystal films

A detailed analysis of classical diffraction on the optically induced phase gratings in thin films of the nematic liquid crystals 5CB, 6CB, and MBBA is presented. It enables a quantitative separation of thermal contributions from reorientational contributions to the gratings to be made by a simple measurement of the gratings' diffraction efficiency with a polarized probe laser.

Determination of the rotational viscosity coefficient γ1of nematic liquid crystals

An apparatus for the measurement of the rotational viscosity coefficient gamma 1 of nematic liquid crystals is described. The commonly used method with a rotating magnetic field is improved by using a rotating sample and a fixed magnetic field, which allows larger field strengths and sample diameters. This technique and a precise determination of the torque on the sample lead to relative errors of measurement of the order of 0.3%. Wall effects, which can cause serious errors, are discussed. Their influence on the gamma 1 values obtained is small if magnetic fields of sufficient strength are used. Kinematic and dynamic rotational viscosity coefficients for the nematic liquid crystals MBBA and 4-n-octyloxyphenyl 4-n-pentyloxybenzoate are presented.

Structural transition at the free surface of the nematic liquid crystals MBBA and EBBA

In this paper we report measurements of the angle θ t between the director and the vertical axis at the free surface of the nematic liquid crystals MBBA and EBBA. The basis of our technique is the same as the one reported by Bouchiat and Langevin-Cruchon for measurements on MBBA and PAA. However some important improvements make our technique more suitable and more accurate so that we can measure θ t to within ±1° over the whole range of the nematic phase. We find a new structural transition occurs in MBBA (and EBBA) when the temperature reaches a critical value T 0 close to the clearing temperature of the nematic L.C. Below T 0 the polar angle θ t tends to zero as (T 0 -T)β, where β≃0.5 is a critical exponent. Information about the anchoring energy at the free surface of MBBA is also obtained. The experimental results agree with two recent models of the free surface (Parsons, Mada) Mesure de l'angle θ t entre le directeur et la normale a la surface libre de MBBA et EBBA. Observation d'une nouvelle transformation structurale a la surface libre pour une valeur critique de la temperature T 0 . Au-dessous de T 0 , decroissance de θ t vers zero suivant (T 0 -T) 1/2 . Accord avec des previsions de Parsons et de Mada

An EPR determination of the helix pitch and relative handedness of some induced cholesteric mesophases

The pitch of weakly cholesteric mesophases obtained by dissolving a chiral compound in the nematic liquid crystal MBBA, has been determined by an EPR spin probe technique. The measured values are in excellent agreement with those obtained by the Grandjean-Cano microscopic method. Once a sufficiently large number of EPR simulated spectra have been computed, the experimental procedure for the determination of the pitch is shown to be faster and as reliable as other methods. The handedness of the mesophase can also be easily established by observing the spectral variations which follow the addition of a cholesteric of known handedness to a given sample.

Alignment of Nematic MBBA on the Cleavage Surface of Ferroelectric TGS

The alignments of molecules of nematic liquid crystal MBBA on the ferroelectric antiparallel domain revealed on the cleavage surface of TGS are investigated using a polarizing microscope. On the positive domain, the alignment is one in which the directors are in a plane nearly perpendicular to the (101) plane of the TGS and are titled at about 55° to the TGS surface. On the negative domain, the alignment is homogeneous with the directors nearly perpendicular to the (001) plane.

Second Harmonic Generation in Nematic Liquid Crystals: Effect of Molecular Symmetry, Nonlinear Susceptibility and Phase-Matching

Abstract Second harmonic generation in the nematic liquid crystal MBBA was experimentally observed both for phase-matched and for non-phase-matched interactions. The nonlinear susceptibility tensor components values were measured for the first time. The possible SHG mechanisms are discussed. The obtained new physical information on liquid crystal structure is emphasized. The experimental results are thoroughly compared with calculations of second harmonic generation parameters. A physical interpretation of observable effects is given.

External Noise Effect on the Onset of Williams Domain in Nematic Liquid Crystals

In order to examine the effect of external noise on the electrohydrodynamic instability, we have measured the wave number spectrum of the pattern of Williams domain in nematic liquid crystals MBBA in the presence of external noise superposed to the a c voltage. The threshold voltage for the onset of Williams domain is found to shift to higher values with increasing noise power. Its noise dependence is interpreted in terms of a theory presented by Dubois-Violette et al.

Dependence of the Electrically Controlled Birefringence on Applied Frequency in a Nematic Liquid Crystal

Abstract Detailed studies on the dependence of the ECB effect on the applied frequency have been made in a nematic liquid crystal MBBA. It is found experimentally that, when the applied voltage is just above the threshold voltage, there are strong dependences of both the delay time and the mean deformation angle of the molecular director subjected to a constant voltage setup on the applied frequency. Those applied frequency dependences of the ECB effect can be interpreted in terms of the dielectric alignment in an electrically conducting nematic liquid crystal. We have measured the capacitance of the deformed liquid crystal cell varying the applied frequency. The experimental results are compared with the Gruler and Cheung theoretical prediction, and good agreements are obtained between them when the applied voltage is just above the threshold voltage.

Wave Number Spectrum of Dissipative Structures in Nematic Liquid Crystals

We have measured the wave number spectrum of dissipative structures in nematic liquid crystals MBBA which undergo successive hydrodynamic instabilities in external electric field. The transition to turbulence is preceded by two successive stationary modes; the first of these corresponds to the two dimensional flow (Williams domain), and the second to the three dimensional flow (grid pattern). The spectrum for the turbulent state is found to obey the Kohogorov's -5/3 power law.

Refractive Index Distribution and Molecular Alignment in Electro-Optical Effect of Nematic Liquid Crystal MBBA

Refractive Index Distribution and Molecular Alignment in Electro-Optical Effect of Nematic Liquid Crystal MBBA

Evidence for Anisotropic Expansion of Secondary Dynamic Scattering

Abstract An experimental study of generation and propagation of the secondary dynamic scattering mode /DSM2/ in the nematic liquid crystal MBBA in the conduction regime is presented, In homogeneously aligned samples a strong anisotropy of the velocity of propagation of DSM2 was found and measured, The frequency dependence of the threshold voltage for DSM2 was determined by light transmission measurements and microscopic observations.

Orientation control of dye molecules in a liquid crystal

Laser dye molecules (coumarin 6) were dissolved in a nematic liquid crystal MBBA + BBCA. Great changes in polarization and fluorescence vs applied voltage were found as well as a change in dielectric constant. Molecular orientation of the liquid crystal under the electric field and association of the dye molecules are identified as having the greatest effect on the fluorescence intensity.

Helical Twisting Power of Chiral Substituted Dibenzo |B,F|1,4-Diazocines in Nematic Liquid Crystals

Abstract Twisting powers of some chiral substituted dibenzo |b, f |l l,4-diazocines were determined in nematic liquid crystal MBBA.