Laser pulse shaping with liquid crystals

A method of unpolarized laser pulses shaping is reported. The basis of the method is the use of an hybrid optical bistable device with nematic liquid-crystals, similar to the one previously reported by us. A sample of the input light constrols, by an asymmetrical electronic comparator, a 1 x 2 electro-optical total switch. The output pulses are reshaped and maintain the same polarization properties as the input light. From triangular input light signals, symmetriacl and asymmetrical output pulses have been obtained. The minimum pulse width achieved was 0.1 msec. A representation of the output versus input light signals gives an hysteresys cycle in the asymmetrical case.

Magnetic behavior of 10 nm-magnetite particles diluted in lyotropic liquid crystals

A magnetic study of 10 nm magnetite nanoparticles diluted in lyotropic liquid crystal and common liquids was carried out. In the liquid crystal the ZFC-FC curves showed a clear irreversible behavior, and it was possible to distinguish the nematic from the isotropic phase since the magnetization followed the dependence of the nematic order parameter with the temperature. This behavior could be mimicked by Monte Carlo simulation. (C) 2011 American Institute of Physics. [doi:10.1063/1.3549616]

An electro-rheological study of the nematic liquid crystal 4-n-heptyl-4'-cyanobiphenyl

An experimental and theoretical study of the electro-rheological effects observed in the nematic phase of 4-n-heptyl-4'-cyanobiphenyl has been conducted. This liquid crystal appears to be a model system, in which the observed rheological behaviour can be interpreted by the Leslie-Ericksen continuum theory for low molecular weight liquid crystals. Flow curves are illustrated at different temperatures and under the influence of an external electric field ranging from 0 to 3 kV mm-1, applied perpendicular to the direction of flow. Also presented is the apparent viscosity as a function of temperature, over similar values of electric field, obtained at different shear rates. A master flow curve has been constructed for each temperature by dividing the shear rate by the square of the electric field and multiplying by the square of a reference value of electric field. In a log-log plot, two Newtonian plateaux are found to appear at low and high shear rates, connected by a shear-thinning region. We have applied the Leslie-Ericksen continuum theory, in which the director alignment angle is a function of the electric field and the flow field boundary conditions are neglected, to determine viscoelastic parameters and the dielectric anisotropy.

Light shutters from nanocrystalline cellulose rods in a nematic liquid crystal

This work reports a recently developed electro-optical (EO) device that can potentially be used as a light shutter or a privacy window. By using nanocrystalline cellulose rods, we were able to improve some of the most relevant parameters characterising the EO behaviour. A brief description of the proposed working mechanism for these devices is presented, and numerical simulations based on this mechanism of both the optical transmission and the cells' electrical capacitance are compared with the obtained results, validating the underlying working model considered.

The additive effects in matrix dispersed liquid crystals

Dissertation presented at Faculdade de Ciências e Tecnologia from Universidade Nova de Lisboa to obtain the degree of Master in Chemical and Biochemical Engineering

Viscous fingering in liquid crystals: Anisotropy and morphological transitions

We show that a minimal model for viscous fingering with a nematic liquid crystal in which anisotropy is considered to enter through two different viscosities in two perpendicular directions can be mapped to a twofold anisotropy in the surface tension. We numerically integrate the dynamics of the resulting problem with the phase-field approach to find and characterize a transition between tip splitting and side branching as a function of both anisotropy and dimensionless surface tension. This anisotropy dependence could explain the experimentally observed (reentrant) transition as temperature and applied pressure are varied. Our observations are also consistent with previous experimental evidence in viscous fingering within an etched cell and simulations of solidification.

Lattice-gas model of orientable molecules: Application to liquid crystals

We have analyzed a two-dimensional lattice-gas model of cylindrical molecules which can exhibit four possible orientations. The Hamiltonian of the model contains positional and orientational energy interaction terms. The ground state of the model has been investigated on the basis of Karl¿s theorem. Monte Carlo simulation results have confirmed the predicted ground state. The model is able to reproduce, with appropriate values of the Hamiltonian parameters, both, a smectic-nematic-like transition and a nematic-isotropic-like transition. We have also analyzed the phase diagram of the system by mean-field techniques and Monte Carlo simulations. Mean-field calculations agree well qualitatively with Monte Carlo results but overestimate transition temperatures.

Use of an open photoacoustic cell for the thermal characterisation of liquid crystals

In this paper, we describe the use of an open cell photoacoustic configuration for the evaluation of the thermal effusivity of liquid crystals. The feasibility, precision and reliability of the method are initially established by measuring the thermal effusivities of water and glycerol, for which the effusivity values are known accurately. In order to demonstrate the use of the present method in the thermal characterization of liquid crystals, we have measured the thermal effusivity values in various mesophases of 4-cyano-4 - octyloxybiphenyl (8OCB) and 4-cyano-4 -heptyloxybiphenyl (7OCB) liquid crystals using a variable temperature open photoacoustic cell. A comparison of the measured values for the two liquid crystals shows that the thermal effusivities of 7OCB in the nematic and isotropic phases are slightly less than those of 8OCB in the corresponding phases

Use of an open photoacoustic cell for the thermal characterisation of liquid crystals

In this paper, we describe the use of an open cell photoacoustic configuration for the evaluation of the thermal effusivity of liquid crystals. The feasibility, precision and reliability of the method are initially established by measuring the thermal effusivities of water and glycerol, for which the effusivity values are known accurately. In order to demonstrate the use of the present method in the thermal characterization of liquid crystals, we have measured the thermal effusivity values in various mesophases of 4-cyano-4 - octyloxybiphenyl (8OCB) and 4-cyano-4 -heptyloxybiphenyl (7OCB) liquid crystals using a variable temperature open photoacoustic cell. A comparison of the measured values for the two liquid crystals shows that the thermal effusivities of 7OCB in the nematic and isotropic phases are slightly less than those of 8OCB in the corresponding phases

Use of an open photoacoustic cell for the thermal characterisation of liquid crystals

In this paper, we describe the use of an open cell photoacoustic configuration for the evaluation of the thermal effusivity of liquid crystals. The feasibility, precision and reliability of the method are initially established by measuring the thermal effusivities of water and glycerol, for which the effusivity values are known accurately. In order to demonstrate the use of the present method in the thermal characterization of liquid crystals, we have measured the thermal effusivity values in various mesophases of 4-cyano-4 - octyloxybiphenyl (8OCB) and 4-cyano-4 -heptyloxybiphenyl (7OCB) liquid crystals using a variable temperature open photoacoustic cell. A comparison of the measured values for the two liquid crystals shows that the thermal effusivities of 7OCB in the nematic and isotropic phases are slightly less than those of 8OCB in the corresponding phases

Use of mirage effect for the detection of phase transitions in liquid crystals

he phenomenon of single beam mirage effect, otherwise known as photothermal deflection (PTD) effect using a He–Ne laser beam has been employed to detect phase transitions in some liquid crystals. It has been observed that anomalous changes in amplitude occur in the PTD signal level near the transition temperature. The experimental details and the results of measurements made in liquid crystals E8, M21 and M24 are given in this paper.

Laser Induced Photoacoustic Technique for the Detection of Phase Transitions in Liquid Crystals

In this paper we report the use of a laser induced phoroacoustic technique for the detection of multiple phase transitions in heptyl-oxy-cyanobiphenyl (70CB) and octyl-oxy-cyanobiphenyl (80CB) liquid crystals. The observed photoacoustic signal amplitude profile carries clear signatures of two tirst order transitions in 70CB and two tirst order and a second order transitions in 80CB. Analysis of the experimental data using Rosencwaig-Gersho theory shows that the sudden decrease in the photoacoustic (PA) signal amplitude during phase transitions is due to a sharp increase in the heat capacity of the samples near the transition temperatures.

Photo-induced phase transitions in azobenzene-doped liquid crystals

The effect of irradiation (UV-visible light) on a nematic liquid crystal doped with a photoactive azobenzene derivative was investigated. The selective irradiation results in either an E implies Z or Z implies E isomerization of the azobenzene unit. The effect of the isomerization is to cause a reversible depression of the liquid crystal to isotropic (LC implies l) phase transition temperature of the doped mixture, which can be monitored optically as an isothermal phase transition. This depression also results in a biphasic liquid crystal+isotropic region which is discussed. The authors investigate the cause and magnitude of the phase depression as a function of the amount of doped 4-butyl-4'-methoxyazobenzene (photoactive unit) in 4-cyano-4'-n-pentylbiphenyl (liquid crystal unit), and as a function of the percentage conversion of E implies Z (caused by isomerization) in the azobenzene. The photostationary state of the doped mixtures achieved by Z implies E isomerization is considered and its effect upon the transition temperature of the mixture and response time of the system is discussed. They discuss the implications of the photostationary state with regards to the reversibility of the photo-induced phase transition and hence potential applications.

Computer simulation of ordering and dynamics in liquid crystals in the bulk and close to the surface

The aim of this PhD thesis is to investigate the orientational and dynamical properties of liquid crystalline systems, at molecular level and using atomistic computer simulations, to reach a better understanding of material behavior from a microscopic point view. In perspective this should allow to clarify the relation between the micro and macroscopic properties with the objective of predicting or confirming experimental results on these systems. In this context, we developed four different lines of work in the thesis. The first one concerns the orientational order and alignment mechanism of rigid solutes of small dimensions dissolved in a nematic phase formed by the 4-pentyl,4 cyanobiphenyl (5CB) nematic liquid crystal. The orientational distribution of solutes have been obtained with Molecular Dynamics Simulation (MD) and have been compared with experimental data reported in literature. we have also verified the agreement between order parameters and dipolar coupling values measured in NMR experiments. The MD determined effective orientational potentials have been compared with the predictions of Maier­Saupe and Surface tensor models. The second line concerns the development of a correct parametrization able to reproduce the phase transition properties of a prototype of the oligothiophene semiconductor family: sexithiophene (T6). T6 forms two crystalline polymorphs largely studied, and possesses liquid crystalline phases still not well characterized, From simulations we detected a phase transition from crystal to liquid crystal at about 580 K, in agreement with available experiments, and in particular we found two LC phases, smectic and nematic. The crystal­smectic transition is associated to a relevant density variation and to strong conformational changes of T6, namely the molecules in the liquid crystal phase easily assume a bent shape, deviating from the planar structure typical of the crystal. The third line explores a new approach for calculating the viscosity in a nematic through a virtual exper- iment resembling the classical falling sphere experiment. The falling sphere is replaced by an hydrogenated silicon nanoparticle of spherical shape suspended in 5CB, and gravity effects are replaced by a constant force applied to the nanoparticle in a selected direction. Once the nanoparticle reaches a constant velocity, the viscosity of the medium can be evaluated using Stokes' law. With this method we successfully reproduced experimental viscosities and viscosity anisotropy for the solvent 5CB. The last line deals with the study of order induction on nematic molecules by an hydrogenated silicon surface. Gaining predicting power for the anchoring behavior of liquid crystals at surfaces will be a very desirable capability, as many properties related to devices depend on molecular organization close to surfaces. Here we studied, by means of atomistic MD simulations, the flat interface between an hydrogenated (001) silicon surface in contact with a sample of 5CB molecules. We found a planar anchoring of the first layers of 5CB where surface interactions are dominating with respect to the mesogen intermolecular interactions. We also analyzed the interface 5CB­vacuum, finding a homeotropic orientation of the nematic at this interface.

Molecular organization of n-cyanobiphenyl liquid crystals on a molybdenite surface

The alignement and anchoring of liquid crystals on solid surfaces is a key problem for modern device technology that until now has been treated empirically, but that can now be tackled by atomistic computer simulations. Molecular dynamics (MD) simulations were used in this thesis work to study two films of 7 and 8 n-alkyl-4’cyanobiphenyl (7CB and 8CB) liquid crystals , with a thickness of 15 nm, confined between two (001) surfaces of MoS2 (molybdenite). The isotropic and nematic phases of both liquid crystals were simulated, and the resulting structures characterized structurally. A new force field was designed to model the interactions between the liquid crystal (LC) molecules and the surface of molybdenite, while an accurate force field developed previously was used to model the 7CB and 8CB molecules. The results show that the (001) molybdenite surface induces a planar orientation in both the liquid crystals. For the nematic phase of 8CB, one of the two solid/LC interfaces is composed of a first layer of molecules aligned parallel to the surface, followed by a second layer of molecules aligned perpendicular to the surface (also called, homeotropic). The effect of the surface appears to be local in nature as it is confined to the first 15 Angström of the LC film. Conversely, for the nematic phase of 7CB, a planar ordering is established into the LC film. The LC molecules at the interface with the molybdenite appear to align preferentially their alkyl chains toward the solid substrate. The resulting tilt angle of molecules was found to be in good agreement with experimental measurements available in literature. Despite the fact that the MD simulations spanned a time range of more than 100 ns, the nematic phases of both 7CB and 8CB were found not to be completely formed. In order to confirm the findings presented in this thesis, we propose to extend the current study.