Anisotropic shear viscosity in nematic liquid crystals: new optical measurement method

We propose a new optical method and the experimental set-up for measuring the anisotropic shear viscosities of nematic liquid crystals (LCs). LC shear viscosities can be optimized to improve liquid crystal display (LCD) response times, e.g. in vertical aligned nematic (VAN) or bistable nematic displays (BND). In this case a strong back-flow effect essentially determines the LCD dynamic characteristics. A number of shear viscosity coefficients defines the LCD response time. The proposed method is based on the special type of a shear flow, namely, the decay flow, in the LC cell with suitably treated substrates instead of magnetic or electric field application. A linear regime of a quasi-stationary director motion induced by a pressure difference and a proper configuration of a LC cell produces decay flow conditions in the LC cell. We determine three principal shear viscosity coefficients by measuring relative time variations of the intensity of the light passed through LC cells. The shear viscosity coefficient measurements provide a new opportunity for the development of new LC mixtures with fast response times in VAN, BND and other important LCD types.     

Rotational viscosity of nematic liquid crystals and their shear viscosity under flow alignment

The influence of molecular properties on the rotational viscosity, γ₁, of nematic liquid crystals is studied. The shear viscosity under flow alignment, η_s, is determined for the same liquid crystals. A significant correlation between both quantities is found. An equation is presented which allows the calculation of γ₁ from η_s with an error of about 20 per cent for the liquid crystals studied.     

Anisotropic attenuation of acoustic waves in nematic liquid crystals

Velocity and attenuation measurements for the 5 MHz ultrasonic wave were carried out in two nematic liquid crystals with a uniform orientation created by a magnetic field. Calculations show agreement of the results with those for the viscosity coefficients measured with non-acoustic methods.     

Out of shear plane deformations in nematic liquid crystals

The stability of the director field to deformations out of the plane of shear is examined by use of the Taylor expansion method based on catastrophe theory. For simple shear flow of nematics, the coming out of the shear plane is found for suitable surface alignment and not too high twist elastic constant. The role of these parameters is pointed out. Non-flow-aligning nematics are also considered, and results consistent with earlier reports are obtained.     

Rotational viscosity and molecular structure of nematic liquid crystals

An electro-optical technique was used to measure the rotational viscosity of nematic liquid crystals, whose structures were chosen to investigate the effects of linking groups and lateral groups on rotational viscosity. These results are compared with the theory developed by Osipov and Terentjev. It is shown that the theory is useful in predicting rotational viscosity for specific molecular structures. The agreement between theory and experiment is found to be good for all the liquid crystals studied.     

Third order optical non-linearities of nematic liquid crystals

Liquid crystals can exhibit large third order optical non-linearities. Using the Z-scan technique, we have measured the intensity dependence of the non-linear absorption and non-linear refractive indices on time-scales ranging from milliseconds to picoseconds for nematic liquid crystals. This method allows the determination of the non-linear absorption coefficients as well as the signs and magnitudes of the non-linear refractive indices for different polarizations. A two pulse technique further enables estimation of the response time of the dominant mechanism. Using CW argon and Q-switched and mode-locked pumped dye lasers, we have carried out Z-scan measurements on aligned liquid crystal samples as a function of temperature, as well as wavelength. In the geometries studied, director reorientation is not expected to take place. On the nanosecond time-scale, all materials studied were self-focusing for polarization perpendicular to the director, and self-defocusing for parallel polarization. On the picosecond timescale, the samples were self-focusing for all polarizations, but strong non-linear birefringence was typically observed. An attempt is made to relate the bulk response to the molecular structures.     

The determination of the viscosity coefficients of nematic liquid crystals

This paper considers the flow generated by driving a sample of nematic liquid crystal through a rectangular capillary by application of a small pressure gradient in the presence of a large aligning magnetic field. A theoretical calculation based on the continuum theory of nematics is presented which makes some allowance for non-uniform alignment induced by flow, and allows a more accurate determination of the viscosities corresponding to the three principal configurations in the plane of shear.     

The measurement of weak chirality using nematic liquid crystals

ABSTRACT

Two recently reported methods for measuring weak chirality using nematic liquid crystals are described. These are between one and two orders of magnitude more sensitive than the standard Grandjean-Cano wedge. Pitch lengths of 50 mm have been measured with prospects for extending this beyond 100 mm; this is equivalent to measuring the presence of 1 ppm of a strong chiral dopant. Results for a number of standard chiral dopants and nematic liquid crystal hosts are reported.     

Carbazole nematic liquid crystals

A number of calamitic 2,7-diary-N-alkyl-substituted carbazoles with an enantiotropic nematic phase have been prepared. Branching of the aliphatic chain attached to the nitrogen atom in the carbazole ring leads to significantly lower liquid crystal transition temperatures. These new materials show a lower ionisation potential than fluorene analogues and blue photoluminescence in solution and as thin solid films.     

Landau theory-based estimates for viscosity coefficients of uniaxial and biaxial nematic liquid crystals

Using Landau theory, it is shown that eight phenomenological parameters are needed to describe and distinguish the twelve viscosity coefficients of a biaxial nematic phase, or the five viscosity coefficients of a uniaxial nematic phase. The dependence of the coefficients on the macroscopic uniaxial and biaxial order parameters is established. Since these order parameters are determined by the anisotropies of the dielectric constant, we show that it should be possible to determine values for all eight of the phenomenological parameters of the theory from measurements of the temperature dependence of the five viscosities of a uniaxial phase.     

Defect-induced melting in nematic liquid crystals

In this paper we employ a relatively simple theory to show how a nematic disclination line can act as a nucleation site for the growth of the isotropic phase. With this theory we are able to find analytical expressions for the critical temperature of nucleation and the behaviour of the core radius as a function of temperature. We are then able to compare these results with a previous numerical model of this effect.     

Defect-induced melting in nematic liquid crystals

In this paper we employ a relatively simple theory to show how a nematic disclination line can act as a nucleation site for the growth of the isotropic phase. With this theory we are able to find analytical expressions for the critical temperature of nucleation and the behaviour of the core radius as a function of temperature. We are then able to compare these results with a previous numerical model of this effect.     

Magnetohydrodynamic effects in nematic liquid crystals

The behaviour of a nematic liquid crystal when it is spun about an axis orthogonal to a magnetic field is predicted to be controlled by the critical angular velocity, ω_c. For spinning speeds below ω_c theory shows that the director makes an increasing angle with the field until at ω_c this angle is 45°. Above ω_c the director should rotate with an angular velocity slightly less than that of the sample. Observation in both regimes allows ω_c to be determined; since it depends on the ratio of the diamagnetic anisotropy to the rotational viscosity coefficient of the nematic, this ratio can be measured. However, an experimental investigation by Eastman et al. [1], suggests that the theoretical relationship between ω_c and this ratio may be in error by a factor of about four. We have reanalysed their data in an attempt to check this important claim and have found that there is in fact good agreement between theory and experiment.     

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light-emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.     

Backflow-affected relaxation in nematic liquid crystals

A complete numerical study of a two-dimensional nematic backflow problem is presented. Nematodynamic equations are reviewed, and characteristic scales are introduced. The relaxation under the application and suppression of a magnetic field is studied in square- and rectangular-shaped cells. Solutions for the flow fields, director fields, and director time derivative fields are given and these are interpreted to gain a qualitative understanding of the problem. The backflow is found to depend critically on the geometry of the cell. The complete solution is compared with the simplified approach in which the backflow is neglected. The discrepancy depends strongly on the cell geometry.     

Oligomeric rod-disc nematic liquid crystals

Six new oligomeric nematic liquid crystals are reported consisting of a triphenylene-based core attached to which are six 4-cyanobiphenyl units via flexible alkyl spacers.     

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light‐emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.     

Orientational optical nonlinearity of nematic liquid crystals induced by high-molecular-mass azo-containing compounds

Processes of light-induced reorientation of nematic liquid-crystalline molecules induced by the addition of low concentrations (0.1–2.0 wt %) of comb-shaped polymers and carbosilane dendrimers containing azobenzene fragments are studied. When the molecular structure of the above compounds becomes more complicated, the induced orientational nonlinearity increases. The introduction of 2G and 3G dendrimers into a nematic has for the first time made it possible to visualize and study a purely optical first-order Freedericksz transition in the field of a linearly polarized wave.     

Nanoparticles in nematic liquid crystals: interactions with nanochannels

A mesoscale theory for the tensor order parameter Q is used to investigate the structures that arise when spherical nanoparticles are suspended in confined nematic liquid crystals (NLCs). The NLC is "sandwiched" between a wall and a small channel. The potential of mean force is determined between particles and the bottom of the channels or between several particles. Our results suggest that strong NLC-mediated interactions between the particles and the sidewalls of the channels, on the order of hundreds of k(B)T, arise when the colloids are inside the channels. The magnitude of the channel-particle interactions is dictated by a combination of two factors, namely, the type of defect structures that develop when a nanoparticle is inside a channel, and the degree of ordering of the nematic in the region between the colloid and the nanochannel. The channel-particle interactions become stronger as the nanoparticle diameter becomes commensurate with the nanochannel width. Nanochannel geometry also affects the channel-particle interactions. Among the different geometries considered, a cylindrical channel seems to provide the strongest interactions. Our calculations suggest that small variations in geometry, such as removing the sharp edges of the channels, can lead to important reductions in channel-particle interactions. Our calculations for systems of several nanoparticles indicate that linear arrays of colloids with Saturn ring defects, which for some physical conditions are not stable in a bulk system, can be stabilized inside the nanochannels. These results suggest that nanochannels and NLCs could be used to direct the assembly of nanoparticles into ordered arrays with unusual morphologies.