Hans Gruler

Electric Field-Induced Deformations in Oriented Liquid Crystals of the Nematic Type

Abstract The influence of an electric field on nematic liquid crystals is investigated for several orientations of the liquid crystal. We give a theoretical treatment of the normal deformation that arises from the interaction of the electric field with the dielectric anisotropy. Our experimental deformations show relatively good agreement with the calculated values. Some anomalous deformations were also investigated. Conditions and threshold voltages for Williams domains are discussed. If the undisturbed optical axis is parallel to the applied electric field and the dielectric anisotropy is positive, no Williams domains appear. This result is in agreement with the Carr-Helfrich model.

Elastic Constants of Nematic Liquid Crystals

Abstract We present a theoretical treatment and give experimental observations of the deformation that occurs in a nematic liquid crystal when electric or magnetic fields are applied. We consider only normal deformations in the nematic material where fluid flow and other dynamic phenomena play no role. Three important sample geometries are considered in the magnetic field, and the experimentally observed deformations are in good agreement with theory. The normal deformation induced by electric fields is of interest from a device standpoint, and we give a solution for the deformation that is valid even for large dielectric anisotropics. This solution has been experimentally verified. We give a detailed comparison of the distortions produced by electric and magnetic fields and show that the deformations are of a similar form even though the field is nonuniform in the electric case. The change in birefringence and electrical capacitance as a function of distortion is discussed as a means of observing the deformation.

Correlation between Electrical Properties and Optical Behaviour of Nematic Liquid Crystals

Abstract The results of simultaneous electrical and optical investigations in p-azoxyanisole and di-n-heptoxy-azobenzene are presented. The role of fluid flows is discussed in connection with their importance for domain formation. The mechanism of domain formation and turbulence occurrence is found to be different for d.c. and very low frequencies than it is for higher frequencies. For strong electric fields the behaviour of azoxyanisole is very different from that of the azo compound, a fact attributed to the different sign of the dielectric anisotropy.

Dielectric alignment in an electrically conducting nematic liquid crystal

For small deformations the local macroscopic electric field strength in a deformed uniaxial crystal is derived. For relative high frequencies (≫ σ/e0e) the electric field strength is determined by the anisotropic dielectric properties and for relative low frequencies (≪ σ/e0e) by the anisotropic electric conductivity. The local electric field strength is frequency independent if the dielectric and the electric conductivity tensor have the same anisotropy. The nonhydrodynamic deformation of a nematic liquid crystal under the influence of an electric field is derived from the torque balance equation in the small‐angle approximation. This dielectric alignment is frequency dependent corresponding to the frequency dependence of the local electric field strength.

Investigations on the Elastic Constants of the Nematic Homologous Series of 4,4′-Di(n-Alkoxy) Azoxybenzene

Abstract The splay and bend elastic constants of the 4,4′-di(n-alkoxy)-azoxybenzenes have been determined by measuring the change in birefringence of parallel oriented samples in a magnetic field. The Maier-Saupe theory shows that the elastic constants should be proportional to S 2 where S is the long range order parameter, but that short range order can also strongly affect these constants. Our measurements on the homologous series do indicate that both long and short range order are important. The temperature dependence of the bend and splay elastic constants, for example, near the nematic→isotropic transition cannot be explained by the long range order. This dependence corresponds to the pretransitional effects seen in the thermal expansion coefficient in this region, which can only be due to short range order. Our measurements also show that near the nematic→smectic C transition the splay (k 11) and bend (k 33) elastic constants abnormally increase. This strong temperature dependence of k 11 and k 33 ...

Periodic free-surface disclination at nematic to smectic C phase transition

Abstract We have observed the sudden formation of parallel, equally spaced line disclinations at the free surface of a liquid crystal just before the nematic to smetic C phase transition, In 4,4′ -di- n -heptyloxy-azoxybenzene the disclinations have a separation of 1.5μ at formation, but this rapidly increases to 35μ over the remaining 0.10°C temperature interval to the smetic phase C. We interpret these disclinations as being analogous to the quantized flux filaments in the Shubnikov or mixed state phase of a type II superconductor.

Williams Domains and Dielectric Alignment in a Nematic Liquid Crystal

Abstract We investigate here the threshold behaviour of Williams domains and of the dielectric alignment in homeotropic and parallel oriented nematic films for different signs and magnitudes of the dielectric anisotropy. The theoretical predictions of the Helfrich theory are in agreement with the experiment.

Fluorescence Probes in LC-Multilayers at the Air-Water Interface

Abstract The ferroelectric liquid crystal molecule HOBACPC formes multilayers at the air-water interface. Measurements of the surface contact potential exhibit an orientation of those molecules adjacent to the liquid phase. Their polar head groups point towards the water. In contrast the molecules in the second and further layers are statistically oriented. The aminostilbazolium dye di5ASPBS serves as a probe molecule which is sensitive to the surrounding dielectric constant. We have inseted the dye in a HOBACPC matrix to examine the change of fluorescence during multilayer formation on the water surface. The fluorescence exhibits some characteristic features as a function of decreasing area: a strong decrease of the amplitude shortly before the formation of the first layer and several maxima at the following layer formations. The measurement at an area above 23 A2/molecule is not reproducible due to domains larger than the measurement spot.

The Elastic Constants of a Nematic Liquid Crystal

Abstract The elastic constants of a nematic liquid and of a solid crystal were derived by comparing the Gibbs free energy with the elastic energy. The expressions for the elastic constants of the nematic and the solid crystal are isomorphic: k ∝ ∣ D ∣ /1. In the nematic phase ∣ D ∣ and I are the mean field energy and the molecular length. In the solid crystal, ∣ D ∣ and l correspond to the curvature of the potential and to the lattice constant, respectively. The measured nematic elastic constants show the predicted l dependence.

Poly[bis(ethylglycoxypropyl)silylene]s and poly[n-alkyl-(ethylglycoxy)silylene]s and their higher homologues, 2. Amphiphilicity and molecular ordering in the solid state†

A series of polyglycol substituted polysilylenes containing up to three ethoxy units in one or both substituents of the silicon atoms were investigated for their bulk and solution properties. In comparison to the analogous nonpolar poly(di-n-alkylsilylenes), these ether functionalized polysilylenes show improved solubility in alcohols and a distinct amphiphilic character depending on the length of side chains and number of oxygen atoms incorporated. Spreading experiments on water/air interfaces demonstrated the formation of stable monolayers. Polysilylene samples with -CH 2 -(CH 2 -CH 2 -O) n -CH 2 -CH 3 n=1, 2 substituents exhibit thermochromism which was correlated to a first order endotherm found by calorimetric measurements. Specimen containing two ethylglycoxypropyl substituents per silicon atom or ethyldiglycoxypropyl side chains showed no temperature or solvent dependence of the UVabsorption.