Self-organized liquid-crystal filamentary forms arise in many mixtures of the smectogen compounds at the transition
from the isotropic melt. In some mixtures, a subsequent phase transition to the crystal or crystal smectic phase occurs
at the core of the filaments. The resulting hard-core fibers act as anisotropic cylindrical lenses composed of the
crystalline core surrounded by the nematic shell. In this work, the filaments, referred to as nematoids, have been
obtained in several binary mixtures based on five mesogens: 4,4'-dipentylazoxybenzene, 4-dodecyloxy-4'-
pentylbiphenyl, 4-hexyl-4'nonyloxybiphenyl, 4-acetyl-4'-dodecylbiphenyl and 4''-pentylcyclohexyl 4-(4'-
pentylcyclohexyl) benzoate within a silicone oil as an inert liquid. To characterize the molecular arrangements within
the nematoids, we present the microinterferometric measurements of the refractive index distribution within the
fibers and its changes at the phase transitions.
Self-organization drives the system to its internal attractors and so it is particularly effective in the soft matter. Self-organized systems are created in the presence of two at least simultaneous and competitive interactions of comparable strength. Such phenomena are the most exciting features of the liquid-crystal state and are frequently observed in multicomponent and multiphase systems as spontaneous periodic superstructures. If some of the periods of the superstructure are comparable to the wavelength of light and a sort of photonic band gap can be determined in it, then the superstructure behaves as a photonic crystal. In chiral structures, the band of a total reflection of circularly polarized light can play the role of an angle-dependent photonic band gap. The mechanisms of the formation of the superstructures as the result of minimization of the free energy, including cross coupling between gradients of the director field around the disclinations and gradients of the concentration of dopants (the thermodynamic force of a concentration gradient), are discussed here. The experimentally observed stable patterns, e.g., spontaneous diffraction gratings with uniformly oriented helical axis of N* at weak anchoring, two-dimensional hexagonal arrays of disclination lines perpendicular to nematic layers doped with small amount of chiral compound (bubble domains), and oriented arrays of TGBA in chiral smectics, are presented.
The methodologies used in simulation studies of liquid crystals with an emphasis on the collective phenomena are shortly outlined together with the attractive and orientational intermolecular interactions.
Two main types of the striped domains in thin layers of chiral nematics can be distinguished: 1 . chiralBloch4ype domain walls placed between two neighbouring homeotropic regions which are indUced in chiral nematic layers by surface interactions ofthe chiral nematic liquid and glass plates; 2. cylindrical myelin filaments freely suspended within homeotropic nematic matrix induced in a chirahnematic thin layer by surface interaction or by an action of external electric (or magnetic) fields. The myelin filaments when oriented perpendicularly to the glass support are visible as the bubble domains and can be hexagonally packed within some regions ofthe layer. The Bloch-type walls have been investigated by T. Akahane and co-workers [1] with the use ofthe differential interference method. The cylindrical chiral myelins are observed at the front of diffusion of the chiral mixture into a homeotropic nematic layer or near the saturation region in the electrically induced phase transition N N. The structure of the myelin filaments has been proposed as a set of coaxial cylinders having the chiral-nematic molecular ordering with radially oriented screw axis [2]. Here, the cylindrical structure of the chiral myelins is confirmed by means of the Pluta fringe-field birefracting niicrointerferometiy. By directly measuring the distribution of the main refractive indices n1 and nas well as their difference n over perpendicular cross-sections of myelin filaments, this technique reveals microscopic features of the molecular arrangement within filaments. Computer models of the molecular structure based on the microinterferometric data are presented.
We show that collective Cotton-Mouton (C-M) effect can be used to determine the anisotropic orientational arrangement of molecules within scattering centers such as colloid and suspension particles. The technique applied to liquid-crystal suspensions indicates that both smectic-nematic and nematic-isotropic transitions can be easily determined within mesogenic droplets. The measurements were performed on mesomorphic 4-cyano-4'-(n-octyl) biphenyl having both smectic A and nematic phases. Using the generalized nth order Langevin's functions L(subscript n%/, the statistical model is presented for the collective C-M effect in the monodisperse suspensions of smectics and nematics.
The authors see an analogy between a molecular beam and light beam13. In the case of optical modelling of an effusion
molecular beam, in a simulation system the light from the source was introduced via a diffusion ground glass plate into
the "effusion" channel. The inside of the chanel was lined with the corrugated aluminium foil of a high reflection
coefficient and the light was reflected from the wall in chaotic directions, just like molecules from the wall of the actual
channel (Fig. 7). The intensity of the light beam distribution was measured with a photodetector. This means that
information about high vacuum processes can be received from investigations performed under normal pressure.
Some multicomponent liquid crystal systems show unusual behavior in the phase transition from the isotropic melt to the mesomorphic state. In these systems, the nucleation is performed in the form of filaments, called nematoids, freely suspended in the isotropic melt. The observed aspect ratio (diameter : length) of the nematoids achieves 1:3000. Matured nematoids are rather unstable and undergo rapid shrinkage to droplets. The main features common to essentially all nematoids in the multicomponent (nematic, smectic B, and non-mesogenic chiral dopant) systems are: they can split into two separate threads surrounding homeotropically oriented smectic 'lake' or can undergo the segmentation or double-spiralling before their transformation to droplets. To confirm the supposition that these processes develop the bifilar organization of the nematoids, the microinterferometric analysis was performed by using the Pluta birefracting microinterferometry. This analysis supports the conception that the nematoids represent multiphase systems and are composed of two parallel planar nematic filaments connected with homeotropically oriented smectic wall (gluon). To adjust the mutually perpendicular orientation of molecules, the peripheral filaments and gluon are assumed to be connected with chiral interface. Presumably, the creation of the nematoids is the result of the phase separation in a system with anisotropic surface tensions. It is possible that instability with respect to the segmentation of the nematoid filaments is caused by the anisotropy of the surface tensions at the interfaces within a nematoid whereas the double-spiraling can be the result of a phase transition of the gluon connecting both the filaments or within the chiral interfaces.
A review is given of our present general classification of the liquid crystals. The liquid crystal structures are divided into two main groups: nematic phases with parallel ordering of the long molecular axes and smectic phases having additional layered structure. An interesting phenomenon is that the liquid crystal phases when formed by optical active molecules develop chiral modifications: chiral nematics and chiral smectics. There are four fundamental smectic phases: A, B, L, and E having skewed analogues: C for smectic A and F, G, H, (with long molecular axes tilted to the side of the hexagon) and I, J, K (tilted to the apex of the hexagon) for smectics B, L, and E, respectively. Chiral nematics N and chiral modifications of smectics with weak interlayer correlation (C, I, and F) form long-range helicoidal structures. We also briefly discuss the main topological defects in liquid crystal structures: dislocations in layered (smectic) or pseudolayered (chiral nematic) phases and disclinations (including focal domains) that are fundamental defects of structures with continuous symmetries.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.