Antoni Adamczyk

Phase Transitions in Freely Suspended Smectic Droplets. Cotton-Mouton Technique, Architecture of Droplets and Formation of Nematoids

Abstract Phase transitions of aqueous suspensions of droplets of smectogens have been studied using the collective Cotton-Mouton (CM) technique. Two compounds were examined: NPOOB and 8OCB. A statistical model (previously developed for suspensions of nematogens) has been adapted to describe the collective CM effect in these systems. It was found that both the phase transitions and the changes of architecture within the droplets cause distinct changes in the CM effect. An attempt was made to investigate suspensions of theliquid crystalline germs created in the isotropic melts of nematic and smectic mesogens mixed with a chiral additive. In such mixtures, nematoids (the liquid crystal analogues of the whiskers of crystalline solids) are formed. It is usually found that nematoids arise as an extension of the tail of a monopole droplet or grow from a focal conic multipole droplet but “free” nematoids (without droplets at their ends) can also occur. The aspect ratio of nematoids varies with the composition, ra...

The Stable Inhomogeneities of Mixtures and the Permanent Liquid-Crystalline Diffraction Gratings

Abstract The stable structural inhomogeneities in nematics/optical active substance solutions have been experimentally investigated. The MBBA or MBBA/EBBA eutectic and abietic acid (AA) as convenient system was chosen. In the solutions of concentrations above some threshold value the creation of separated cholesteric filament regions immersed in nematics were observed, i. e. the distribution of the AA is not homogeneous in nematic liquid. Attempts have been made to interpret teh filament structure in terms of teh lowest energy configuration of the mixtures. The regular band systems of the filament and the lamellar structures play the role of permanent liquid-crystalline diffraction gratings of phase type. The grating constants lie in the range from 0.3 to 3 microns and more. The diffraction is of the Bragg type below the boundary value of the diffraction grating constant and is of the Raman-Nath type above this value. The boundary value was estimated.

Nematoid Filaments: New Mixtures, Cluster Morphology and Comparative Microinterferometry

Abstract New three-component liquid crystal systems creating spontaneously nematoid-type filaments at the transition from the isotropic melt and formation of the nematoid-type filaments at the nematic - smectic A transition in dilute solutions of smectogen in silicon oil are reported. We have observed over 50 new three-component systems, composed of smectogen, nematogen and chiral dopant, in which the formation of the nematoids occurs. The plate-type nucleation of the mesomorphic phases has been observed at the transition from the isotropic melt for pure smectogens and also for three-component systems. In addition, creation of the nematoid-type filaments in dilute solutions of mesogen (NPOOB) in sicon oil has been observed here. Early stages of the plate-type nucleation have been examined using Nomarski contrast. The refractive profiles of the filaments have been examined using double-refracting microinterferometers.

Plate-to-nematoid transition of self-organized layers in liquid crystal systems

Abstract We observed self-organized plates growing in some mesogenic mixtures as the first stage of nucleation of the mesomorphic phase and transformation of the plates into multiphase bifilar filaments (called nematoids). The liquid crystal plates and subsequent nematoids are freely suspended inside the isotropic melt. The creation of the plates and their transition to nematoids were observed in mixtures of nematic, smectic B and non-mesogenic chiral dopant. The maximum size of the plates depends on the concentration of the mixtures and varies from 0.1 to more than 1nm. Study of the fringe deflection profiles in Pluta birefracting microinterferometry revealed that the nematoids have bifilar internal organization. Two modes of creation of the nematoids can be distinguished: (a) formation of initial toroidal droplets and then nematoids and (b) intermediate formation of nematoids by rolling of the plates. Four main types of temporal and thermal evolution of the nematoids can be specified: (a) immediate fast shrinkage to droplet, (b) segmentation, (c) splitting along the longitudinal axis, and (d) double-spiralling. In all cases, the final stage of evolution is a liquid crystalline droplet which can be a source of secondary nematoids growing at diminished temperature.

Basic Liquid Crystal Structures, Polymorphisms and Defects

Abstract 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 (...

Cotton-Mouton Effect in Monodisperse Suspensions of Liquid Crystals

Abstract The collective Cotton-Mouton (C-M) effect in the system of micron-size liquid-crystal droplets freely suspended in the isotropic liquid is studied experimentally and theoretically. The collective C-M constant in liquid-crystal suspensions is found to be from 104 to 108 times higher than the respective “molecular” C-M constant in the isotropic phase of the same substance. These large values cause the saturation in the C-M effect at relatively low magnetic fields. Using the generalized nth order Langevins functions L, n the statistical therory is presented for the collective C-M effect in the monodisperse liquid-crystal suspensions. As a result, the induced birefringence is obtained in the form Δn = 1/2(Δn)∞ (3L 2 – 1), where (Δn)∞ is the induced birefringence at saturation. The collective C-M constant is C c = CN s S g where N 8 is the number of molecules in the droplet, S 8 is the orientational order parameter of the long molecular axes in the droplet, and C is the “molecular” C-M constant. The ...

Phase Transitions in Liquid Crystal Filamentary Structures

Abstract Instability of liquid crystal membranes with respect to spontaneous rolling up into cylinders or double cylinders and their subsequent very complicated evolution and also phase transition within cores are reported in the present paper. We have investigated here the formation and spontaneous evolution of liquid crystal membranes and filamentary structures in nematosmectogen 4-nitrophenyl-4′-octyloxybenzoate (NPOOB) dissolved in hydrophobic silicone oil H (SO). The first-type phase transitions are observed here within the filaments produced in system composed of 4-acetyl-4′-dodecylbiphenyl (B8) and 4,4″-pentylcyclohexyl(4′-pentylcyclohexyl) benzoate (B10) dissolved in SO. We discuss crude continuum model ofthe instability of liquid crystal membranes with asymmetric surface tension and the influence of the pressure exerted by such a tension on the phase behavior within the filaments. The fringe-field microinterferometric analysis has provided decisive evidence ofthe homeotropic organization ofthe membranes, planar ordering within the cylinders and of bifilar model of the filaments.

Self-organized liquid photonic crystals

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.

Refractive profiles of chiral-nematic myelin filaments

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.

Computer simulations of collective processes in liquid crystals

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.