Dimer structures formed in smectic films by inclusions with parallel and antiparallel topological dipole moments

Self-organization of cholesteric and nematic droplets in smectic free-standing films is investigated. Strong tangential anchoring at the droplet boundary leads to nucleation of a topological defect and formation of a topological dipole from the droplet and the defect. The interaction between droplets results in assembly of the droplets in dimers and line chains. Topological dipoles of the droplet-defect pairs can be oriented in dimers with polar (ferroelectric) and nonpolar (antiferroelectric) ordering. In this paper we found novel dimer structures formed by droplets with different handedness of the c-director field near the droplets. Depending on the relative magnitude of the droplet dipoles the resulting structure may be dipolar or quadrupolar. Formation of antiferroelectric dimers is discussed on the basis of electromagnetic analogy.     

Collapse of islands in freely suspended smectic nanofilms

A time-nonlinear stage of the collapse of islands in freely suspended smectic nanofilms is observed and investigated. Islands thicker than a nanofilm are prepared and studied, which are unstable inside the dislocation loops, since they increase the energy of the film. Such instability leads to the decrease in the size of islands and is terminated by their collapse. The time dependence of the size of islands is measured experimentally. It is shown that the found dependence is in agreement with the theory of the dynamics of dislocation loops in smectic films developed earlier with allowance for the dissipation of energy in the film and in the meniscus. A nontrivial dynamic coupling between islands in a film resembling Ostwald ripening is also found, though the nonequilibrium kinetics of unstable islands, at which the hydrodynamic flow through a film leads to the decrease in sizes of one island and the increase in those of the other, rather than of the growth of the nucleation centers in the thermodynamically stable phase from the metastable state of the system (described by the Lifshitz–Slezov theory in films), is studied in our experiments.     

Different mechanisms of nucleation and self-organization of droplets in ferroelectric smectic membranes

New mechanisms of droplet nucleation and self-organization in ferroelectric membranes are described. The droplets may be accompanied by different number of topological defects (zero, one, two) whose location may be on the droplet boundary or in the membrane. Nucleation and self-organization of droplets with total topological charge S = 0 , S = + 1 and S = - 1 were investigated. We found that an S = - 1 topological defect may be the center of both droplet nucleation and chain formation. This mechanism of chaining drastically differs from the droplet self-organization described earlier which is realized by attraction of droplet-defect pairs. Our observations demonstrate new possibilities for manipulating the inclusions and their self-organization in smectic membranes.     

Ferrielectric smectic phase with a layer-by-layer change of the two-component order parameter

One of the most remarkable properties of smectics is the wide variety of possible equilibrium structures. In this paper, based on the Landau theory of the phase transitions, the transitions between ferroelectric and antiferroelectric phases and the structure formed by smectic layers with different azimuthal and polar orientations of the molecules were calculated. This unique structure has been predicted [P.V. Dolganov et al., JETP Lett. 76, 498 (2002)] using the minimization of the free energy with respect to the phase and modulus of the two-component order parameter, but never before detected. Recently, a nonresonant Bragg reflection, consistent with the predictions of the model, was found [P. Fernandes et al., Eur. Phys. J. E 20, 81 (2006)] in the ferrielectric smectic C* _FI1(SmC* _FI1) phase. In the three-layer ferrielectric structure with a macroscopic helical pitch, the modulus of the order parameter is larger in anticlinic-like layers and smaller in layers with mixed ordering. The values of the interlayer interactions were determined for smectic liquid-crystalline materials forming different polar structures. The text was submitted by the authors in English.     

Polarization-luminescence studies of the structure and molecular dynamics of smectic and nematic mesophases

The liquid crystal 50B7A in the smectic A and nematic phases was studied by the method of polarization luminescence. The order parameters P₂ and P₄ were determined from the experimental values of the luminescence spectrum both neglecting and taking into account relaxation processes in the liquid crystal. The orientational distribution function of the molecules in the ground and excited states are constructed. The orientational relaxation time (10^–9 sec for the nematic and smectic A phases) is calculated. The results obtained are compared with calculation of the rotational relaxation time determined from the coefficient of diffusion for the related compound EBBA.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 39–43, August, 1987.     

Photo- and thermo-induced variation of photonic properties of cholesteric liquid crystal containing azobenzene-based chiral dopant

Photooptical properties of a new cholesteric mixture containing azobenzene-based chiral dopant were investigated. Presence of a photosensitive dopant in the investigated liquid crystal enabled to effectively manipulate optical characteristics of the photonic band upon temperature change and illumination. Temperature dependence of the optical anisotropy and the orientational order parameter of the liquid crystal were determined. The obtained results are compared with existing theories. Discrete multistable change of the spectral position of the photonic band on illumination is realized.     

Inelastic light scattering from the liquid crystal and solid phases of MBBA

Brillouin spectra from 4-methoxybenzylidene-4'-n-butylaniline were measured for the nematic phase, a quenched liquid crystal (phase C₀), and a partially ordered structure (phase C₁). The difference in the velocities of the longitudinal hyper-sound waves in the nematic and the quenched liquid crystal structures is associated with temperature and relaxation effects.     

Free-standing smectic films at high temperature

Optical reflectivity studies on free-standing liquid crystal films above the bulk smectic temperature range have revealed different melting phenomena. Our measurements are performed on tilted smectic phases (smectic C*, smectic C) using optical microscopy in polarized light in order to visualize the changes of the film structure. We observe the formation of twodimensional defect structures from string-like lines in very thick (about 1000 layers) as well as in thin (about 20 layers) films. In thick films these structures nucleate around the temperature of the bulk smectic-cholesteric phase transition, while in thin films the formation of the defects occurs well above this temperature and just before the thinning transitions. In thick and intermediate thickness films, cholesteric or nematic droplets and a ‘quasi-smectic’ structure are observed. The films exhibiting the ‘quasi-smectic’ structure definitely exist at higher temperatures than the smectic films with the same thickness.     

Redox-induced reversible transformation of a system quinone-dianion based on 2,2′-(anthracene-9,10-diylidene)bis(4,5-diphenyl-2H-imidazole)

2,2′-(Anthracene-9,10-diylidene)bis(4,5-diphenyl-2H-imidazole) was synthesized and subjected to a study by cyclic voltammetry. This compound is capable of undergoing repeated reversible redox-transformations quinone-dianion.