Smectic Films Research Articles

Chiral symmetry breaking and the soliton-stripe pattern in Langmuir monolayers and smectic films

Chiral symmetry breaking in Langmuir monolayers and smectic films may be modelled by a system of two coupled fields: a scalar field that represents the chirality of the molecules and a vector field that represents the tilt direction of the molecules. In a particular parameter range, we prove the existence of a soliton-stripe pattern using the Γ-limit theory in perturbative variational calculus. This pattern, modelled by one-dimensional local minimizers of the free energy of the system, consists of stripes of molecules with distinct chirality in the film delineated by sharp domain walls.

Effects of Electric Fields as a Function of depth in a Smectic Film

The response of liquid crystals (LC) to applied electric fields is the basis for most display applications. Optically it is known that the bulk portion of a LC cell responds to an applied field. The remainder of the film is assumed to consist of LC molecules and layers that do not reorient due to surface interactions. The inactive interfacial layer has not been accessed directly nor characterized. We have used a 8CB smectic-A cell consisting of two ITO coated glass plates to look into the layers with grazing incidence X-ray diffraction. The plates are connected to a voltage source with a maximum output of 40.7 V DC. The changes in lattice spacing vary as a function of depth within the sample, which suggests the presence of different regions of layer reorientation within the film.

Manipulation of Disk-Shaped Islands on Freely Suspended Smectic Films and Bubbles Using Optical Tweezers

We have demonstrated that disk-shaped islands on freely suspended smectic films and bubbles can be manipulated using optical tweezers. We have measured the optical trapping forces experimentally and have studied the trapping properties theoretically using an optical model with ray, wave, and electromagnetic components. The observations confirm our prediction that the trapping force is maximized when the edge of the island finds itself in the focal region of a highly divergent laser beam.

Surface and Interface Tensions Determined From Isotropic Droplets in Freely Suspended Smectic Films

Isotropic droplets in freely suspended smectic films are studied by means of optical microscopy. Droplet shapes are determined with an interference method and described with a model that considers the surface and interface tensions of the smectic, isotropic liquid and gas phase. In uniform film regions, droplets adopt the shape of flat lenses. Deformations of the circular droplet contour are observed for droplets lined up at thickness steps in inhomogeneous films. PACS numbers: 61.30.−v 68.15. + e 87.64.Rr

Dislocation loop dynamics in freestanding smectic films: the role of the disjoining pressure and of the finite permeability of the meniscus.

When a dislocation loop nucleates in a freestanding film, it collapses or grows depending on whether its radius r is smaller or larger than a critical radius r(c). In this paper, we analyze the growth dynamics of a dislocation loop in the limit of r>>r(c). Experiments with pure octylcyanobiphenyl show that the dislocation velocity is constant in thick films (more than 100 layers) regardless of their thicknesses, and only depends on the pressure in the meniscus. At intermediate thickness (between 100 and 15 layers), the velocity is no longer constant and tends to decrease in time on account of the finite permeability of the meniscus. In very thin films (less than 15 layers), the dislocations move faster than in thick films, although their velocities continue to decrease in time. The thinner the film, the larger the global acceleration is. This effect is linked to a supplementary force acting on the dislocations caused by the attraction between the free surfaces (where the smectic order parameter is enhanced). The progressive deceleration is due to the finite permeability of the meniscus.

Continuous Phase Change Mediated by an Unstable State at Freely Suspended Smectic Film

Phase transitions of freely suspended chiral smectic films exhibiting a first order phase transition in a bulk system are studied in the framework of a phenomenological theory, in which such a widely observed finding that the surface layer orders at a temperature higher than the bulk critical temperature is taken into account. While the inner portion of the film exhibits the first order phase transition at a temperature below that of the surface layer in the system of sufficient thickness, a continuous change occurs for the system with a thickness smaller than a critical thickness. The mechanism of the continuous change is elucidated on the basis of the phase diagram of the bulk system on the field versus temperature plane, where nonuniformity due to the ordered surface layers is replaced by an effective field and the high-temperature phase is connected continuously to the low-temperature one by the insertion of an unstable state between them.

1C03 バルクで一次相転移を示すスメクティック液晶の自己保持膜の相転移(物理・物性)

1C03 バルクで一次相転移を示すスメクティック液晶の自己保持膜の相転移(物理・物性)

Publisher’s Note: Dependence of film tension on the thickness of smectic films [Phys. Rev. E67, 021707 (2003)

Publisher’s Note: Dependence of film tension on the thickness of smectic films [Phys. Rev. E<b>67</b>, 021707 (2003)

Dependence of film tension on the thickness of smectic films.

The film tension tau of free standing S(A) films has been measured for films with thicknesses between 2 and 150 layers. There is a clear increase of tau with the thickness for very thin films and a nonlinear slower increase for high thickness. The nonlinearity depends on the amount of liquid crystal accessible to the meniscus of the film during the drawing process. Several models are discussed that describe these effects.

Structure and fluctuations of smectic membranes

Smectic membranes are perfect model systems for studying low-dimensional phase transitions and the associated fluctuations. During the last two decades we have seen important progress in the understanding of the structure and fluctuation behavior of these systems, driven by both new experimental techniques and theoretical developments. Phase transitions are reviewed involving liquid, hexatic, and crystalline layers, which provide several types of model system for low-dimensional melting. The authors discuss the influence of the surfaces on the physical properties of the membranes as well as the crossover from threeto two-dimensional behavior. The layer-displacement fluctuations in smectic membranes have been investigated by specular and diffuse x-ray reflectivity. Theoretical and experimental aspects of the displacement-displacement correlation function are discussed. Of special interest is the quenching or enhancement of fluctuations at surfaces, which is directly related to the phenomenon of surface ordering. The authors consider the conditions under which fluctuations are conformal throughout a membrane, and then the dynamic aspects of the layer-displacement correlation function, which include the effects of finite size, surface tension, and viscous dissipation. This leads in smectic membranes to a discrete spectrum of elastic and viscous relaxation modes, which have been studied experimentally with coherent x rays at third-generation synchrotron sources. The fluctuating character of crystalline-B membranes is also considered. Finally, the article looks briefly at thinning transitions, smectic membranes of chiral molecules, smectic films on substrates, and applications to biologically relevant systems. Open questions and future trends in the field are discussed.

Phase behavior of α, ω-dihexyl- α-quaterthiophene and ordering on a textured substrate

Ordering of α, ω-dihexyl- α-quaterthiophene (DH α4T) thin films on substrates including friction-transferred polytetrafluoroethylene (PTFE) is explored. DH α4T exhibits a high-temperature mesophase from 81 to 181 °C. Textural analysis reveals this phase to be a tilted, highly ordered smectic phase. Upon cooling from the isotropic melt, the smectic phase grows layer by layer on many surfaces. Smectic thin films grown on friction-transferred PTFE exhibit a high degree of alignment with edge dislocations being the primary defect. Upon crystallization from the mesophase, the material retains for the most part the order of the mesophase. Growth of the smectic phase on directional surfaces such as friction-transferred PTFE appears to be a route to control of surface ordering of DH α4T.

Dinamicheskie svoistva smekticheskikh plenok

Dinamicheskie svoistva smekticheskikh plenok

Electro-optical response of reflected light from freely suspended smectic liquid-crystal films

We have studied the electro-optical response of reflected light from the surface of freely suspended liquid-crystal films (FSFs) in SmA, SmC*, and SmCA* phases. Upon application of an electric field parallel to FSFs, the film vibrational spectrum generally contains a second-harmonic component of the applied frequency. In the SmA phase, only a second-harmonic component appeared in the fast Fourier transform (FFT) spectra of the reflected light. In the SmC* phase, however, many other components also appeared in the FFT spectra of the reflected light. These complicated characteristics observed in the ferroelectric phase might be attributed to the contribution of the transient scattering mode. On the other hand, in the SmCA* phase, only the second-harmonic component appeared, similar to the case of the SmA phase. In the electric field high enough to cause an antiferroelectric–ferroelectric phase transition, however, many other components appear in the spectrum, in the same manner as that in the SmC* phase.

Isotropic Droplets in Thin Free Standing Smectic Films

Isotropic liquid droplets are formed by different mechanisms near the smectic−isotropic phase transition in smectic freely suspended films. Because of the layered structure of the smectic films, the interplay of tensions that determines the droplet geometry is essentially different from that of liquid droplets on solid or isotropic liquid surfaces. By means of interferometric measurements, the three-dimensional droplet geometry is determined. A model is proposed that describes the droplet shapes, and the relevant parameters are discussed on the basis of experimental data. We derive surface and interface tensions of low molecular mass and polymeric mesogens and provide a molecular interpretation. In particular, the small difference between surface tensions in the smectic and isotropic phases is accessible with the method; it can be related to the structure of the isotropic liquid to air interface of smectogens. Capillary forces causing a lateral motion of droplets in inhomogeneously thick films are discuss...

Theory of the anticlinic pretilted surface phase in tilted chiral smectic films.

In freely suspended chiral smectic films, the molecules tilt at the surfaces above the bulk ferroelectric transition temperature. The tilt angle at the two surfaces can be either equal (synclinic surface phase) or opposite (anticlinic surface phase) on each surface of the film. We propose a realistic theoretical model accounting for the stability and properties of these two structures, based on competing surface interactions. The anticlinic phase is stable for sufficiently thin samples and high temperatures. We predict a reentrant SmA phase merging with the surface phases at a three-phase point.

Photoinduced linear and/or circular birefringences from light propagation through amorphous or smectic azopolymer films

The self-induced rotation of the azimuth of elliptically polarized light passing through birefringent azopolymer thin films is investigated. The experiments were carried out on thin films of the amorphous p(DR1M-co-MMA) and p(DR1M) azopolymer samples and of the p(6MAN) derivative in its glassy and liquid-crystalline phases. In fact, using various controlled input light ellipticities, linear birefringence (LB) and/or circular birefringence (CB) measurements were performed separately and in conjunction with polarization analyses of the transmitted pump beam. According to a general theoretical analysis based on Jones’ matrix formalism, it is thus shown that the induced rotation angle through the films depends mainly on the ellipticity of the input light, on the generated LB level and, to a lesser extent, on the CB photoinduced in the liquid-crystalline phase. In the latter case, it is concluded that irradiation with circularly polarized light does induce a chiral arrangement in the polymer film, although the photosensitive chromophores do not contain any optically active group.

Surface effects on the amplitude of fluctuation-induced interactions in smectic films.

Within a quadratic functional integral approach, we investigate the role played by surface terms in the fluctuation-induced surface-surface interaction of free standing smectic liquid crystals. We show that the typical 1/l decay of the Casimir-type contribution to the free energy of a film with thickness l is replaced by a faster 1/l3 decay at a characteristic surface tension. An intermediate 1/l2 decay can also take place for specific surface parameters with unlike boundary conditions. In all the investigated cases, a repulsive long-range force appears only for mixed boundary conditions with strong anchoring at one surface and weak anchoring at the opposite one. Further, the amplitude of the thermal Casimir energy, besides being influenced by the applied surface tension, depicts a nonmonotonic dependence on the coupling between the outermost film layers, reflecting a crossover between strong and weak anchoring regimes.

Structure and elastic properties of smectic liquid crystalline elastomer films.

Mechanical measurements, x-ray investigations, and optical microscopy are employed to characterize the interplay of chemical composition, network topology, and elastic response of smectic liquid crystalline elastomers (LCEs) in various mesophases. Macroscopically ordered elastomer films of submicrometer thicknesses were prepared by cross linking freely suspended smectic polymer films. The cross-linked material preserves the mesomorphism and phase transitions of the precursor polymer. The elastic response of the smectic LCE is entropic, and the corresponding elastic moduli are of the order of MPa. In the tilted ferroelectric smectic-C* phase, the network structure plays an important role. Due to the coupling of elastic network deformations to the orientation of the mesogenic groups in interlayer cross-linked materials (mesogenic cross-linker units), the stress-strain characteristics is found to differ qualitatively from that in the other phases.

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.

Dislocation loops in overheated free-standing smectic films.

Static and dynamic phenomena in overheated free-standing smectic-A films are studied theoretically. The work is based on a generalization, introduced recently by the authors, of de Gennes' theory for a confined presmectic liquid. In this approach, smectic ordering in an overheated film is caused by an intrinsic surface contribution to the film free energy and vanishes at some temperature depending on the number of layers. Here the theory is further generalized to study the dynamics of films with planar inhomogeneities. A static application is to determine the profile of the film meniscus and the meniscus contact angle, the results being compared with those of a recent study employing de Gennes' original theory. The dynamical generalization of the theory is based on a time-dependent Ginzburg-Landau approach. This is used to compare two modes for layer-thinning transitions in overheated free-standing films, namely, "uniform thinning" versus nucleation of dislocation loops. It is concluded that the nucleation mechanism dominates provided there is a sufficiently large pressure difference arising from meniscus curvature. Properties such as the line tension and velocity of a moving dislocation line are evaluated self-consistently by the theory.