Smectic Films Research Articles

Corona patterns around inclusions in freely suspended smectic films

We discuss the structure and physical origin of corona patterns observed around solid or liquid spherical inclusions in freely suspended smectic films. Such patterns are observed when droplets or solid beads of micrometer size are sprayed onto the films. They are found in the smectic C phase and in the smectic A phase above such a smectic C phase, but disappear, for example, at the transition into a lower-temperature smectic B phase. We show that these structures are equivalent to splay domains found in the meniscus of freely suspended films, originating from surface-induced spontaneous splay.

Influence of anchoring strength on focal conic domains in smectic films

We present a study of focal conic domains in smectic- A liquid-crystal films on solid substrates. The antagonistic anchoring conditions of the film surfaces, random planar at the substrate interface and homeotropic at the air interface, enforce the formation of focal conic domains the lateral size of which is dependent on the film thickness. The strength of the planar anchoring on the solid substrate is systematically varied by coating the substrate with special alkoxysilane compounds. For each anchoring strength value, the relation between the size of the focal conic domains and the film thickness is determined. Increasing the planar anchoring strength influences the size-thickness relation and leads to the formation of larger focal conic domains.

Dynamic charge separation in a liquid crystalline meniscus

Oscillating electric fields can sustain a macroscopic and steady separation of electrostatic charges. The control over the dynamic charge separation (dyCHASE) is presented for the example of circular menisci of thin, free standing smectic films. These films are subject to an in-plane, alternating radial electric field. The boundaries of the menisci become charged and unstable in the electric field and deform into pulsating, flower-like shapes. This instability ensues only at frequencies of the electric field that are lower than a critical one. The critical frequency is a linear function of the strength of the electric field. Since the speed of electrophoretic drift of ions is also linearly related to the strength of the field, the linear relation between critical frequency and the amplitude of the field sets a characteristic length scale in the system. We postulate that dyCHASE is due to (i) electrophoretic motion of ions in the liquid crystalline (LC) film, (ii) microscopic separation of charges over distances similar in magnitude to the Debye screening length, and (iii) further, macroscopic separation of charges through an electro-hydrodynamic instability. Interestingly, the electrophoretic motion of ions couples with the macroscopic motion of the LC material that can be observed with the use of simple optical microscopy.

Influence of chirality on director configuration and droplet interaction in ferroelectric free-standing films

Droplets in smectic free-standing films interact due to elastic distortion of the c-director field and formation of topological defects. Our experiments show that chirality of the liquid-crystal medium plays a key role in the interaction between droplets. We find that the configuration of the c-director field near the droplets and the position of the topological defects on the droplet boundary depend on droplet size and film polarity. An intermediate c-director configuration between dipolar and quadrupolar is formed near droplets in smectic films due to the competition between elasticity and chirality. We observed that the distance between droplets in self-organized structures depends on the position of defects on the droplet boundary and significantly changes with respect to that for dipolar droplets. Our results open the way to modify the droplet interaction and the structures formed by droplets.

Controlling Smectic Focal Conic Domains by Substrate Patterning

We describe a simple method to control the generation and the dimensions of focal conic domains in smectic liquid-crystal films. The surface of silicon substrates is structured in a way that areas with homeotropic anchoring conditions alternate with areas possessing random planar anchoring conditions. In smectic films on such substrates, the anchoring pattern translates into a presence-and-absence pattern of focal conic domains. The lateral dimensions of the planar anchoring areas determine an upper limit of the diameter of the focal conic domains. Thus, an almost arbitrary two-dimensional arrangement of focal conic domains can be achieved by controlling the size and position of individual domains.

Simulation of Surface-Enhanced Ordering in Smectic Films

A Landau-de Gennes type model for the direct isotropic/smectic A phase transition is used to study surface-enhanced smectic ordering in the stable smectic temperature regime. A unified surface-free energy functional is proposed which can be utilized for homeotropic and planar surface anchoring. The time-dependent complex Landau-Ginzburg evolution equations and boundary conditions are derived for thin-film geometry. Simulation results are presented for the two types of anchoring and compared to observations from experiments and previous simulations. Simple visualization software for smectic layering was developed and is also presented that is compatible with discretized numerical solutions of the model.

Homeotropic surface anchoring and the layer-thinning transition in free-standing films

In this work, we investigate the interplay between surface anchoring and finite-size effects on the smectic-isotropic transition in free-standing smectic films. Using an extended McMillan model, we study how a homeotropic anchoring stabilizes the smectic order above the bulk transition temperature. In particular, we determine how the transition temperature depends on the surface ordering and film thickness. We identify a characteristic anchoring for which the transition temperature does not depend on the film thickness. For strong surface ordering, we found that the thickness dependence of the transition temperature can be well represented by a power-law relation. The power-law exponent exhibits a weak dependence on the range of film thicknesses, as well as on the molecular alkyl tail length. Our results reproduce the main experimental findings concerning the layer-thinning transitions in free-standing smectic films.

Inclusions in free standing smectic liquid crystal films.

Colloidal inclusions in thin free standing liquid crystal films are ideal model systems for 2D anisotropic dispersions. Different types of self-organization in chain and lattice structures have been observed. The orientational elasticity of the anisotropic matrix and capillary forces are the dominating interaction mechanisms between solid or liquid inclusions, the director field, and dislocations of the films. We give an overview of the progress in this field, focussing on different inclusion types and their interactions in thermotropic smectic films.

Spontaneous bend patterns in homochiral ferroelectric SmCP films: evidence for a negative effective bend constant

We describe the spontaneous formation of striped and labyrinthine textures of the c-director in freely suspended polar smectic SmCPF films of a bent-core mesogen. Those films develop an instability of the uniform director field. The texture decomposes into local domains of alternating bend. Both right- and left-handed bends of the director occur with equal probability, independently of the overall chirality defined by the local structure of the SmCPF phase. Thus, despite the overall chirality, the system prefers a ground state with bend deformations of both handedness (ambidextrous) of the director. In this paper we suggest a theoretical explanation of this spontaneous bend of the c-director. We assume a negative effective bend elastic constant, in order to explain the formation and optical appearance of the observed stripe and labyrinthine patterns.

Modelling Polarization Effects in a Freely Suspended Smectic Liquid Crystal Film

We consider a smectic liquid crystal film freely suspended between two electrodes and subject to an electric field. For a fixed voltage the system can exhibit qualitatively different types of equilibrium director profile, depending on the spontaneous polarization and flexoelectric contributions to the bulk free energy. We examine the stability of the calculated director profiles and determine which configurations are energetically favourable. Furthermore, we also observe that, at critical values of the polarization parameters, there exists a bifurcation corresponding to a change in the number of equilibrium profiles. Generally, any configuration where the electric field and spontaneous polarization act in opposing directions is energetically unfavourable. However, we show that these profiles can become metastable and local energy minima when the flexoelectric parameters are relatively large.

Colloidal inclusions in smectic films with spontaneous bend

Inclusions in free-standing smectic films are simple model systems for two-dimensional anisotropic dispersions. From theory and experiment, different topologies of elastic distortions of the embedding liquid crystal are known. Quadrupolar and different dipolar defect configurations in the vicinity of the inclusion are possible, and these configurations determine the type of interactions between the inclusions. The quadrupolar configuration is often energetically preferred. We show, however, that dipolar director configurations around inclusions can be energetically favourable over quadrupolar arrangements in chiral smectics, as a consequence of a spontaneous-bend term in the elastic-energy formulation. As the inclusion size influences the selection of the deformation types, the corresponding spontaneous-bend constant can be estimated for the strong anchoring limit if the c -director fields around inclusions of different diameters are taken into account.

Role of electrostatics in the texture of islands in free-standing ferroelectric liquid crystal films

Curved textures of ferroelectric smectic-C* liquid crystals produce space charge when they involve divergence of the spontaneous polarization field. Impurity ions can partially screen this space charge, reducing long-range interactions to local ones. Through studies of the textures of islands on very thin free-standing smectic films, we see evidence of this effect, in which materials with a large spontaneous polarization have static structures described by a large effective bend elastic constant. To address this issue, we calculated the electrostatic free energy of a free-standing film of ferroelectric liquid crystal, showing how the screened Coulomb interaction contributes a term to the effective bend elastic constant, in the static long-wavelength limit. We report experiments which support the main features of this model.

FTIR spectroscopy of smectic elastomer films under lateral strain

Polarized Fourier transform infrared (FTIR) spectroscopy is used to study the strain‐induced compression of molecular layers in oriented smectic liquid crystal elastomer films. A reversible change of the smectic layer thickness in SmA and SmC* films in response to external strain was revealed earlier by optical reflectometry and X‐ray measurements. However, these methods cannot probe the mechanism of layer compression on a molecular level. Polarized FTIR spectra show that the induced mesogenic tilt, one of the possible mechanisms, is too small to provide the dominating contribution to the layer shrinkage. The FTIR absorbance spectra of stretched samples are also evidence that there are no significant changes of the order parameter. Apparently, layer compression is achieved by a certain interpenetration of neighbouring layers, and/or compression of the interstitial backbone and spacer layers.

Stick-slip dynamics around a topological defect in free-standing smectic films

We study the orientational relaxation of a spiral pattern around a central defect with topological strength S=+1 in free-standing smectic films. Instead of a continuous unwinding, a characteristic stick-slip relaxation is observed, where the elastic anisotropy plays the dominant role for the director anchoring in the vicinity of the defect. A model derived from nematic continuum theory is used to interpret the experimental observations.

C-director relaxation around a vortex of strength +1 in free-standing smectic-C films

The relaxation of director fields in freely suspended smectic films is studied experimentally by means of polarizing microscopy, and analyzed by solving the torque balance equation under appropriate initial and boundary conditions. We consider in particular the role of anchoring conditions of the c-director at particles and defects in the film. The structure of regular relaxation patterns allows to determine the elastic anisotropy of smectic materials. The splay elastic constant can exceed the bend constant by a factor of two and more. A remarkable consequence of this anisotropy is the stick-slip-like relaxation around a central defect of topological strength s = + 1.

Vortex Flow in Freestanding Smectic Films Driven by Elastic Relaxation of thecDirector

We report a simple experiment in freestanding smectic films in which elastic distortions of the c director drive macroscopic flow. The flow field is visualized with tracer particles. Measurements are compared to predictions of a model that employs the coupled dynamic equations for director and velocity fields. Relaxation dynamics depends on the topology of the film center: for defect-free target patterns, shear flow provides the dominating contribution to the c director dynamics. In presence of a central topological defect of strength S = + 1, the influence of flow on the relaxation dynamics is practically negligible, while for a central S = - 1 defect, the influence of vortex flow on the c-director relaxation is roughly twice as large as for the defect-free state.

Measurement of the viscosity coefficientη3in free-standing smectic films

Thinner parts (the so-called holes) and thicker parts (islands) move in free-standing smectic films due to gravity if the film is inclined. The velocity of the movement depends on the viscosity coefficient eta3 of the film. Therefore, this measurement is a direct method to determine eta3 in smectic phases. This paper contains the corresponding hydrodynamic calculations for the flow patterns in and around a circular hole in a circular two-dimensional film under various conditions. The viscous force on the hole is calculated. Experiments are performed with the liquid crystal n-octyl-cyano-biphenyl (8CB) in the smectic phase. The influence of the disclination which surrounds the hole and the influence of the gas which surrounds the film on the movement is studied. It is shown that the influence of these effects on the determination of eta3 can be neglected if the film is thick and the difference in the thicknesses between film and hole is small. The temperature dependence of eta3 is determined for 8CB.

Energetics of 2D colloids in free-standing smectic-C films

The formation of regular colloid patterns in free-standing smectic films at the transition from the smectic-C to the isotropic or nematic phase is well known experimentally. The self-organization of isotropic or nematic droplets is caused by their mutual interaction, mediated by elastic distortions of the local director in the surrounding liquid crystal. These distortions are related to the anchoring conditions of the director at the droplet border. We describe analytically the energetics of the liquid crystal environment of a single droplet in one-constant approximation. A method of complex analysis, Conformal Mapping, is employed. Following a suggestion of Dolganov et al. (Phys. Rev. E. 73, 041706 (2006)), energetics of chain and grid patterns built from the colloids are investigated numerically in order to explain experimentally observed formations and their director fields.

Colloids on free-standing smectic films

We study the structure of a free-standing smectic-A film around a micron-size polystyrene colloid adsorbed onto the film. We find that a colloid or a cluster of colloids is surrounded by an optically distinct and radially decorated meniscus ending with a sharp edge. The observed strong and finite-range attraction between the adsorbed colloids is driven by the fusion of menisci. We interpret the structure of the smectic meniscus in terms of a model dominated by the surface free energy and we argue that the characteristic appearance of the meniscus is due to layer undulations.

Temperature dependence of the Casimir-like force in free-standing smectic films

The thermal Casimir-like force in free-standing liquid crystal films close to the smectic-A-nematic transition temperature is computed using a quadratic functional approach. In the framework of a microscopic mean-field model of free-standing smectic-A films, the temperature dependence of the order parameter profiles is computed and later used to estimate the elastic coupling variability in the vicinity of first- and second-order bulk smectic-A-nematic phase transitions. The strong nonuniformity of the coupling constant profiles promotes a significant increase of the fluctuation-induced force over three orders of magnitude, especially in thin films. This result reinforces the possible predominance of the thermal Casimir force as compared to the standard van der Waals interaction in thin smectic-A liquid crystal films.