Ring-pattern dynamics in smectic-C* and smectic-C*A freely suspended liquid crystal films

Director Fluctuations in Two-Dimensional Liquid Crystal Disclinations

We present analytical calculations of the energies and eigenfunctions of all normal modes of excitation of charge +1 two-dimensional splay (bend) disclinations confined to an annular region with inner radius R1 and outer radius R2 and with perpendicular (tangential) boundary conditions on the region’s inner and outer perimeters. Defects such as these appear in islands in smectic-C films and can in principle be created in bolaamphiphilic nematic films. Under perpendicular boundary conditions on the two surfaces and when the ratio β=Ks/Kb of the splay to bend 2D Frank constants is less than one, the splay configuration is stable for all values μ=R2/R1. When β>1, the splay configuration is stable only for μ less than a critical value μc(β), becoming unstable to a “spiral” mixed splay-bend configuration for μ>μc. The same behavior occurs in trapped bend defects with tangential boundary conditions but with Ks and Kb interchanged. By calculating free energies, we verify that the transition from a splay or bend configuration to a mixed one is continuous. We discuss the differences between our calculations that yield expressions for experimentally observable excitation energies and other calculations that produce the same critical points and spiral configurations as ours but not the same excitation energies. We also calculate measurable correlation functions and associated decay times of angular fluctuations.

Marangoni Flow in Freely Suspended Liquid Films

We demonstrate controlled material transport driven by temperature differences in thin freely suspended smectic films. Films with submicrometer thicknesses and lateral extensions of several millimeters were studied in microgravity during suborbital rocket flights. In-plane temperature differences cause two specific Marangoni effects, directed flow and convection patterns. At low gradients, practically thresholdless, flow transports material with a normal (negative) temperature coefficient of the surface tension dσ/dT<0 from the hot to the cold film edge, it accumulates at the cold film edge. In materials with dσ/dT>0, the reverse transport from the cold to the hot edge is observed. We present a model that describes the effect quantitatively. It predicts that not the temperature gradient in the film plane but the temperature difference between the thermopads is relevant for the effect.

Phase winding of a nematic liquid crystal by dynamic localized reorientation of an azo-based self-assembled monolayer

Azobenzene-based molecules forming a self-assembled monolayer (SAM) tethered to a glass surface are highly photosensitive and readily reorient liquid crystals in contact with them when illuminated with polarized actinic light. We probe the coupling of such monolayers to nematic liquid crystal in a hybrid cell by studying the dynamics of liquid crystal reorientation in response to local orientational changes of the monolayer induced by a focused actinic laser with a rotating polarization. The steady increase in the azimuth of the mean molecular orientation of the SAM around the laser beam locally reorients the nematic, winding up an extended set of nested rings of splay-bend nematic director reorientation until the cumulative elastic torque exceeds that of the surface coupling within the beam, after which the nematic director starts to slip. Quantitative analyses of the ring dynamics allow measurements of the anchoring strength of the azo-SAM and its interaction with the nematic liquid crystal.

Generation of umbilics by Poiseuille flows

The Fredericks transition in homeotropic nematic layers submitted to an electric field can be described in terms of a 2D vectorial order parameter [Formula: see text]. We have shown previously that umbilics which are point defects of the field [Formula: see text] can be generated in a controlled manner by magnetic fields. Here we report on a serendipitous discovery of generation of umbilics during cleaning of a sample and on a series of experiments which were performed with the aim to unveil conditions necessary for a controlled reproduction of this phenomenon. From the analysis of all circumstances in which umbilics were generated we conclude that umbilics are generated by Poiseuille flows and appear in π-walls in points where these walls are orthogonal to the flow direction. We discuss theoretical aspects of the action of Poiseuille flows on the field [Formula: see text] and unveil the relationship [Formula: see text]→ system π-walls. Finally we explain why and where umbilics are generated in π-walls.

Textures in thin films of nematic liquid crystals induced by strongly focusing a circularly polarized laser

We investigate the rearrangement of the director in thin films of nematic liquid crystals caused by tightly focusing circularly polarized laser beams. We find either target or spiral patterns, depending on the topology of the director configuration at the position of the beam focus. The induced rearrangements of the director are governed by the viscosity of the media, the handedness of circular polarization, and the irradiation power of the laser. Experimental observations are interpreted using a model derived from nematic continuum theory.

Triclinic fluid order

Among the condensed phases, those of lowest point group symmetry are the triclinic crystals, which have only the identity element or the identity and inversion elements. Such low symmetry is stabilized by the specificity of molecular interaction, which is weakened with increasing disorder, so that known phases with fluid degrees of freedom are more symmetric. Here we report triclinic order, appearing as a broken symmetry in a single, isolated, fluid smectic liquid crystal layer freely suspended in air, showing that none of its principal dielectric axes lies either normal or parallel to the layer plane.

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.

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.

Vortex flow in freestanding smectic films driven by elastic relaxation of the c director

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.

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.

Buckling instability of droplet chains in freely suspended smectic films

A buckling instability of chains of isotropic droplets in smectic films is investigated. The c -director field in a free-standing film is prepared as a target pattern with a continuous radial deformation. In such a pattern, isotropic liquid droplets are induced by light irradiation of the photochromic mesogenic material. The droplets align tangentially in regular chains in the regular structure of the c -director field. Incorporation of additional droplets lengthens the chains at a given ring diameter until they form complete rings. Further chain growth introduces a reversible buckling with a characteristic wave length. The phenomenon is similar in many respects to growth processes in biosystems or Euler buckling in polymer foils. A simple model of the wavelength selection mechanism is introduced.

Effect of high spontaneous polarization on defect structures and orientational dynamics of tilted chiral smectic freely suspended films

The director structure around topological defects and in 2pi walls in the two-dimensional orientation field of thin freely suspended films of tilted chiral smectic liquid crystal is observed to minimize splay of the spontaneous polarization. Concentric ring patterns in the director field unwind more slowly in higher polarization films. These experiments confirm that polarization space charge increases the effective elasticity of static polarization-splay distortions and that it attracts ionic charge, leading to an increase in the effective orientational viscosity of the director field.

Layering transitions and Schlieren textures in Langmuir films of two organic radicals

Two paramagnetic radicals have been investigated in terms of their film-forming properties at the air-water interface. Although the radicals failed to display any mesomorphic behavior in the bulk, they were found prone to built-up multilayer films on the Langmuir trough. The molecules seem to dimerize in the upper layers of the films that exhibit striking Schlieren textures when observed with Brewster angle microscopy. These Schlieren textures, together with the ability to form multilayers, indicate that the molecules came close to displaying smectic mesomorphism. A tentative model of the layers' structure is proposed, and a suggestion for synthesizing new molecules with actual mesomorphism is offered. The presented results show that the study of the behavior of molecules at the air-water interface can shed a new light on their behavior in the bulk and help in the design of new magnetic mesogens.

2pi and pi walls in antiferroelectric smectic-C*A and smectic-C free-standing films

We observed, in free-standing films of antiferroelectric liquid crystal, dependence of the wall structure on orientation of walls with respect to the external field direction and on parity of number of smectic layer in the film. These observations are explained by anisotropy of two-dimensional elastic constants and by dependence of polarization direction on parity of the film. Investigations in electric and magnetic field allow us to determine the values of elastic constants and longitudinal polarization of the films. Different mechanisms of c -director reorientation were observed when the electric-field direction was reversed or magnetic field orientation was changed with respect to the film plane.

Thickness-dependent smectic-A–smectic-C*transition in chiral smectic free-standing liquid-crystal films

We have conducted heat-capacity measurements near the smectic-A -smectic-C* phase transition in free-standing films of a chiral liquid-crystal compound. The heat-capacity behavior confirms the evolution of the transition in the interior layers from first to second order as a function of film thickness suggested earlier in tilt angle measurements. The asymmetry in the heat-capacity anomaly exhibits an interesting crossover in thinner films. We also observed the phase transitions leading to the crystallization to the tilted crystal-G phase in these films at lower temperatures. The surface ordering strength in these films is found to be much larger than that in other orthogonal-smectic films, which we speculate is due to the molecular tilt.

Ferroelectric smectic phase formed by achiral straight core mesogens

We report electro-optic experiments in liquid crystalline freestanding films of achiral hockey stick shaped mesogens with a straight aromatic core. The material forms two smectic mesophases. In the higher temperature phase, a spontaneous polarization exists in the smectic layer plane and the films show polar switching in electric fields. It is the first example of a ferroelectric phase formed by nearly rodlike achiral mesogens. Mirror symmetry of the phase is spontaneously broken. We propose a molecular configuration similar to a synclinic ferroelectric (C(S)P(F)) high temperature phase and an anticlinic, probably antiferroelectric (C(A)P(A)) low temperature phase.

Electro-optic study of antiferroelectric freely suspended films of bent-core mesogens in the B2 phase

We report the electro-optic investigation of a liquid crystalline free standing film formed by bent-core (banana-shaped) mesogens. The ground state of the B2 phase in the films is chiral and antiferroelectric, although the mesogenic molecules themselves lack chirality. The films can be switched in lateral electric fields. In the antiferroelectric ground state, the external electric field in the film plane couples to a small residual spontaneous polarization which we attribute to the incomplete compensation of polarizations of neighboring smectic layers, due to a nonzero twist normal to the layers. We derive viscoelastic parameters from the uniform switching dynamics and the structure of domain walls. Dynamic patterns in rotating fields are observed and analyzed.

Sequence of morphological transitions in two-dimensional pattern growth from aqueous ascorbic Acid solutions

A sequence of morphological transitions in two-dimensional dehydration patterns of aqueous solutions of ascorbic acid is observed with humidity as a control parameter. Change in morphology occurs due to humidity induced variation in the concentration of the metastable supersaturated solution phase formed after initial solvent evaporation. As percent humidity is varied from 40 to 80, patterns change from compact circular --> radial --> density modulated radial (a new morphology) --> density modulated circular --> density modulated dendritic (a new morphology) --> dense branching.