Smectic Liquid Crystal Films Research Articles

Ultra-thin films of polymerised smectic liquid crystals. A study using AFM and ellipsometry

Langmuir–Blodgett multilayers very often consist of 2D crystallites which scatter light. One way of avoiding this problem is to use preformed polymers but even here there is a difficulty as multilayers formed from them have a poor degree of order in the direction normal to the plane of the film. This defect is exemplified by the fact that a low-angle X-ray diffractogram obtained from such films consists, usually, of 3 or less Bragg peaks and, at the most, 5 observable peaks. In an effort to obtain better order in multilayers both within the plane and normal to it, we have examined the properties of multilayers of a compound which can exist in the smectic A phase and can be polymerised with the aid of UV light while they are in this phase. We have studied the thickness of a number of multilayers of this compound as they go through the phase change using ellipsometric techniques and indeed find a thickness change which corresponds to the change in monolayer thickness found by X-ray diffraction and reported in our earlier studies. The ellipsometer that we have used gives the average thickness of a circular region having a diameter of about 1 mm. In order to obtain a better understanding of the behaviour of films on a much smaller horizontal scale, we have used atomic force microscopy. We display a number of typical cross sections of AFM scans and show that, on a molecular scale, the films are smoother in the smectic phase than they are in the crystalline phase and that polymerisation tends to maintain a film in a smooth condition.

Polar Smectic Films

We report on a new experimental procedure for forming and studying polar smectic liquid crystal films. A free standing smectic film is put in contact with a liquid drop, so that the film has one liquid crystal/liquid interface and one liquid crystal/air interface. This polar environment results in changes in the textures observed in the film, including a boojum texture and a previously unobserved spiral texture in which the winding direction of the spiral reverses at a finite radius from its center. Some aspects of these textures are explained by the presence of a Ksb term in the bulk elastic free energy density that favors a combination of splay and bend deformations.

Coupling of Polarization and Dislocation in Ferroelectric Smectic Liquid-Crystal Films

The polarization vector in ferroelectric smectic- ${C}^{*}$ films preferably aligns along a dislocation line due to the Coulomb interaction between polarization charges. The electric field locally perpendicular to the dislocation line distorts it, and in the case of a dislocation loop an $n$-finger structure is formed, with $n$ depending on the applied voltage. This phenomenon has been observed in an experiment in which the screening effect of ion impurities has been partially lifted in a low-frequency (3 Hz) electric field. A characteristic length scale related to this phenomenon is of the order of ${10}^{4}\AA{}$.

Ultrathin Films of Polymerized Smectic Liquid Crystals. A Study of the Polymerization Process

We have studied the photopolymerization of an amphiphilic compound containing a biphenyl group which has, at one end, a fumarate propyl ester and, at the other, a vinyl group. We have photopolymerized the compound when it was spread as a monolayer at the air/water interface and have then assembled it into multilayer structures by the Langmuir−Blodgett technique. We have also formed multilayers of the monomers and then photopolymerized them. Both kinds of films have been characterized by X-ray diffraction, by polarizing microscopy, and by UV absorption spectroscopy. When monomers are assembled into a layer structure and are then irradiated with UV light, they will only polymerize when in the smectic phase, which can be produced by heating the film to a temperature above 54 °C, but when the multilayer consists of two-dimensional crystals, it will not polymerize. Because polymerization can take place at the air/water interface, we think it likely that the polymerization process takes place within layers rather than between layers.

Anticlinic structures at the surface of freely suspended smectic-Cfilms

Ellipsometric studies have been conducted on freely suspended films of chiral smectic liquid crystals. In the temperature range of the smectic-$C$ phase, we observe sharp steplike changes of the ellipsometric quantity $\ensuremath{\Delta}$ indicating sudden inversions of the molecular tilt direction in single smectic layers near the surface. These tilt inversion transitions are strongly hysteretic with temperature differences up to 10 K between heating and cooling runs. Although the films possess two identical free surfaces, metastable states occur in which the tilt is inverted at only one surface.

Evidence for the Presence of a Twist-Grain-Boundary Structure in Confined Hybrid Non-Chiral Smectic Liquid Crystal Films

AbstractWe have used Grazing Incidence X-ray Scattering to study the depth dependence of the interfacial structure of smectic liquid crystal (8CB) hybrid films. These films are bounded by a grated glass substrate and air, and have been studied as a function of both temperature and thickness. Our measurements indicate the presence of both a chevron structure and a structure similar to the helical twist-grain-boundary (TGB) phase. Our measurements constitute the first to show that this structure can also be observed in hybrid smectic liquid crystal films. The twisted structure is the result of anchoring at two dissimilar competing confining surfaces.

Surface order effects on the dynamics in thin free standing films of smectic liquid crystals

AbstractThe Goldstone mode has been measured in a homeotropically oriented ferroelectric liquid crystal in SSFLC cells and in free standing films by means of optical reflectivity. In cells the Goldstone mode was found only in the smectic C* phase as expected. In the free standing films, however, the Goldstone mode was still detectable several degrees above the bulk phase transition temperature to the smectic A phase. This effect is due to a molecular tilt in the boundary layers of the smectic A film caused by surface ordering.

Surface tension measurements in freely suspended bubbles of thermotropic smectic liquid crystals

We present an experimental technique for the inflation of freely suspended films of thermotropic smectic liquid crystals to macroscopic bubbles. These bubbles have many properties in common with well known lyotropic soap bubbles, but on the other hand represent a completely different class of material with different physical properties and new phenomena. We describe the generation of such smectic bubbles and compare their physical properties with those of lyotropic soap bubbles. The measurement of inner pressure versus curvature is used for the determination of surface tensions of the smectic materials.

Model of Layer-By-Layer Thinning Transition in Thin Films

Free-standing smectic liquid-crystal films exhibit an interesting phenomenon of layer-by-layer thinning transition where, instead of abrupt rupture of the film at or below Sm-A–isotropic transition temperature T c, discrete jumps in film thickness are observed when temperature is increased, and the film eventually breaks at a temperature above the bulk value T, [4]. Starting with a molecular pair interaction potential of the McMillan form and deriving therefiom a single effective potential, the behavior of thin films with changing temperature is studied. The film thickness experimentally found [4] to exhibit a clear functional relationship to temperatures of layer transitions, h(t) = l 0 t −v, can be explained qualitatively, its exact form depending strongly on the parameters of the potential. Fluctuation profile of the order parameters within smectic layers is considered showing a critical behavior in the vicinity of the transition.

Weakly nonlinear analysis of electroconvection in a suspended fluid film

It has been experimentally observed that weakly conducting suspended films of smectic liquid crystals undergo electroconvection when subjected to a large enough potential difference. The resulting counter-rotating vortices form a very simple convection pattern and exhibit a variety of interesting nonlinear effects. The linear stability problem for this system has recently been solved. The convection mechanism, which involves charge separation at the free surfaces of the film, is applicable to any sufficiently two-dimensional fluid. In this paper, we derive an amplitude equation which describes the weakly nonlinear regime, by starting from the basic electrohydrodynamic equations. This regime has been the subject of several recent experimental studies. The lowest order amplitude equation we derive is of the Ginzburg-Landau form, and describes a forward bifurcation as is observed experimentally. The coefficients of the amplitude equation are calculated and compared with the values independently deduced from the linear stability calculation.

X-ray reflectivity study of layering transitions and the internal multilayer structure of films of three-block organosiloxane amphiphilic smectic liquid crystals at the air-water interface

The structure and layering transitions of ultrathin films of the 4-(5-pentamethyldisiloxypentoxy), ${4}^{\ensuremath{'}}$-cyanobiphenyl smectic liquid crystal $(5AB)$ at the air-water interface has been studied by x-ray reflectivity. By adjusting the molecular area in a Langmuir trough the reversible, layer-by-layer growth starting from a single monolayer was controlled and analyzed. The films consist of a monolayer in direct contact with the aqueous subphase and bilayers on top of it. The thickness of the monolayer $(\ensuremath{\cong}12$ \AA{}) suggests a molecular tilt of about 58$\ifmmode^\circ\else\textdegree\fi{}$. The bilayers are about 36 \AA{} thick, which is approximately the thickness of the bulk smectic ${A}_{d}$ phase. Hence, the molecules are presumably in the same interdigitated arrangement of the aromatic cores as in the bulk. A careful analysis of the reflectivity data further indicates a slightly different molecular ordering in the successive bilayers. The interfacial contrast was optionally enhanced, via salt addition, in order to increase the information content and the reliability of the analysis. In these cases ion depletion layers between the bulk subphase and the film were detected. The aromatic parts of the monolayer are found to be partially immersed in water. The bilayer structure itself is not influenced significantly by the addition of ions.

Electroconvection patterns in smectic films at and above onset

We have studied patterns of electrically driven convective vortices in thin freely suspended films of smectic liquid crystal at and above the onset of convection. We present measurements of the convective amplitude above onset, the spatial variation of the amplitude due to the presence of end walls, and the relaxation of the amplitude following a sudden change in the experimental control parameter. Our experimental results are shown to agree with predictions based on the Ginzburg-Landau equation, and we find values for the dimensionaless length and time scales of the Ginzburg-Landau equation.

Surface-Freezing Transitions and Novel Tilted Hexatic Phases in Smectic Liquid-Crystal Films

We report the first unambiguous identification in a thermotropic liquid crystal of a hexatic smectic- $L$, a phase with molecular tilt direction intermediate between those of the smectic- $I$ and smectic- $F$, using electron diffraction in free-standing films of 5-(4\ensuremath{'}\ensuremath{'}-hexyl,3\ensuremath{'}-fluoro-p-terphenyl-4-oxy)-pentanoic acid ethyl ester. Our structural data directly support the proposed explanation of the unusual stripe phase observed optically. We also describe multiple surface-freezing transitions which occur at lower temperatures and involve novel tilted crystal phases.

Boundary-induced wavelength selection in a one-dimensional pattern-forming system.

We have measured the stability boundary for steady electroconvection in thin, freely suspended smectic liquid crystal films. As the voltage across the film, or the film length, is varied, convective vortices are created or destroyed at the ends of the film to keep the pattern wave number within a stable range. The measured stability boundary lies substantially inside the Eckhaus boundary for this system. Our results are consistent with a mechanism for boundary-induced wavelength selection proposed by Cross et al. [Phys. Rev. Lett. 45, 898 (1980)].

Ferroelectricity in free standing films of smectic liquid crystals

Abstract The spontaneous polarization Ps has been measured in free standing films of a smectic C* phase by a new experimental setup consisting of two micro electrodes which were dipped directly into the films.

A Structural Study of Temperature Influence on the Effect of Grated Substrates on Smectic Liquid Crystal Films

Abstract We present a preliminary analysis of the results of our structural measurements on the influence of temperature variation on the orientation of smectic liquid crystal films deposited on photolithographed gratings on glass gratings. These gratings have periods of 10μm with depths ranging up to 2–3μm. We previously found that both silicon and glass gratings are able to align the liquid crystal along the grating direction for film thick-nesses above 15μm, in the grating period range 664nm–24μm, as determined by using both x-ray diffraction and optical microscopy. The size of the top layer and the grating period range are of the order of magnitude of the correlation length for the smectic liquid crystal film. In this study, the region just above the critical thickness in smA droplets was studied as a function of temperature beyond the smA - N transition. Special attention was given to the region of thickness around 40μm, a region where the competition between the air-liquid crystal interface and the ...

Dislocation loops in finite systems.

The distortions induced by dislocation loops are studied in finite systems. The influence of finite size, D, of the system, surface tension, and surface-bending elastic constant on the nematic-smectic (NA) phase transition is discussed. In thin freely suspended smectic liquid-crystal films the nematic-smectic transition temperature, ${\mathit{T}}_{\mathrm{NA}}$(D), is proportional to 1/ \ensuremath{\surd}D , providing the transition is initiated by the unbinding of the dislocation loops. The smectic phase is stabilized in films, i.e., ${\mathit{T}}_{\mathrm{NA}}$(D)g${\mathit{T}}_{\mathrm{NA}}$(\ensuremath{\infty}). The loop-unbinding mechanism for the NA transition is completely suppressed in films sandwiched between solid boundaries, because the distortion energy in this case is proportional to the area of the loop, while the loop configurational entropy is proportional to its length. For temperatures T\ensuremath{\ge}${\mathit{T}}_{\mathrm{NA}}$(\ensuremath{\infty}) the equilibrium size of the loop, ${\mathit{R}}_{\mathrm{eq}}$, is proportional to the distance between solid boundaries, D. It could grow to infinity only for D\ensuremath{\rightarrow}\ensuremath{\infty} and T\ensuremath{\ge}${\mathit{T}}_{\mathrm{NA}}$(\ensuremath{\infty}).

INFLUENCE OF DIMENSIONALITY AND SURFACE ORDERING ON PHASE TRANSITIONS: STUDIES OF FREELY-SUSPENDED LIQUID-CRYSTAL FILMS

Freely-suspended films of smectic liquid crystals can be regarded as membranes consisting of a stack of an integral number of molecular (smectic) layers with the layer planes being parallel to the two free surfaces. Because of their variable thickness (between thousands and only two layers) and the large variety of phase transitions between different smectic phases, freely-suspended films are excellent systems to study the influence of the dimensional cross-over from three to two dimensions on phase transitions. Further, because the free surface of a liquid crystal has a strong ordering effect (contrary to solids which exhibit generally surface-induced disorder), freely-suspended films are well-suited for the study of the effect of enhanced surface order on phase transitions. A review of the corresponding experimental work is given.

Temperature dependence of XY-like order parameters in thin free-standing smectic liquid-crystal films.

The in-plane projection of the director in smectic-C liquid crystals and the hexatic order parameter in hexatic smectic liquid crystals are both analogous to the magnetization in an XY ferromagnet. Recent experimental results and theoretical arguments suggest that two-dimensional XY ferromagnets have a universal finite-size-induced effective magnetization exponent \ensuremath{\beta}\ensuremath{\approxeq}0.23 [S. T. Bramwell and P. C. W. Holdsworth, J. Phys. Condens. Matter 5, L53 (1993)]. From these results, we predict that XY-like order parameters in thin free-standing smectic liquid-crystal films should behave as \ensuremath{\propto}(${\mathit{T}}_{\mathit{C}}$-T${)}^{\mathrm{\ensuremath{\beta}}}$ in an intermediate temperature range below the relevant transition temperature ${\mathit{T}}_{\mathit{C}}$ if they exhibit a Kosterlitz-Thouless transition of a single XY-like order parameter.

Novel dynamical mode in a tilted smectic liquid-crystal film.

We report on light-scattering experiments in the smectic-C phase of a two-dimensional, freely suspended liquid-crystal film at wave vectors intermediate between pure bend and splay director fluctuations. The dynamic signal reveals the presence of two diffusive modes, one of which is the usual hydrodynamic mode for in-plane director fluctuations and one of which has not been previously reported. The wave-vector dependence of the scattered intensity for the new mode is well described by the selection rules for out-of-plane fluctuations (undulations) of the film. However, the overdamped dynamics reveal a nonhydrodynamic and extremely slow decay, in contrast to the usual picture for smectic layer undulations. Near the smectic-C to smectic-I (hexatic) transition, the elastic moduli and decay constants associated with the two modes show a remarkably similar temperature dependence.