Smectic Liquid Crystal Films Research Articles

Detection of islands and droplets on smectic films using machine learning

Machine learning techniques have been developed to identify inclusions on the surface of freely suspended smectic liquid crystal films imaged by reflected light microscopy. The experimental images are preprocessed using Canny edge detection and then passed to a radial kernel support vector machine (SVM) trained to recognize circular islands and droplets. The SVM is able to identify these objects of interest with an accuracy that far exceeds that of conventional tracking software, especially when the background image is non-uniform or when the target features are in close proximity to one another. This method could be applied to tracking objects in a variety of visually inhomogeneous fluid environments, in order to study growth dynamics, the development of spatial order, and hydrodynamic behavior.

Coalescence of islands in freely suspended smectic films

Smectic liquid crystal films a few molecular layers thick that are freely suspended in air are used as a model system to study the coalescence of fluids in two dimensions. High-speed video microscopy is used to observe the coalescence of islands, which are thicker, disk-shaped regions of the film, in a process driven by the line tension associated with edge dislocations along the island boundaries and limited by viscous dissipation in the liquid crystal and in the surrounding air. The early time growth of the bridge connecting the merging islands reveals much slower dynamics than predicted by Hopper's classical hydrodynamic model of coalescence of two infinitely long, fluid cylinders in vacuum, a discrepancy proposed to be due to significant dissipation in the background film and in the air that is not included in Hopper's theory. At late times, the elliptical merged island relaxes exponentially to a circular shape, at rates that are described quantitatively by a model originally developed for the evolution of fluid domains in Langmuir films.

Analogy between periodic patterns in thin smectic liquid crystal films and the intermediate state of superconductors

Spontaneous breaking of symmetry in liquid crystal (LC) films often reveals itself as a microscopic pattern of molecular alignment. In a smectic-A LC, the emergence of positional order at the transition from the nematic phase leads to periodic textures that can be used as optical microarrays, templates for soft lithography, and ordering matrices for the organization and manipulation of functional nanoparticles. While both 1d and 2d patterns have been obtained as a function of the LC film thickness and applied fields, the connection has not been made between pattern formation and the peculiar critical behavior of LCs at the nematic-smectic transition, still eluding a comprehensive theoretical explanation. In this article, we demonstrate that an intense bend distortion applied to the LC molecular director while cooling from the nematic phase produces a frustrated smectic phase with depressed transition temperature, and the characteristic 1d periodic texture previously observed in thin films and under applied electric fields. In light of De Gennes' analogy with the normal-superconductor transition of a metal, we identify the 1d texture as the equivalent of the intermediate state in type I superconductors. The bend distortion is analog to the magnetic field in metals and penetrates in the frustrated phase as an array of undercooled nematic domains, periodically intermixed with bend-free smectic-A domains. Our findings provide fundamental evidence for theories of the nematic-smectic transition, highlighting the deep connection between phase frustration and pattern formation, and perspectives on the design of functional smectic microarrays.

Pattern-Directed Phase Transitions and VOC Sensing of Liquid Crystal Films

Solvent vapor exposure could transform a crystalline or smectic liquid crystal (LC) film into nematic and isotropic phases under ambient conditions. The average time for such phase transitions is found to linearly reduce with an increase in vapor pressure and reduction in the molecular weight of solvents. Such responses of solvent vapor-annealed phase transitions of a nanoparticle-loaded LC droplet were then converted into an electrical signal, wherein the electrical resistance reduced (increased) with time upon destruction (restoration) of the orientational order of the LC matrix. Variation in the electrical response was used to identify the volatile organic vapors, phase transition of LCs, rate of diffusion–absorption of solvent into LCs, and rate of desorption–evaporation of solvent from LCs. Pattern-directed phase transitions on physically heterogeneous surfaces showed a faster (slower) kinetics on thinner (thicker) patterns. However, for chemically heterogeneous surfaces, weaker (stronger) anchoring of LCs on hydrophobic (hydrophilic) patches ensured a faster (slower) transition.

Nanohybrid Materials with Tunable Birefringence via Cation Exchange in Polymer Films

AbstractIn this work, a nanohybrid material based on a freestanding polymeric liquid crystal network capable of postmodification via cation exchange to tune birefringence is proposed. The smectic liquid crystal films can be infiltrated with a variety of cations, thereby changing the refractive indices (ne and no) and the effective birefringence (Δn) of the nanohybrid material, with reversible cation infiltration occurring within minutes. Birefringence could be tuned between values of 0.06 and 0.19, depending on the cation infiltrated into the network. Upon infiltration, a decrease in the smectic layer spacing is found with layer contraction independent of the induced change in birefringence. Potential applications are in the field of specialty optical devices, such as flexible, retunable reflective filters.

Polarization-Modulated Bent-Core Liquid Crystal Thin Films without Layer Undulation.

Spatial confinement is known to affect molecular organizations of soft matter. We present an important manifestation of this statement for thin films of bent-core smectic liquid crystals. Prior freeze-fracture transmission electron microscopy (FFTEM) studies carried out on nitro-substituted bent-core mesogens (n-OPIMB-NO_{2}) revealed an undulated smectic layer structure with an undulation periodicity of ∼8 nm. We report cryogenic TEM measurements on ∼100 nm thick 8-OPIMB-NO_{2} films. In contrast to FFTEM results, our studies show only density modulation with periodicity b=16.2 nm, and no smectic layer undulation. We show that the discrepancy between the FFTEM and cryogenic transmission electron microscopy (cryo-TEM) results can be attributed to the different sample thicknesses used in the experiments. FFTEM monitors cracked surfaces of a relatively thick (5-10 μm) frozen sample, whereas cryo-TEM visualizes the volume of a thin (0.1 μm) film that was quenched from its partially fluid phase. These results have importance in possible photovoltaics and organic electronics applications where submicron thin films are used.

Persistence of Smectic-A Oily Streaks into the Nematic Phase by UV Irradiation of Reactive Mesogens

Thin smectic liquid crystal films with competing boundary conditions (planar and homeotropic at opposing surfaces) form well-known striated structures known as “oily streaks”, which are a series of hemicylindrical caps that run perpendicular to the easy axis of the planar substrate. The streaks vanish on heating into the nematic phase, where the film becomes uniform and exhibits hybrid alignment. On adding sufficient reactive mesogen and polymerizing, the oily streak texture is maintained on heating through the entire nematic phase until reaching the bulk isotropic phase, above which the texture vanishes. Depending on the liquid crystal thickness, the oily streak structure may be retrieved after cooling, which demonstrates the strong impact of the polymer backbone on the liquid crystal texture. Polarizing optical, atomic force, and scanning electron microscopy data are presented.

Realization of hydrodynamic experiments on quasi-2D liquid crystal films in microgravity

Freely suspended films of smectic liquid crystals are unique examples of quasi two-dimensional fluids. Mechanically stable and with quantized thickness of the order of only a few molecular layers, smectic films are ideal systems for studying fundamental fluid physics, such as collective molecular ordering, defect and fluctuation phenomena, hydrodynamics, and nonequilibrium behavior in two dimensions (2D), including serving as models of complex biological membranes. Smectic films can be drawn across openings in planar supports resulting in thin, meniscus-bounded membranes, and can also be prepared as bubbles, either supported on an inflation tube or floating freely. The quantized layering renders smectic films uniquely useful in 2D fluid physics. The OASIS team has pursued a variety of ground-based and microgravity applications of thin liquid crystal films to fluid structure and hydrodynamic problems in 2D and quasi-2D systems. Parabolic flights and sounding rocket experiments were carried out in order to explore the shape evolution of free floating smectic bubbles, and to probe Marangoni effects in flat films. The dynamics of emulsions of smectic islands (thicker regions on thin background films) and of microdroplet inclusions in spherical films, as well as thermocapillary effects, were studied over extended periods within the OASIS (Observation and Analysis of Smectic Islands in Space) project on the International Space Station. We summarize the technical details of the OASIS hardware and give preliminary examples of key observations.

Active microrheology of smectic membranes.

Thin fluid membranes embedded in a bulk fluid of different viscosity are of fundamental interest as experimental realizations of quasi-two-dimensional fluids and as models of biological membranes. We have probed the hydrodynamics of thin fluid membranes by active microrheology using small tracer particles to observe the highly anisotropic flow fields generated around a rigid oscillating post inserted into a freely suspended smectic liquid crystal film that is surrounded by air. In general, at distances more than a few Saffman lengths from the meniscus around the post, the measured velocities are larger than the flow computed by modeling a moving disklike inclusion of finite extent by superposing Levine-MacKintosh response functions for pointlike inclusions in a viscous membrane. The observed discrepancy is attributed to additional coupling of the film with the air below the film that is displaced directly by the shaft of the moving post.

Acoustic streaming in two-dimensional freely suspended smectic liquid crystal films

We study horizontal streaming excited by means of a low-frequency and low-intensity acoustic wave in 2D freely suspended films of thermotropic smectic liquid crystals. Acoustic pressure induces fast periodic transverse oscillations of the film, which produce in-plane stationary couples of vortices slowly rotating in opposite directions owing to hydrodynamic nonlinearity. The parameters of the vortices are measured using a new method, based on tracking solidlike disk-shaped islands. The horizontal motion occurs only when the amplitude of the acoustic pressure exceeds the threshold value, which can be explained by Bingham-like behavior of the smectic film. The measurements above threshold are in good agreement with existing theoretical predictions. We demonstrate experimentally that in-plane flow is well controlled by changing the acoustic pressure, excitation frequency, and geometry of the film. The observations open the way to using the phenomenon in nondisplay applications.

Two-dimensional island emulsions in ultrathin, freely-suspended smectic liquid crystal films.

We report a novel type of two-dimensional colloidal emulsion, in which arrays of disc-shaped liquid crystal domains are created in ultrathin, freely-suspended, fluid smectic C liquid crystal films. After a film has been drawn across an aperture, an island emulsion is produced by repeatedly compressing and expanding the film while maintaining vigorous shear and extensional air flow across its area. Once formed, these emulsions restructure over a period of a few minutes to a stable state that then changes only slowly, over the course of several days. This stability enables study of the sedimentation of the emulsion under in-plane gravitation produced by tilting the film, during which the original island emulsion segregates into regions with different kinds of emulsions distinguished by the size, density, and degree of order of the islands. We observe a rich array of phenomena that includes the formation of chains of islands organized into two-dimensional smectics in the dilute phase, and island deformation and coalescence in the condensed phase.

Hydrodynamic interactions in freely suspended liquid crystal films.

Hydrodynamic interactions play an important role in biological processes in cellular membranes, a large separation of length scales often allowing such membranes to be treated as continuous, two-dimensional (2D) fluids. We study experimentally and theoretically the hydrodynamic interaction of pairs of inclusions in two-dimensional, fluid smectic liquid crystal films suspended in air. Such smectic membranes are ideal systems for performing controlled experiments as they are mechanically stable, of highly uniform structure, and have well-defined, variable thickness, enabling experimental investigation of the crossover from 2D to 3D hydrodynamics. Our theoretical model generalizes the Levine-MacKintosh theory of point-force response functions and uses a boundary-element approach to calculate the mobility matrix for inclusions of finite extent. We describe in detail the theoretical and computational approach previously outlined in Z. Qi et al., Phys. Rev. Lett. 113, 128304 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.128304 and extend the method to study the mutual mobilities of inclusions with asymmetric shapes. The model predicts well the observed mutual mobilities of pairs of circular inclusions in films and the self-mobility of a circular inclusion in the vicinity of a linear boundary.

Self-organized arrays of dislocations in thin smectic liquid crystal films.

Combining optical microscopy, synchrotron X-ray diffraction and ellipsometry, we studied the internal structure of linear defect domains (oily streaks) in films of a smectic liquid crystal 8CB with thicknesses in the range of 100-300 nm. These films are confined between air and a rubbed PVA polymer substrate which imposes hybrid anchoring conditions (normal and unidirectional planar, respectively). We show how the presence or absence of dislocations controls the structure of highly deformed thin smectic films. Each domain contains smectic layers curved in the shape of flattened hemicylinders to satisfy both anchoring conditions, together with grain boundaries whose size and shape are controlled by the presence of dislocation lines. A flat grain boundary normal to the interface connects neighboring hemicylinders, while a rotating grain boundary (RGB) is located near the axis of curvature of the cylinders. The RGB shape appears such that dislocation lines are concentrated at its summit close to the air interface. The smectic layers reach the polymer substrate via a transition region where the smectic layer orientation satisfies the planar anchoring conditions over the entire polymer substrate and whose thickness does not depend on that of the film. The strength of planar anchoring appears to be high, larger than 10(-2) mJ m(-2), compensating for the high energy cost of creating an additional 2D defect between a horizontal smectic layer and perpendicular ones of the transition region. This 2D defect may be melted, in order to avoid the creation of a transition region structure composed of a large number of dislocations. As a result, linear defect domains can be considered as arrays of oriented defects, straight dislocations of various Burger vectors, whose location is now known, and 2D nematic defects. The possibility of easy variation between the present structure with a moderate amount of dislocations and a structure with a large number of dislocations is also demonstrated.

Microdroplets Impinging on Freely Suspended Smectic Films: Three Impact Regimes.

We employ high-speed video imaging to study microdroplets of a few picoliters volume impacting freely suspended smectic liquid-crystal films. Depending on the impact parameters, in particular, droplet velocity and mass, three different regimes are observed such as trapping, rebounding, and tunneling. Fast droplets penetrate the films completely. After they have passed the film, they are coated with a layer of film material while the original smectic film remains intact. Droplets in a certain intermediate velocity range bounce back from the film. After impact, the film deforms and hurls the droplet back, depleting a substantial share of the initial kinetic energy. Slow droplets are caught and embedded in the film. During impact and tunneling, an appreciable amount of kinetic energy is lost. The energy is partially dissipated during droplet impact and during subsequent mechanical vibrations and oscillations of the film and the droplet. The tunneling process of high-speed droplets can be exploited to prepare smectic shells of well-defined sizes that enclose picoliters of an immiscible liquid.

Nanoparticle Aggregation and Fractal Growth in Fluid Smectic Membranes

We observe directly the diffusion and aggregation of buckyball clusters dispersed in thin, freely-suspended films of smectic liquid crystal using reflected light microscopy. The buckyballs at early times are barely resolvable, nanoscale clusters, which diffuse stochastically in the film. Clusters eventually coalesce to form micron-scale, fractal aggregates whose effective radius increases approximately linearly with time, so that after several days, millimeter-size fractals extend across the entire film. The measured fractal dimension of these final networks suggests that the aggregation of the buckyball clusters in the film is a diffusion-limited process.

Controlling the Self‐Assembly of Periodic Defect Patterns in Smectic Liquid Crystal Films with Electric Fields

Large‐area periodic defect patterns are produced in smectic A liquid crystals confined between rigid plate electrodes that impose conflicting parallel and normal anchoring conditions, inducing the formation of topological defects. Highly oriented stripe patterns are created in samples thinner than 2 μm due to self‐assembly of linear defect domains with period smaller than 4 μm, whereas hexagonal lattices of focal conic domains appear for thicker samples. The pattern type (1d/2d) and period can be controlled at the nematic–smectic phase transition by applying an electric field, which confines the defect domains to a thin surface layer with thickness comparable to the nematic coherence length. The pattern morphology persists in the smectic phase even after varying the field or switching it off. Bistable, non‐equilibrium patterns are stabilized by topological constraints of the smectic phase that hinder the rearrangement of defects in response to field variations.

Negative dispersion of birefringence in two-dimensionally self-organized smectic liquid crystal and monomer thin film.

We suggest a method to obtain a negative dispersion (ND) of birefringence using a two-dimensional self-organization of smectic liquid crystal (LC) and monomer molecules. The averaged orientation of the smectic LC was the layer normal direction with the extraordinary refractive index n(e). Meanwhile, the orientation of the monomer molecules was templated by the host-smectic LC and parallel to the layer plane corresponding to the ordinary refractive index n(o). We selected the LC molecules absorbing a shorter wavelength of UV light rather than the polymerized monomers, hence n(e) was more smoothly decreased than n(o) in the visible-wavelength range. Consequently, the birefringence Δn≡n(e)-n(o) was increased with a longer wavelength, thus giving a ND of birefringence. Using the proposed method, the ND of birefringence could be obtained in a single layer, which is desirable for thin flexible applications.

Nanoscale viscoplastic behavior of smectic liquid crystals and its application in nanolithography

We report a unique combination of properties of smectic liquid crystal films that can be described as a viscoplastic behavior on the nanoscale: On the one hand, the films preserve imprinted surface patterns despite being permeated by a directed molecular flow of the film material. On the other hand, their surface morphology can easily be manipulated using an atomic force microscopy tip. Our results demonstrate a controlled molecular-scale deposition of material on the film surface, thereby enabling nanolithographic surface modification of a fluid material.

Surface energetics of freely suspended fluid molecular monolayer and multilayer smectic liquid crystal films

A study of the surface energetics of the thinnest substrate-free liquid films, fluid molecular monolayer and multilayer smectic liquid crystal films suspended in air, is reported. In films having monolayer and multilayer domains, the monolayer areas contract, contrary to predictions from the van der Waals disjoining pressure of thin uniform slabs. This discrepancy is accounted for by modeling the environmental asymmetry of the surface layers in multilayer films, leading to the possibility that preferential end-for-end polar ordering of the rod shaped molecules can reduce the surface energy of multilayers relative to that of the monolayer, which is inherently symmetric.

Field-induced dependence of anchoring energy and surface tension in smectic liquid crystal films deposited on a solid surface

We have carried out a numerical study of the structural, thermodynamic, anchoring and surface tension properties of thin smectic films deposited in vacuo on a solid surface and subjected to external electric field E. The effects both of surface enhanced pair interactions in the bounding LC/solid and LC/vacuum layers and of E on the orientational and translational order parameters, as well as on Helmholtz free and anchoring energies, entropy, and surface tension of smectic films have been investigated. Calculations, based upon the extended McMillan’s approach with anisotropic forces, shows a weak effect of E on the anchoring energy. A reasonable agreement between the theoretically predicted and the experimentally obtained data on the anchoring energy and surface tension of 4-n-octyl-4′-cyanobiphenyl films has been obtained.