Nematic Droplets Research Articles

Self-assembled block copolymers as matrix for multifunctional materials modified with low-molecular-weight liquid crystals

Meso/nanocomposite films made of low-molecular-weight nematic 4′-(hexyloxy)-4-biphenyl-carbonitrile (HOBC) embedded in poly(styrene- block-ethylene oxide) (PSEO) diblock copolymer have been prepared by using solvent-induced phase separation at room temperature. Their phase separation behavior has been investigated by morphological and viscoelastic analyses as a function of HOBC content and preparation conditions. Self-assembly of block copolymer in HOBC/PSEO systems containing 5, 10, 30 and 50 wt.% HOBC leads to submicron phase segregation of HOBC. Moreover, narrower droplet size and good distribution of nematic liquid crystal droplets in the self-assembled PSEO matrix have been observed. Thus solvent-induced microphase separation of poly(ethylene oxide)-block provokes segregation of the liquid crystal as a mesophase. Self-assembled HOBC/PSEO systems maintain generated droplet-like morphology of the HOBC phase even after 3 months, thus confirming the possibility of obtaining multifunctional materials with stable and well-defined morphologies based on block copolymer-dispersed liquid crystals as optically transparent matrices. This kind of material generates thermally reversible material for optoelectronics based on room temperature self-assembling block copolymers matrices.

Kinetics of the phase transition in an isotropic liquid crystalline dimer subjected to a deep temperature quench

Kinetics of the ordered phase growth in a liquid crystalline azomethine dimer under a deep temperature quench was studied using a polarizing optical microscope at different cooling rates. The nematic droplets revealed in the optical images across the phase transition were treated statistically and the resulting statistical size distributions were described using principles of irreversible thermodynamics. After analysis of the mean droplet diameter evolving with time, two regimes of the nematic phase growth were recognized: (i) nucleation and rapid nucleus growth and (ii) nucleus coarsening. Both regimes were quantitatively described with the universal scaling function.

Analysis of the surface order parameter of liquid crystal on a polymer surface using the phase transition droplet method

The essence of the anisotropic alignment of liquid crystal (LC) molecules at the substrate surface is derived from the distribution of the molecular direction of the LC, which is best described in terms of the order parameter. The order parameter of LC molecules at a polymer surface can be determined by measuring the contact angle of a nematic LC droplet at the nematic-isotropic phase transition temperature. In this study, we quantified the relationship between the surface order parameter and contact angle of the droplet for a homogeneous alignment LC cell. Based on the results, we propose a model for a droplet on a polymer surface. Its validity was confirmed by measuring the polarized infrared absorption.

Liquid crystal droplet production in a microfluidic device

Liquid crystal drops dispersed in a continuous phase of silicone oil are generated with a narrow distribution in droplet size in microfluidic devices both above and below the nematic‐to‐isotropic transition temperature. Our experiments show that the surface properties of the channels can be critical for droplet formation. We observe different dynamics in liquid crystal droplet generation and coalescence, and distinct droplet morphology on altering the microchannel surface energy. This is explained by the thermodynamic description of the wetting dynamics of the system. The effect of the nematic‐to‐isotropic transition on the formation of liquid crystal droplets is also observed and related to the capillary number. We also investigate how the nematic droplet size varies with the flow rate ratio and compare this behaviour with a Newtonian reference system. The effect of the defect structures of the nematic liquid crystal can lead to distinctly different scaling of droplet size in comparison with the Newtonian system. When the nematic liquid crystal phase is stretched into a thin filament before entering the orifice, different defect structures and numbers of defect lines can introduce scatter in the drop size. Capillary instabilities in thin nematic liquid crystal filament have an additional contribution from anisotropic effects such as surface gradients of bending stress, which can provide extra instability modes compared with that of isotropic fluids.

Growth and structure of nematic spherulites under shallow thermal quenches

The growth kinetics, shape, interfacial and internal orientation texture of a submicron nematic spherulite arising during the isotropic-to-nematic liquid crystal phase transformation under shallow thermal quenches is analyzed using theory, scaling, and numerical simulations based on the Landau – de Gennes model (The Physics of Liquid Crystals, 2nd edn. Clarendon, Oxford). The numerical computations from this model yield interfacial cusp formation that relaxes through the nucleation of two disclination lines of topological charge +1/2 and subsequently leads to intra-droplet texturing and a net topological charge within the spherulite of +1. The timing of these events suggests that cusp formation at the interface is intimately associated with the interfacial defect shedding mechanism (J. Chem. Phys. 124:244902, 2006) for shallow quenches. These results are different than predictions for deep quenches (J. Chem. Phys. 124:244902, 2006) where interfacial defect shedding leads to four defects and a net topological charge of +2. A liquid crystal dynamic shape equation is derived from the Landau – de Gennes model to account for the interface shape changes in terms of surface viscosity, the driving forces due to the uniaxial nematic-isotropic free energy difference, capillary forces, and friction forces, and used to semi-quantitatively show that during cusp formation and defect shedding, gradient elasticity, capillary forces and friction play significant roles in decelerating and accelerating the surface. An interfacial eigenvalue analysis shows that during the shallow quench, disclination lines nucleate within the interface itself and then texturize the nematic droplet as they migrate from within the interface to the bulk of the growing nematic droplet. After defect shedding, the spherulite is nearly circular and grows with constant velocity, in agreement with experiments. The results shed new light on intra-spherulite texturing mechanisms in phase ordering under weak driving forces.

Study of PDCLC Materials Using Fabry Perot Scattering Technique

Polymer dispersed cholesteric liquid crystals (PDCLCs) consist of liquid crystal (nematic and Cholesteric) droplets that are dispersed in a solid polymer matrix. PDCLCs are a relatively new class of materials that hold promise for many applications ranging from switchable windows to projection displays. These materials, which are simply a combined application of polymers and liquid crystals, are the focus of extensive research in the display industry.

Topological Changes in Bipolar Nematic Droplets under Flow

Bipolar liquid crystal drops moving inside microchannels exhibit periodic director field transformations due to induced circulating flows inside them. These modifications are characterized by changes in the type of point surface disclinations; they periodically change from splay to bend disclinations, implying the drop changes between bipolar and escaped concentric configurations. Upon stopping the flow, this structure does not relax to the lower energy bipolar configuration; we argue this is due to drop flattening inside the channels.

Friedericksz threshold field in bipolar nematic droplets with strong surface anchoring

A numerical method has been developed for calculating the director configuration in ellipsoidal droplets of a nematic liquid crystal with strong tangential anchoring in a uniform magnetic field of an arbitrary orientation. A relation has been obtained for determining the Friedericksz threshold corresponding to the beginning of the reorientation of the central region of a droplet when the field is orthogonal to the biopolar axis. The effect of the breaking of the orthogonal condition on the threshold character of the orientation process is considered. The reorientation of the ensemble of bipolar droplets of the 5CB nematic liquid crystal dispersed in polyvinyl butyral has been studied by the magneto-optical method. Comparative analysis of calculation data and measured values of the threshold field has been performed.

Anisotropic diffusion of light in polymer dispersed liquid crystals

Anisotropic diffusion of light was experimentally first observed in bulk nematic liquid crystals. The origin of anisotropy in such a system is due to the optical anisotropy of nematic liquid crystal and to anisotropic scattering of light from nematic orientational fluctuations. In polymer dispersed liquid crystals (PDLCs) anisotropic scatterers-i.e., nematic droplets-are embedded in an optically isotropic polymer. The anisotropy of diffusion of light in this system therefore depends only on the anisotropic scattering from a single nematic droplet and on the average orientation of the droplets in PDLCs. In the absence of an external field droplets in PDLCs are randomly oriented and diffusion of light is isotropic. In an external field the droplets are on average oriented in the direction of the external field and the diffusion of light becomes anisotropic. We calculated the diffusion tensor and expected anisotropy of the diffusion of light as a function of an external field, kR , and volume fraction using the theory developed for bulk nematic liquid crystals. Our calculations show that the anisotropy of diffusion tensor can have both the signs and that the magnitude of diffusion constants increase with increasing external field.

Focusing of light by polymer-dispersed liquid-crystal films with nanosized droplets

An analysis is presented of polarization-independent electrically tunable light focusing by polymerdispersed liquid-crystal films with nanosized liquid-crystal droplets. Polymer-dispersed liquid-crystal films with axially symmetric distributions of liquid-crystal droplet concentration and layers with axially symmetric thickness profiles are considered. The paraxial, Rayleigh, and Rayleigh-Gans approximations, as well as the Foldy-Twersky equation, are used to examine the dependence of focal length on lens geometry, droplet size, concentration of nematic liquid-crystal droplets, and applied field. The tunable focusing ranges are evaluated for both lens types considered in the study. Dependence of the transmittance of polymer-dispersed liquid-crystal film on its characteristics is analyzed. The results obtained are compared with those available from the literature.

Fractionation in polydisperse systems of spherocylindrical rods: the influence of attractive interactions and adjacent substrates

Systems of spherocylindrical rods with polydisperse length distribution are investigated with the help of Monte Carlo simulations. For a sufficiently high pressure, a demixing of long and short rods is observed. The degree of fractionation increases with the strength of the attractive interaction. In systems of attractive rods, the largest rods arrange themselves in a hexatic monolayer, whereas in hard rod systems, a less-ordered nematic droplet forms with a broader length distribution. Fractionation effects are also investigated for a rod system in contact with a smooth planar substrate. At low pressures, the density of long rods close to the substrate is reduced. For a sufficiently high pressure, the enhanced order close to the substrate leads to an aggregation of long rods in the vicinity of the substrate.

New cellulose derivatives composites for electro-optical sensors

In this work we used hydroxypropylcellulose (HPC) and acetoxypropylcellulose (APC) to produce free standing solid films (∼60 μm) for electro-optical devices. Cellulose derivatives films were prepared from crosslinked both HPC and APC isotropic and liquid crystalline solutions without and with a low molecular weight nematic liquid crystal mixture (LC) E7. Films prepared from anisotropic solutions presented a band texture and E7 droplets of micron and submicron size were found to coexist with the band texture in the films prepared with E7. The optical cells were composed by the cellulose derivatives films covered on both free surfaces by a layer of the LC E7 and placed between two transparent conducting substrates. All the electro-optical cells prepared showed switching times in the range of some milliseconds to hundreds of milliseconds. The on-voltage ( V on) is lowered for cells prepared with the ester derivative film obtained from isotropic solutions. The maximum transmission values show a clearly tendency to decrease for the cellulosic matrix prepared from anisotropic solutions with and without nematic liquid crystal droplets.

Self-Propulsion of Nematic Drops: Novel Phase Separation Dynamics in Impurity-Doped Nematogens

We report a novel phase separation dynamics, mediated by self-propelled motion of the nucleated drops, in a mixture of a nematogen and an isotropic dopant. We show that surface flow, induced by the gradient in the concentration of the dopant expelled by the growing drops, provides the driving force for the propulsion of nematic droplets. While the liquid crystal-isotropic transition is used here to demonstrate the phenomenon, self-propulsion should be observable in many other systems in which the dynamics of a conserved order parameter is coupled to a nonconserved order parameter.

Kinetics of the nematic ordered phase growth during a temperature quench of an isotropic siloxane‐azomethine polymer

Kinetics of the nucleus growth during a deep temperature quench across the isotropic to nematic phase transition was experimentally investigated for a siloxane‐azomethine polyether at cooling rates of 10 and 20°C min−1. Nematic droplets revealed in the optical images during the phase separation were treated statistically and the resulting statistical size distributions were described using the model of reversible aggregation. Analysis of the time‐dependent distribution parameters allowed two processes involved in liquid crystal phase ordering to be identified: nucleus growth and nucleus coarsening. Both regimes are quantitatively described using the universal growth law.

Structure formation and fractionation in systems of polydisperse attractive rods

Fractionation effects and the formation of structured domains are investigated in polydisperse systems of attractive spherocylinders with the help of Monte Carlo simulations. For sufficiently high attractive interaction strength and pressure, the large rods in the system aggregate and form a highly ordered hexatic monolayer that coexists with an isotropic fluid of smaller rods. Fractionation diminishes with decreasing interaction strength but is still observed for hard rod systems, in which the large rods form a nematic droplet rather than a monolayer. Results for polydisperse systems are accompanied by phase diagrams for monodisperse systems of attractive spherocylinders. Here, the phase behavior is shown as a function of rod length and pressure.

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.

Polarization and phase of light transmitted through polymer‐dispersed liquid‐crystal film

Abstract— A method for calculating the transmission coefficient, phase shift, and characteristics of the polarization state of light, transmitted through a PDLC film with nanosized nematic droplets, is considered. Functional dependences of these values on the morphological characteristics of a film are obtained and graphically illustrated. Good agreement between the theoretical and available experimental data is established.

New Photovoltaic Concept: Liquid‐Crystal Solar Cells Using a Nematic Gel Template

Liquid-crystal (LC) gels are normally used as insulating, scattering media in LC displays. Organic solar cells generate electrical power by the separation of photocreated electrons and holes at the interface between electron-donating and electron-accepting organic semiconductors. We combine these two technologies to create a new type of plastic solar cell. The efficiency of solar cells can be improved by enlarging the interface area to maximize charge separation and by vertically separating the two materials to provide different paths for electrons and holes to the electrodes. We use a new electron-donating, visible-light absorbing, LC gel to form a surface with nanometer-sized grooves with a large interface area to an overlying electron-accepting layer. This forms a photovoltaic cell, which exhibits a monochromatic power conversion efficiency of 0.6 % at an incident intensity of 45 mW cm. Improvements will provide low-cost, robust solar cells compatible with plastic electronics and roll-to-roll processing. Most organic photovoltaics use either main-chain conjugated polymers, often in conjunction with fullerenes, or low-molecular-weight glasses, achieving power-conversion efficiencies up to 4.2 %. Charge separation is achieved in solar cells by ionization of the photogenerated exciton at a heterointerface between electron-donating material with a low ionization potential (IP) and electron-accepting species with a high electron affinity (EA). The diffusion length of the exciton before recombination is of the order of 100–200 A, so that distributed, rather than planar, heterointerfaces have been used to maximize charge separation. These have been formed by phase-separated blends of electron-donating and electronaccepting organic materials. The bulk-distributed heterojunctions are formed randomly and do not guarantee a continuous pathway for charge carriers to the electrodes. The required vertical segregation of the blend materials can be obtained by surface treatment or in diffuse bilayer photovoltaics. A blend of an electron-donating columnar LC, hexabenzocoronene, and an electron-accepting perylene dye vertically segregates spontaneously to give an efficient photovoltaic effect. Unfortunately, the spatial scale of the phase separation cannot be controlled reproducibly in these devices. Another problem for phase-separated configurations is poor stability of the domain size lowering the device efficiency. To date the photovoltaic effect has only been demonstrated with calamitic LCs via an ionic mechanism, although electronic photoconductivity has been well researched. Polymerizable LCs, with two polymerizable groups attached through flexible aliphatic spacers to an aromatic core, are attracting interest as organic semiconductors for electroluminescence and organic transistors. Their advantages include spontaneous self-assembly, relatively high charge mobility (1 × 10 cm V s), easy deposition by spin-coating or ink-jet printing and lithographic photopatterning to form multilayer polymer networks. We now report the use of reactive mesogens in a novel way to demonstrate a new approach to photovoltaics. Liquid-crystalline gels are formed from a homogeneous mixture of a reactive mesogen and a non-polymerizable LC. Polymerization results in controlled phase separation of the two components and the formation of a polymer network matrix around the nematic droplets. To date, transparent LC gels have typically been used in direct-view reflective displays, switchable polarizers, directional reflectors, and so forth. Polymerization-induced phase separation of a homogeneous mixture of a LC and a polymer-forming material brings the concept of a paintable display closer. A key step in our new approach is the use of mixtures of hole-transporting reactive mesogens with a low IP and analogous LCs with the same aromatic core to form an electron-donating LC gel. An ideal LC composite photovoltaic is illustrated in Figure 1. It has a vertically separated, large-area interface between electron-donating and electron-accepting layers providing complete pathways for transport of holes and electrons. An electron-blocking polymer network of the electron-donating reactive mesogen is deposited first on an electrode surface by spin-coating from solution and then crosslinking in order to provide a high open circuit. A mixture of the same electron-donating reactive mesogen and the corresponding non-polymerizable analogue are then deposited in a similar C O M M U N IC A TI O N S

Holographic polymer dispersed liquid crystal reflection gratings formed by visible light initiated thiol-ene photopolymerization

Visible laser light of wavelengths 442, 488 and 532 nm was used to write holographic reflection gratings with notches into the infra-red spectral region in polymer dispersed liquid crystals (H-PDLC). The gratings were formed by the spatially periodic phase separation of nematic liquid crystal during thiol-ene photopolymerization. An organic titanocene based complex, Rhodamine 6G (Rh6G) and Pyrromethene 597 (Pym) dyes in combination with benzoyl peroxide, were used as initiators. Using visible laser light and a single prism, electrically switchable reflection gratings with notches in the blue, green, yellow, red, and near infra-red (NIR) spectral regions were fabricated using angle tuning. The photophysics of the initiator dyes was studied by absorption and fluorescence spectroscopy. Morphology studies by TEM showed a periodic distribution of discrete nematic droplets of nearly spherical shape. The development of visible photoinitiator systems broadens the range of fabrication wavelengths of dynamic reflection filters using inexpensive visible lasers.

Director configuration and self-organization of inclusions in two-dimensional smectic membranes

The behavior of isolated inclusions (nematic droplets, smectic islands) and formation of chains and clusters from inclusions in oriented smectic membranes have been studied. Investigations of inclusions were performed in membranes in which the molecular ordering was oriented by an external magnetic field. At planar boundary conditions on the interface between the membrane and inclusions different configurations of the c-director field were observed: Coulombic, dipolar, quadrupolar, and mixed. The observed orientation of inclusions and their interactions and self-organization correlate with the predictions of the theory based on the electromagnetic analogy. Chaining and formation of superstructures differ in oriented and nonoriented membranes.