Topological changes in bipolar nematic droplets under flow

Topology of ferroelectric nematic droplets: the case driven by flexoelectricity or depolarization field

The recent discovery of ferroelectric nematics provides new opportunities for exploring polar topology in liquid matter. Here, we report numerous potential polarization topological states (e.g., polar vortex-like and line disclination mediated structures) in confined ferroelectric nematics with similar free-energy levels. In the experiment, they appear according to the confinement size and surface anchoring conditions. Based on a minimal analytical approach, we reveal that the topological transformation is balanced among the nematic elasticity, the polarization gradient, the flexoelectric and the depolarization interactions.

Spontaneous electric-polarization topology in confined ferroelectric nematics

Topological textures have fascinated people in different areas of physics and technologies. However, the observations are limited in magnetic and solid-state ferroelectric systems. Ferroelectric nematic is the first liquid-state ferroelectric that would carry many possibilities of spatially-distributed polarization fields. Contrary to traditional magnetic or crystalline systems, anisotropic liquid crystal interactions can compete with the polarization counterparts, thereby setting a challenge in understating their interplays and the resultant topologies. Here, we discover chiral polarization meron-like structures, which appear during the emergence and growth of quasi-2D ferroelectric nematic domains. The chirality can emerge spontaneously in polar textures and can be additionally biased by introducing chiral dopants. Such micrometre-scale polarization textures are the modified electric variants of the magnetic merons. Both experimental and an extended mean-field modelling reveal that the polarization strength plays a dedicated role in determining polarization topology, providing a guide for exploring diverse polar textures in strongly-polarized liquid crystals.

Overview of Liquid Crystal Biosensors: From Basic Theory to Advanced Applications

Liquid crystals (LCs), as the remarkable optical materials possessing stimuli-responsive property and optical modulation property simultaneously, have been utilized to fabricate a wide variety of optical devices. Integrating the LCs and receptors together, LC biosensors aimed at detecting various biomolecules have been extensively explored. Compared with the traditional biosensing technologies, the LC biosensors are simple, visualized, and efficient. Owning to the irreplaceable superiorities, the research enthusiasm for the LC biosensors is rapidly rising. As a result, it is necessary to overview the development of the LC biosensors to guide future work. This article reviews the basic theory and advanced applications of LC biosensors. We first discuss different mesophases and geometries employed to fabricate LC biosensors, after which we introduce various detecting mechanisms involved in biomolecular detection. We then focus on diverse detection targets such as proteins, enzymes, nucleic acids, glucose, cholesterol, bile acids, and lipopolysaccharides. For each of these targets, the development history and state-of-the-art work are exhibited in detail. Finally, the current challenges and potential development directions of the LC biosensors are introduced briefly.

Lattice Boltzmann and Jones matrix calculations for the determination of the director field structure in self-propelling nematic droplets

Nematic droplets immersed in aqueous surfactant solutions can show a self-propelled motion induced by a Marangoni flow in the droplet surface. In addition to the self-propulsion, the Marangoni flow induces within the droplet a convective flow which considerably influences the nematic director field of the droplet. We report numerical simulations aiming at the determination of the director field in the self-propelling droplet. The convective flow and the resulting structure of director field are described by a lattice Boltzmann model. The reliability of the obtained structures is proved by subsequent Jones matrix calculations which enable the direct comparison of experimental polarizing microscopy images of self-propelling droplets with calculated images based on the simulated structures.

Applications of Microfluidics in Liquid Crystal-Based Biosensors

Liquid crystals (LCs) with stimuli-responsive configuration transition and optical anisotropic properties have attracted enormous interest in the development of simple and label-free biosensors. The combination of microfluidics and the LCs offers great advantages over traditional LC-based biosensors including small sample consumption, fast analysis and low cost. Moreover, microfluidic techniques provide a promising tool to fabricate uniform and reproducible LC-based sensing platforms. In this review, we emphasize the recent development of microfluidics in the fabrication and integration of LC-based biosensors, including LC planar sensing platforms and LC droplets. Fabrication and integration of LC-based planar platforms with microfluidics for biosensing applications are first introduced. The generation and entrapment of monodisperse LC droplets with different microfluidic structures, as well as their applications in the detection of chemical and biological species, are then summarized. Finally, the challenges and future perspectives of the development of LC-based microfluidic biosensors are proposed. This review will promote the understanding of microfluidic techniques in LC-based biosensors and facilitate the development of LC-based microfluidic biosensing devices with high performance.

Structured Liquid Droplets as Chemical Sensors that Function Inside Living Cells

We report that micrometer-scale droplets of thermotropic liquid crystals (LCs) can be positioned inside living mammalian cells and deployed as chemical sensors to report the presence of toxins in extracellular environments. Our approach exploits droplets of LC enclosed in semi-permeable polymer capsules that enable internalization by cells. The LC droplets are stable in intracellular environments, but undergo optical changes upon exposure of cells to low, sub-lethal concentrations of toxic amphiphiles. Remarkably, LC droplets in intracellular environments respond to extracellular analytes that do not generate an LC response in the absence of cellular internalization. They also do not respond to other chemical stimuli or processes associated with cell growth or manipulation in culture. Our results suggest that droplet activation involves the transport and co-adsorption of amphiphilic toxins and other lipophilic cell components to the surfaces of internalized droplets. This work provides fundamentally new designs of biotic-abiotic systems that can report sensitively and selectively on the presence of select chemical agents outside cells and provides a foundation for the design of structured liquid droplets that can sense and report on other biochemical or metabolic processes inside cells.

Liquid Crystals in Curved Confined Geometries: Microfluidics Bring New Capabilities for Photonic Applications and Beyond

The quest for interesting properties and phenomena in liquid crystals toward their employment in nondisplay application is an intense and vibrant endeavor. Remarkable progress has recently been achieved with regard to liquid crystals in curved confined geometries, typically represented as enclosed spherical geometries and cylindrical geometries with an infinitely extended axial-symmetrical space. Liquid-crystal emulsion droplets and fibers are intriguing examples from these fields and have attracted considerable attention. It is especially noteworthy that the rapid development of microfluidics brings about new capabilities to generate complex soft microstructures composed of both thermotropic and lyotropic liquid crystals. This review attempts to outline the recent developments related to the liquid crystals in curved confined geometries by focusing on microfluidics-mediated approaches. We highlight a wealth of novel photonic applications and beyond and also offer perspectives on the challenges, opportunities, and new directions for future development in this emerging research area.

Self-excited oscillation of the director field in cholesteric liquid crystalline droplets under a temperature gradient

In this study, we demonstrate a self-excited oscillation induced in cholesteric liquid crystalline droplets under a temperature gradient. At equilibrium, a winding Maltese cross pattern with a point defect was observed via polarised microscopy in the droplets dispersed in an isotropic solvent. When the temperature gradient was applied, the pattern was deformed owing to the Marangoni convection induced by the gradient. Here, when both the droplet size and temperature gradient were sufficiently large, the periodic movement of the defect together with the pattern deformation was observed, which demonstrated the self-excited oscillation of the director field. To describe this phenomenon, we theoretically analysed the flow and director fields by using Onsager's variational principle. This principle enabled the simplified description of the phenomenon; consequently, the time evolution of the director field could be expressed by the phenomenological equations for the two parameters characterising the field. These equations represented the van der Pol equation, which well expressed the mechanism of the self-excited oscillation.

Stable and Metastable Patterns in Chromonic Nematic Liquid Crystal Droplets Forced with Static and Dynamic Magnetic Fields

Spherical confinement of nematic liquid crystals leads to the formation of equilibrium director field configurations that include point and line defects. Driving these materials with flows or dynamic fields often results in the formation of alternative metastable states. In this article, we study the effect of magnetic field alignment, both under static and dynamic conditions, of nematic gems (nematic droplets in coexistence with the isotropic phase) and emulsified nematic droplets of a lyotropic chromonic liquid crystal. We use a custom polarizing optical microscopy assembly that incorporates a permanent magnet whose strength and orientation can be dynamically changed. By comparing simulated optical patterns with microscopy images, we measure an equilibrium twisted bipolar pattern within nematic gems that is only marginally different from the one reported for emulsified droplets. Both systems evolve to concentric configurations upon application of a static magnetic field, but behave very differently when the field is rotated. While the concentric texture within the emulsified droplets is preserved and only displays asynchronous oscillations for high rotating speeds, the nematic gems transform into a metastable untwisted bipolar configuration that is memorized by the system when the field is removed. Our results demonstrate the importance of boundary conditions in determining the dynamic behavior of confined liquid crystals even for configurations that share similar equilibrium bulk structures.

Horizontal transportation of a Maltese cross pattern in nematic liquid crystalline droplets under a temperature gradient

Flow and director fields strongly couple with each other in liquid crystalline systems, and herein we discuss the coupling effect in cylindrical and spherical-cap droplets formed by nematic liquid crystal. Applying a temperature gradient to droplets dispersed in a liquid solvent, we observed a crosslike texture in the droplets moved toward the high-temperature side, indicating that the director field was deformed from equilibrium. Additionally, measurement of the flow field revealed that a convective flow was induced in the droplets under temperature gradient. These results suggested that the director deformation in the droplet was induced by convection. By designing a simplified model based on this, we theoretically analyzed the above phenomenon based on Onsager's variational principle. The results show that the phenomenon was well described by a balance of surface energy gradient with viscous and elastic forces.

Thermally reconfigurable Janus droplets with nematic liquid crystalline and isotropic perfluorocarbon oil compartments

We report that mixtures of perfluorocarbon oils and hydrocarbon mesogens can be used to prepare multi-compartment (Janus) emulsion drops comprising coexisting nematic liquid crystalline (LC) and isotropic oil phases. The droplets exhibit stable spherical shapes with internal Janus-type morphologies that can be tuned widely through changes in temperature or adsorbates. In particular, we observe evidence of preferential adsorption of hydrocarbon or fluorocarbon surfactants on the interfaces of nematic versus isotropic domains, respectively, providing added control over the droplet structure. Comparisons of experiments and numerical simulations using a Landau-de Gennes continuum model provide insight into the relative importance of the LC elasticity and orientational-dependent interfacial energies on droplet morphologies and properties. We show that the hierarchical organization of the LC compartments generates optical properties and responsiveness not found in emulsions of isotropic oils.

Liquid crystalline droplets in aqueous environments: electrostatic effects

We demonstrate the strong impact of electrostatic properties on radial-bipolar structural transitions in nematic liquid crystal (LC) droplets dispersed in different aqueous environments. In the experimental part of the study, we systematically changed the electrostatic properties of both LC droplets and aqueous solutions. Mixtures of nematics were studied by combining LC materials with negative (azoxybenzene compounds) and strongly positive (cyanobiphenyl) dielectric anisotropy. The aqueous solutions were manipulated by introducing either polyvinyl alcohol, glycerol, electrolyte or amphiphilic anionic surfactant SDS into water. In the supporting theoretical study, we identified the key parameters influencing the dielectric constant and the electric field strength of aqueous solutions. We also estimated the impact of different electrolytes on the Debye length at the LC-aqueous interface. The obtained results are further analysed for chemical and biological sensing applications.

Cylindrical nematic liquid crystal shell: effect of saddle-splay elasticity

This study introduces cylindrical nematic liquid crystal (LC) shells. Shells as confinement can provide soft matter with intriguing topology and geometry. Indeed, in spherical shells of LCs, rich defect structures have been reported. Avoiding the inherent Plateau-Rayleigh instability of cylindrical liquid-liquid interfaces, we realize the cylindrical nematic LC shell by two different methods: the phase separation in the nematic-isotropic coexistence phase and a cylindrical cavity with a glass rod suspended in the middle. Specifically, the director configurations of lyotropic chromonic LCs (LCLCs) in the cylindrical shell and their energetics are investigated theoretically and experimentally. Unusual elastic properties of LCLCs, i.e., a large saddle-splay modulus, and a shell geometry with both concave and convex curvatures, result in a double-twist director configuration.

Phase Transition-Driven Nanoparticle Assembly in Liquid Crystal Droplets

When nanoparticle self-assembly takes place in an anisotropic liquid crystal environment, fascinating new effects can arise. The presence of elastic anisotropy and topological defects can direct spatial organization. An important goal in nanoscience is to direct the assembly of nanoparticles over large length scales to produce macroscopic composite materials; however, limitations on spatial ordering exist due to the inherent disorder of fluid-based methods. In this paper we demonstrate the formation of quantum dot clusters and spherical capsules suspended within spherical liquid crystal droplets as a method to position nanoparticle clusters at defined locations. Our experiments demonstrate that particle sorting at the isotropic–nematic phase front can dominate over topological defect-based assembly. Notably, we find that assembly at the nematic phase front can force nanoparticle clustering at energetically unfavorable locations in the droplets to form stable hollow capsules and fractal clusters at the droplet centers.

Patterned surface anchoring of nematic droplets at miscible liquid-liquid interfaces

We report on the internal configurations of droplets of nematic liquid crystals (LCs; 10-50 μm-in-diameter; comprised of 4-cyano-4'-pentylbiphenyl and 4-(3-acryloyloxypropyloxy)benzoic acid 2-methyl-1,4-phenylene ester) sedimented from aqueous solutions of sodium dodecyl sulfate (SDS) onto interfaces formed with pure glycerol. We observed a family of internal LC droplet configurations and topological defects consistent with a remarkably abrupt transition from homeotropic (perpendicular) to tangential anchoring on the surface of the LC droplets in the interfacial environment. Calculations of the interdiffusion of water and glycerol at the aqueous-glycerol interface revealed the thickness of the diffuse interfacial region of the two miscible liquids to be small (0.2-0.5 μm) compared to the diameters of the LC droplets on the experimental time-scale (15-120 minutes), leading us to hypothesize that the patterned surface anchoring was induced by gradients in concentration of SDS and glycerol across the diameter of the LC droplets in the interfacial region. This hypothesis received additional support from experiments in which the time of sedimentation of the LC droplets onto the interface was systematically increased and the droplets were photo-polymerized to preserve their configurations: the configurations of the LC droplets were consistent with a time-dependent decrease in the fraction of the surface area of each droplet exhibiting homeotropic anchoring. Specifically, LC droplets with <10% surface area with tangential anchoring exhibited a bulk point defect within the LC droplet, whereas droplets with >10% surface area with tangential anchoring exhibited a boojum defect within the tangential region and a disclination loop separated the regions with tangential and homeotropic anchoring. The topological charge of these LC droplet configurations was found to be consistent with the geometrical theorems of Poincaré and Gauss and also well-described by computer simulations performed by minimization of a Landau-de Gennes free energy. Additional experimental observations (e.g., formation of "Janus-like" particles with one hemisphere exhibiting tangential anchoring and the other perpendicular anchoring) and simulations (e.g., a size-dependent set of LC droplet configurations with <10% surface area exhibiting tangential anchoring) support our general conclusion that placement of LC droplets into miscible liquid-liquid interfacial environments with compositional gradients can lead to a rich set of LC droplet configurations with symmetries and optical characteristics that are not encountered in LC droplet systems in homogeneous, bulk environments. Our results also reveal that translocation of LC droplets across liquid-liquid interfaces can define new transition pathways that connect distinct configurations of LC droplets.

Ultrasensitive Multi-Functional Flexible Sensors Based on Organic Field-Effect Transistors with Polymer-Dispersed Liquid Crystal Sensing Layers

Ultrasensitive flexible sensors with multi-sensing functions are required for various applications in flexible electronics era. Here we demonstrate flexible polymer-dispersed liquid crystal (PDLC)-integrated-organic field-effect transistors (OFETs) (PDLC-i-OFETs), which sensitively respond to various stimulations including weak gas (air) flow, direct physical touch, light, and heat. The flexible PDLC-i-OFETs were fabricated by spin-coating the poly(methyl methacrylate) (PMMA)-dispersed 4,4'-pentyl-cyanobiphenyl (5CB) layers on the poly(3-hexylthiophene) (P3HT) channel layers of OFETs with 200 μm-thick poly(ethylene naphthalate) (PEN) substrates. The flexible PDLC-i-OFET devices could sense very weak nitrogen gas flow (0.3 sccm), which cannot be felt by human skins, and stably responded to direct physical touches (0.6~4.8 g load). In addition, the present devices showed very sensitive photoresponses to a visible light and exhibited excellent heat-sensing characteristics at a temperature of 25~70 °C. In particular, the present flexible PDLC-i-OFET devices could sense two different stimulations at the same time, indicative of promising multi-sensing capabilities.

Liquid crystals in micron-scale droplets, shells and fibers

The extraordinary responsiveness and large diversity of self-assembled structures of liquid crystals are well documented and they have been extensively used in devices like displays. For long, this application route strongly influenced academic research, which frequently focused on the performance of liquid crystals in display-like geometries, typically between flat, rigid substrates of glass or similar solids. Today a new trend is clearly visible, where liquid crystals confined within curved, often soft and flexible, interfaces are in focus. Innovation in microfluidic technology has opened for high-throughput production of liquid crystal droplets or shells with exquisite monodispersity, and modern characterization methods allow detailed analysis of complex director arrangements. The introduction of electrospinning in liquid crystal research has enabled encapsulation in optically transparent polymeric cylinders with very small radius, allowing studies of confinement effects that were not easily accessible before. It also opened the prospect of functionalizing textile fibers with liquid crystals in the core, triggering activities that target wearable devices with true textile form factor for seamless integration in clothing. Together, these developments have brought issues center stage that might previously have been considered esoteric, like the interaction of topological defects on spherical surfaces, saddle-splay curvature-induced spontaneous chiral symmetry breaking, or the non-trivial shape changes of curved liquid crystal elastomers with non-uniform director fields that undergo a phase transition to an isotropic state. The new research thrusts are motivated equally by the intriguing soft matter physics showcased by liquid crystals in these unconventional geometries, and by the many novel application opportunities that arise when we can reproducibly manufacture these systems on a commercial scale. This review attempts to summarize the current understanding of liquid crystals in spherical and cylindrical geometry, the state of the art of producing such samples, as well as the perspectives for innovative applications that have been put forward.

Straining soft colloids in aqueous nematic liquid crystals

Liquid crystals (LCs), because of their long-range molecular ordering, are anisotropic, elastic fluids. Herein, we report that elastic stresses imparted by nematic LCs can dynamically shape soft colloids and tune their physical properties. Specifically, we use giant unilamellar vesicles (GUVs) as soft colloids and explore the interplay of mechanical strain when the GUVs are confined within aqueous chromonic LC phases. Accompanying thermal quenching from isotropic to LC phases, we observe the elasticity of the LC phases to transform initially spherical GUVs (diameters of 2-50 µm) into two distinct populations of GUVs with spindle-like shapes and aspect ratios as large as 10. Large GUVs are strained to a small extent (R/r < 1.54, where R and r are the major and minor radii, respectively), consistent with an LC elasticity-induced expansion of lipid membrane surface area of up to 3% and conservation of the internal GUV volume. Small GUVs, in contrast, form highly elongated spindles (1.54 < R/r < 10) that arise from an efflux of LCs from the GUVs during the shape transformation, consistent with LC-induced straining of the membrane leading to transient membrane pore formation. A thermodynamic analysis of both populations of GUVs reveals that the final shapes adopted by these soft colloids are dominated by a competition between the LC elasticity and an energy (∼0.01 mN/m) associated with the GUV-LC interface. Overall, these results provide insight into the coupling of strain in soft materials and suggest previously unidentified designs of LC-based responsive and reconfigurable materials.

Nematic director reorientation at solid and liquid interfaces under flow: SAXS studies in a microfluidic device

In this work we investigate the interplay between flow and boundary condition effects on the orientation field of a thermotropic nematic liquid crystal under flow and confinement in a microfluidic device. Two types of experiments were performed using synchrotron small-angle X-ray-scattering (SAXS). In the first, a nematic liquid crystal flows through a square-channel cross section at varying flow rates, while the nematic director orientation projected onto the velocity/velocity gradient plane is measured using a 2D detector. At moderate-to-high flow rates, the nematic director is predominantly aligned in the flow direction, but with a small tilt angle of ∼±11° in the velocity gradient direction. The director tilt angle is constant throughout most of the channel width but switches sign when crossing the center of the channel, in agreement with the Ericksen-Leslie-Parodi (ELP) theory. At low flow rates, boundary conditions begin to dominate, and a flow profile resembling the escaped radial director configuration is observed, where the director is seen to vary more smoothly from the edges (with homeotropic alignment) to the center of the channel. In the second experiment, hydrodynamic focusing is employed to confine the nematic phase into a sheet of liquid sandwiched between two layers of Triton X-100 aqueous solutions. The average nematic director orientation shifts to some extent from the flow direction toward the liquid boundaries, although it remains unclear if one tilt angle is dominant through most of the nematic sheet (with abrupt jumps near the boundaries) or if the tilt angle varies smoothly between two extreme values (∼90 and 0°). The technique presented here could be applied to perform high-throughput measurements for assessing the influence of different surfactants on the orientation of nematic phases and may lead to further improvements in areas such as boundary lubrication and clarifying the nature of defect structures in LC displays.

Optical anisotropy in packed isotropic spherical particles: indication of nanometer scale anisotropy in packing structure

We investigated the origin of birefringence in colloidal films of spherical silica particles. Although each particle is optically isotropic in shape, colloidal films formed by drop drying demonstrated birefringence. While periodic particle structures were observed in silica colloidal films, no regular pattern was found in blended films of silica and latex particles. However, since both films showed birefringence, regular film structure patterns were not required to exhibit birefringence. Instead, we propose that nanometer-scale film structure anisotropy causes birefringence. Due to capillary flow from the center to the edge of a cast suspension, particles are more tightly packed in the radial direction. Directional packing results in nanometer-scale anisotropy. The difference in the interparticle distance between radial and circumferential axes was estimated to be 10 nm at most. Nanometer-scale anisotropy in colloidal films and the subsequent optical properties are discussed.

Solubilization of thermotropic liquid crystal compounds in aqueous surfactant solutions

We study the micellar solubilization of three thermotropic liquid crystal compounds by immersing single drops in aqueous solutions of the ionic surfactant tetradecyltrimethylammonium bromide. For both nematic and isotropic drops, we observe a linear decrease of the drop size with time as well as convective flows and self-propelled motions. The solubilization is accompanied by the appearance of small aqueous droplets within the nematic or isotropic drop. At low temperatures, nematic drops expell small nematic droplets into the aqueous environment. Smectic drops show the spontaneous formation of filament-like structures which resemble the myelin figures observed in lyotropic lamellar systems. In all cases, the liquid crystal drops become completely solubilized, provided the weight fraction of the liquid crystal in the system is not larger than a few percent. The solubilization of the liquid crystal drops is compared with earlier studies of the solubilization of alkanes in ionic surfactant solutions.

Surface order at surfactant-laden interfaces between isotropic liquid crystals and liquid phases with different polarity

We present an ellipsometry study of the interface between thermotropic liquid crystals and liquid phases consisting of various binary mixtures of water and glycerol. The liquid-crystal samples contain a small constant amount of a surfactant which induces a homeotropic anchoring at the interface. We determine the smectic or nematic order at the interface in the temperature range above the liquid-crystal-isotropic transition while the water to glycerol ratio is varied, corresponding to a systematic modification of the polarity of the liquid phase. The surface-induced order becomes less pronounced with increasing glycerol concentration in the liquid phase. The observed behavior is compared with previous studies in which the surfactant concentration in the liquid-crystal bulk phase was varied. The results indicate that in both cases the magnitude of the surfactant coverage at the interface is the key quantity which determines the liquid-crystal surface order at the interface.

Recent advances in colloidal and interfacial phenomena involving liquid crystals

This feature article describes recent advances in several areas of research involving the interfacial ordering of liquid crystals (LCs). The first advance revolves around the ordering of LCs at bio/chemically functionalized surfaces. Whereas the majority of past studies of surface-induced ordering of LCs have involved surfaces of solids that present a limited diversity of chemical functional groups (surfaces at which van der Waals forces dominate surface-induced ordering), recent studies have moved to investigate the ordering of LCs on chemically complex surfaces. For example, surfaces decorated with biomolecules (e.g., oligopeptides and proteins) and transition-metal ions have been investigated, leading to an understanding of the roles that metal-ligand coordination interactions, electrical double layers, acid-base interactions, and hydrogen bonding can play in the interfacial ordering of LCs. The opportunity to create chemically responsive LCs capable of undergoing ordering transitions in the presence of targeted molecular events (e.g., ligand exchange around a metal center) has emerged from these fundamental studies. A second advance has focused on investigations of the ordering of LCs at interfaces with immiscible isotropic fluids, particularly water. In contrast to prior studies of surface-induced ordering of LCs on solid surfaces, LC-aqueous interfaces are deformable and molecules at these interfaces exhibit high levels of mobility and thus can reorganize in response to changes in the interfacial environment. A range of fundamental investigations involving these LC-aqueous interfaces have revealed that (i) the spatial and temporal characteristics of assemblies formed from biomolecular interactions can be reported by surface-driven ordering transitions in the LCs, (ii) the interfacial phase behavior of molecules and colloids can be coupled to (and manipulated via) the ordering (and nematic elasticity) of LCs, and (iii) the confinement of LCs leads to unanticipated size-dependent ordering (particularly in the context of LC emulsion droplets). The third and final advance addressed in this article involves interactions between colloids mediated by LCs. Recent experiments involving microparticles deposited at the LC-aqueous interface have revealed that LC-mediated interactions can drive interfacial assemblies of particles through reversible ordering transitions (e.g., from 1D chains to 2D arrays with local hexagonal symmetry). In addition, recent single-nanoparticle measurements suggest that the ordering of LCs about nanoparticles differs substantially from micrometer-sized particles and that the interactions between nanoparticles mediated by the LCs are far weaker than predicted by theory (sufficiently weak that the interactions are reversible and thus enable self-assembly). Finally, LC-mediated interactions between colloidal particles have also been shown to lead to the formation of colloid-in-LC gels that possess mechanical properties relevant to the design of materials that interface with living biological systems. Overall, these three topics serve to illustrate the broad opportunities that exist to do fundamental interfacial science and discovery-oriented research involving LCs.

Tactoids of plate-like particles: size, shape, and director field

We studied, by means of polarized light microscopy, the shape and director field of nematic tactoids as a function of their size in dispersions of colloidal gibbsite platelets in polar and apolar solvents. Because of the homeotropic anchoring of the platelets to the interface, we found large tactoids to be spherical with a radial director field, whereas small tactoids turn out to have an oblate shape and a homogeneous director field, in accordance with theoretical predictions. The transition from a radial to a homogeneous director field seems to proceed via two different routes depending in our case on the solvent. In one route, the what presumably is a hedgehog point defect in the center of the tactoid transforms into a ring defect with a radius that presumably goes to infinity with decreasing drop size. In the other route, the hedgehog defect is displaced from the center to the edge of the tactoid, where it becomes virtual again going to infinity with decreasing drop size. Furthermore, quantitative analysis of the tactoid properties provides us with useful information on the ratio of the splay elastic constant and the anchoring strength and the ratio of the anchoring strength and the surface tension.

Director configuration of liquid-crystal droplets encapsulated by polyelectrolytes

Liquid-crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) droplets dispersed in aqueous solution are prepared by the assembly of poly(styrenesulfonic acid) (PSSH) and poly(styrenesulfonate sodium) (PSSNa) at the 5CB/water interface. The micrometer sized PSSH-coated 5CB droplets in the space confinement formed by two parallel glass slides break up into submicrometer sized droplets under evaporation-induced flow. We find that the size reduction of the PSSH-coated droplets is accompanied by the bipolar-to-radial configuration transition of the 5CB in the droplets, while the PSSNa-coated 5CB droplets show no size-dependent configuration transition in the same size range. Our results suggest that the size-dependent director configuration of liquid-crystal droplets encapsulated by polyelectrolytes can be modulated by changing the interface conditions, which is important in designing liquid-crystal droplets for optical and biological applications.

Formation of optically anisotropic films from spherical colloidal particles

Colloidal silica films, formed by the drop evaporation method, showed birefringent spherulite optical properties. They displayed a Maltese cross pattern under crossed polarizers, and interference colors, such as blue and orange-red, under crossed polarizers with a compensator. The difference in refractive index was estimated to be 9x10(-4) from the interference colors. Scanning electron microscopy (SEM) results revealed anisotropic structures in the colloidal films. Particles formed radially ordered hexagonal arrays. The drop evaporation method used in this report, which dries from the edge to the center, resulted in a radially ordered colloidal film. When a colloidal silica film was prepared using a unidirectional drying method, particles were packed in an ordered structure corresponding to the drying direction and the resulting film showed different birefringent optical properties. Our results show that a variety of birefringent films can be obtained from spherical colloidal dispersions through control of the drying method.

Colloidal transport on magnetic garnet films

This article reports several recent discoveries related to the controlled transport of paramagnetic colloidal particles above magnetic garnet films. The garnet films are thin uniaxial ferromagnetic films in which ferromagnetic domains can be organized into symmetric patterns consisting of stripes or bubbles and generate strong local magnetic field gradients. Application of an external homogeneous magnetic field on a larger scale compared to the spatial periodicity of the magnetic pattern in the film modulates the potential generated at its surface and induces the controlled motion of colloidal particles placed above the film. Several novel dynamical regimes are observed and reported, from localized trajectories to direct particle transport, depending on the geometry of the underlying magnetic pattern and on the parameters, which control the external driving field, such as frequency, strength and direction. Moreover, we show that this strategy allows separation and sorting of bi-disperse particle systems based on the particle size as well as the transport of chemical or biological cargoes attached to the colloidal carriers. Controlled transport of micro-sized cargoes (chemical or biological) by colloidal particle carriers in a microfluidic environment can bring significant contributions in several fields from targeted drug delivery to the realization of precise fluid-based micro-scale devices.

Characterization of adsorbate-induced ordering transitions of liquid crystals within monodisperse droplets

The ordering of liquid crystals (LCs) within micrometer-sized droplets is known to depend strongly on the presence of interfacial adsorbates, although the exact sequence of ordered equilibrium states that accompany a change in interfacial anchoring from tangential to perpendicular has not been established. In this paper, we report use of a methodology that permits the preparation of monodisperse LC droplets in aqueous phases to investigate ordering transitions in the LC droplets that accompany the adsorption of amphiphiles at the aqueous-LC droplet interface. By using an amphiphile that undergoes reversible adsorption at the aqueous-LC interface (sodium dodecylsulfate, SDS), we identified six distinct topologically ordered states of the LC droplets as a function of increasing concentration of SDS. We exploited the reversible adsorption of the SDS to LC droplets with diameters of 8.0+/-0.2 microm to confirm that these topological states are equilibrium ones. We also exposed LC droplets to a continuous gradient in concentration of SDS to document the continuous transitions between topological states and to confirm the absence of additional, intermediate topological states. The formation of the LC droplets as aqueous dispersions also enabled an investigation of ordering transitions in LC droplets driven by biomolecular interactions. Surprisingly, enzymatic hydrolysis of the phospholipid L-dipalmitoyl phosphatidylcholine (L-DLPC) by phospholipase A2 at the interfaces of the LC droplets was observed to trigger the same progression of topologically ordered states of the LC as was observed with SDS. Overall, the results presented in this paper resolve prior conflicting data in the literature by providing an unambiguous set of observations regarding topologically ordered states encountered in LC droplets. This paper provides a data set against which future theories and simulations of LCs can be compared to develop a fundamental understanding of the competition between volumetric and interfacial effects in droplets. The results also suggest that topological ordering transitions in LC droplets can be exploited to report interfacial enzymatic reactions.

Topological transformations in bipolar shells of nematic liquid crystals

We investigate how defects in bipolar shells of liquid crystal evolve when the boundary conditions for the nematic director n at the outer surface of the shell are changed from tangential to homeotropic. When the boundary conditions for n are tangential, these bipolar shells are characterized by the presence of two pairs of boojums, one pair on each spherical surface. However, when n is tangential at the inner surface and perpendicular at the outer surface, only the innermost pair of defects remains. Interestingly, there are two possible routes from one shell to the other and both involve the generation of a disclination ring that shrinks with time, eventually disappearing. Although the process is reminiscent of that encountered in bulk nematic droplets, in the case of nematic shells the two defects on the inner surface play a relevant role in the overall evolution process.

Colloidal shape controlled by molecular adsorption at liquid crystal interfaces

The ability to control finely the structure of materials remains a central issue in colloidal science. Due to their elastic properties, liquid crystals (LC) are increasingly used to organize matter at the micrometer scale in soft composites. Textures and shapes of LC droplets are currently controlled by the competition between elasticity and anchoring, hydrodynamic flows, or external fields. Molecules adsorbed specifically at LC interfaces are known to orient LC molecules (anchoring effect), but other induced effects have been poorly explored. Using specifically designed amphitropic surfactants, we demonstrate that large-shape transformations can be achieved in direct LC/water emulsions. In particular, we focus on unusual nematic filaments formed from spherical droplets. These results suggest new approaches to design new soft LC composite materials through the adsorption of molecules at liquid crystal interfaces.

Nanoparticles in nematic liquid crystals: Interactions with nanochannels

A mesoscale theory for the tensor order parameter Q is used to investigate the structures that arise when spherical nanoparticles are suspended in confined nematic liquid crystals (NLCs). The NLC is "sandwiched" between a wall and a small channel. The potential of mean force is determined between particles and the bottom of the channels or between several particles. Our results suggest that strong NLC-mediated interactions between the particles and the sidewalls of the channels, on the order of hundreds of k(B)T, arise when the colloids are inside the channels. The magnitude of the channel-particle interactions is dictated by a combination of two factors, namely, the type of defect structures that develop when a nanoparticle is inside a channel, and the degree of ordering of the nematic in the region between the colloid and the nanochannel. The channel-particle interactions become stronger as the nanoparticle diameter becomes commensurate with the nanochannel width. Nanochannel geometry also affects the channel-particle interactions. Among the different geometries considered, a cylindrical channel seems to provide the strongest interactions. Our calculations suggest that small variations in geometry, such as removing the sharp edges of the channels, can lead to important reductions in channel-particle interactions. Our calculations for systems of several nanoparticles indicate that linear arrays of colloids with Saturn ring defects, which for some physical conditions are not stable in a bulk system, can be stabilized inside the nanochannels. These results suggest that nanochannels and NLCs could be used to direct the assembly of nanoparticles into ordered arrays with unusual morphologies.

Levitation, lift, and bidirectional motion of colloidal particles in an electrically driven nematic liquid crystal

We study electric-field-induced dynamics of colloids in a nematic cell, experimentally and by computer simulations. Solid particles in the nematic bulk create director distortions of dipolar type. Elastic repulsion from the walls keeps the particles in the middle of cell. The ac electric field reorients the dipoles and lifts them to top or bottom, depending on dipole orientation. Once near the walls, the colloids are carried along two antiparallel horizontal directions by nematic backflow. Computer simulations of the backflow agree with the experiment.