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

A Monte Carlo simulation study of a Janus discotic liquid crystal droplet

The study of discotic liquid crystals (DLCs) under spherical confinement has gained considerable significance due to its relevance in the design and optimization of advanced materials with tailored properties. The unique characteristics of DLC fluids, coupled with confinement within a spherical Janus surface, offer a compelling avenue for exploring novel behaviors and emergent phenomena. In this study, Monte Carlo simulations within the NpT ensemble are employed to investigate the behavior of a DLC fluid confined by a spherical Janus surface. The Janus surface is characterized by distinct hemispheres, with one promoting homeotropic (face-on) anchoring and the other planar (edge-on) anchoring. Our analysis reveals the emergence of two topological defects: one exclusively on the edge-anchoring hemisphere and the other at the boundary of both anchorings. Each topological defect possessing a topological charge ofk= +1/2. We observe that as the temperature transitions the central region of the droplet into a nematic phase, a disclination line forms, linking the two surface defects. By investigating droplets of three different sizes, we confirm that the isotropic-nematic transition is first-order for the larger droplet studied. However, this transition becomes continuous under strong confinement conditions. In contrast, the nematic-columnar transition remains first order even for smaller systems.

Liquid Crystal-Templated Porous Microparticles via Photopolymerization of Temperature-Induced Droplets in a Binary Liquid Mixture

Porous polymeric microspheres are an emerging class of materials, offering stimuli-responsive cargo uptake and release. Herein, we describe a new approach to fabricate porous microspheres based on temperature-induced droplet formation and light-induced polymerization. Microparticles were prepared by exploiting the partial miscibility of a thermotropic liquid crystal (LC) mixture composed of 4-cyano-4'-pentylbiphenyl (5CB, unreactive mesogens) with 2-methyl-1,4-phenylene bis4-[3-(acryloyloxy)propoxy] benzoate (RM257, reactive mesogens) in methanol (MeOH). Isotropic 5CB/RM257-rich droplets were generated by cooling below the binodal curve (20 °C), and the isotropic-to-nematic transition occurred after cooling below 0 °C. The resulting 5CB/RM257-rich droplets with radial configuration were subsequently polymerized under UV light, resulting in nematic microparticles. Upon heating the mixture, the 5CB mesogens underwent a nematic-isotropic transition and eventually became homogeneous with MeOH, while the polymerized RM257 preserved its radial configuration. Repeated cycles of cooling and heating resulted in swelling and shrinking of the porous microparticles. The use of a reversible materials templating approach to obtain porous microparticles provides new insights into binary liquid manipulation and potential for microparticle production.

Liquid-Crystalline Polymer Particles Prepared by Classical Polymerization Techniques

Liquid-crystalline polymer particles prepared by classical polymerization techniques are receiving increased attention as promising candidates for use in a variety of applications including micro-actuators, structurally colored objects, and absorbents. These particles have anisotropic molecular order and liquid-crystalline phases that distinguish them from conventional polymer particles. In this minireview, the preparation of liquid-crystalline polymer particles from classical suspension, (mini-)emulsion, dispersion, and precipitation polymerization reactions are discussed. The particle sizes, molecular orientations, and liquid-crystalline phases produced by each technique are summarized and compared. We conclude with a discussion of the challenges and prospects of the preparation of liquid-crystalline polymer particles by classical polymerization techniques.

Transformation between elastic dipoles, quadrupoles, octupoles, and hexadecapoles driven by surfactant self-assembly in nematic emulsion

Emulsions comprising isotropic fluid drops within a nematic host are of interest for applications ranging from biodetection to smart windows, which rely on changes of molecular alignment structures around the drops in response to chemical, thermal, electric, and other stimuli. We show that absorption or desorption of trace amounts of common surfactants can drive continuous transformations of elastic multipoles induced by the droplets within the uniformly aligned nematic host. Out-of-equilibrium dynamics of director structures emerge from a controlled self-assembly or desorption of different surfactants at the drop-nematic interfaces, with ensuing forward and reverse transformations between elastic dipoles, quadrupoles, octupoles, and hexadecapoles. We characterize intertransformations of droplet-induced surface and bulk defects, probe elastic pair interactions, and discuss emergent prospects for fundamental science and applications of the reconfigurable nematic emulsions.

Control of Director Fields in Phospholipid-Coated Liquid Crystal Droplets

In liquid crystal (LC) droplets, small changes in surface anchoring energy can produce large changes in the director field which result in readily detectable optical effects. This makes them attractive for use as biosensors. Coating LC droplets with a phospholipid monolayer provides a bridge between the hydrophobic world of LCs and the water-based world of biology and makes it possible to incorporate naturally occurring biosensor systems. However, phospholipids promote strong perpendicular (homeotropic) anchoring that can inhibit switching of the director field. We show that the tendency for phospholipid layers to promote perpendicular anchoring can be suppressed by using synthetic phospholipids in which the acyl chains are terminated with bulky tert-butyl or ferrocenyl groups; the larger these end-group(s), the less likely the system is to be perpendicular/radial. Additionally, the droplet director field is found to be dependent on the nature of the LC, particularly its intrinsic surface properties, but not (apparently) on the sign of the dielectric anisotropy, the proximity to the melting/isotropic phase transition, the surface tension (in air), or the values of the Frank elastic constants.

Liquid Crystal Templates Combined with Photolithography Enable Synthesis of Chiral Twisted Polymeric Microparticles

Liquid crystals (LC), when combined with photolithography, enable synthesis of microparticles with 2D and 3D shapes and internal complexities. Films of nematic LCs are prepared using mixtures of reactive (RM257) and non-reactive mesogens with controlled alignment of LCs at the confining surfaces, photo-polymerized the RM257 using a photomask, and then extracted the unreacted mesogens to yield the polymeric microparticles. The extraction results in a controlled anisotropic shrinkage amount dependent on the RM257 content and the direction dependent on LC alignment. Control over the aspect ratio, size, and thickness of the microparticles are obtained with a coefficient of variance less than 2%. In addition, non-parallel LC anchoring at the two surfaces results in a controllable right- or left-handed twisting of microparticles. These methods may find substantial use in applications including drug delivery, emulsions, separations, and sensors, besides their potential in revealing new fundamental concepts in self-assembly and colloidal interactions.

Effects of droplet size and surfactants on anchoring in liquid crystal nanodroplets

Liquid crystal (LC) droplets attract scientific attention for many advanced applications, including, but not limited to optical and sensing devices. To aid experimental advancements, theoretical calculations have been conducted to quantify molecular driving forces responsible for the collective behaviour of LC molecules within micrometer-size spherical droplets. To quantify the LC molecular anchoring within spherical physical constraints, molecular simulations at atomistic resolution would be useful. In an attempt to bridge the gap between computational capabilities and experimental interest, coarse-grained simulations are used here to study nematic LC nanodroplets dispersed in water. A LC phase diagram is generated as a function of droplet size and temperature. The effect of adding surfactants on LC anchoring was quantified, considering surfactants of different molecular features. When few surfactants are present, they self-assemble at the droplet boojums regardless of their molecular features. All surfactants tested shifted LC orientation from bipolar to uniaxial. When the surfactants have a hydrophobic tail of sufficient length, they cause deviations from the spherical symmetry of LC droplets. Increasing the concentration of these surfactants enhances such phenomenon. Simulations were also conducted to assess the ability of the surfactants to prevent the agglomeration between two LC droplets. The results showed that coalescence was inevitable at all conditions and suggested that large enough surfactant concentrations can delay the phenomenon. The results presented could be helpful for designing novel surface-active compounds to develop optical and/or sensing devices at conditions in which mutual solubility between water and LCs is low.

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.

Research on a Dual-Mode Infrared Liquid-Crystal Device for Simultaneous Electrically Adjusted Filtering and Zooming

A new dual-mode liquid-crystal (LC) micro-device constructed by incorporating a Fabry⁻Perot (FP) cavity and an arrayed LC micro-lens for performing simultaneous electrically adjusted filtering and zooming in infrared wavelength range is presented in this paper. The main micro-structure is a micro-cavity consisting of two parallel zinc selenide (ZnSe) substrates that are pre-coated with ~20-nm aluminum (Al) layers which served as their high-reflection films and electrodes. In particular, the top electrode of the device is patterned by 44 × 38 circular micro-holes of 120 μm diameter, which also means a 44 × 38 micro-lens array. The micro-cavity with a typical depth of ~12 μm is fully filled by LC materials. The experimental results show that the spectral component with needed frequency or wavelength can be selected effectively from incident micro-beams, and both the transmission spectrum and the point spread function can be adjusted simultaneously by simply varying the root-mean-square value of the signal voltage applied, so as to demonstrate a closely correlated feature of filtering and zooming. In addition, the maximum transmittance is already up to ~20% according the peak-to-valley value of the spectral transmittance curves, which exhibits nearly twice the increment compared with that of the ordinary LC-FP filtering without micro-lenses.

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.

Light-Responsive Microstructures in Droplets of the Twist-Bend Nematic Phase

We report on the structure and optical manipulation of the director configurations in emulsions of liquid-crystalline droplets of a compound exhibiting the nematic (N) and the twist-bend nematic (N_TB) phases. We demonstrate a decrease in the ratio of the bent elastic constant K₃₃ to the splay constant K₁₁ by nearly 2 orders of magnitude with decreasing temperature in the N phase. The director structures in liquid-crystal droplets doped with a photoswitchable surfactant without and under ultraviolet (UV) light are discussed in light of the strong elastic anisotropy of the investigated compound. We also compare our findings with the results obtained in doped nematic droplets of a conventional 4-cyano-4'-pentylbiphenyl (5CB) liquid crystal. The dynamics of droplets in the N_TB phase by UV light irradiation are also studied using polarizing and confocal microscopies.

Nematic colloidal knots in topological environments

The role of environment in shaping material properties is of great significance, but less is known about how non-trivial topology of the environment couples to material states, which can be of non-trivial topology themselves. In this paper, we demonstrate the role of the topology of the environment on the formation of complex nematic fields and defect structures, specifically in the system of nematic colloidal knots. The topological environments around knotted colloidal particles are suggested to exist as spherical surface-patterned nematic cavities imposing radial, uniform or hyperbolic nematic profiles. We show that topologically different nematic environments significantly affect and create differences in the colloidal field structure created within the environment, such as the location, profile and number of topological defects. Specifically, we demonstrate that topological environments in combination with knotted colloidal particles of non-trivial topology lead to the formation of diverse nematic knotted and linked fields. These fields are different adaptations of the knotted shape of the colloidal particles, creating knots and links of topological defects as well as escaped-core defect-like solitonic structures. These are observed in chiral nematic media but here are stabilised in achiral nematic media as a result of the distinct shape of the knotted colloidal particle, with a double helix segment and nematic environmental patterns. More generally, this paper is a contribution towards understanding the role of environment, especially its topology, on the response and defect formation in elastic fields, such as in nematic liquid crystal colloids.

Multi-Scale Responses of Liquid Crystals Triggered by Interfacial Assemblies of Cleavable Homopolymers

Liquid crystals (LCs) offer the basis of stimuli-responsive materials that can amplify targeted molecular events into macroscopic outputs. However, general and versatile design principles are needed to realize the full potential of these materials. To this end, we report the synthesis of two homopolymers with mesogenic side chains that can be cleaved upon exposure to either H₂ O₂ (polymer P1) or UV light (polymer P2). Optical measurements reveal that the polymers dissolve in bulk LC and spontaneously assemble at nematic LC-aqueous interfaces to impose a perpendicular orientation on the LCs. Subsequent addition of H₂ O₂ to the aqueous phase or exposure of the LC to UV was shown to trigger a surface-driven ordering transition to a planar orientation and an accompanying macroscopic optical output. Differences in the dynamics of the response to each stimulus are consistent with sequential processing of P1 at the LC-aqueous interface (H₂ O₂ ) and simultaneous transformation of P2 within the LC (UV). The versatility of the approach is demonstrated by creating stimuli-responsive LCs as films or microdroplets, and by dissolving mixtures of P1 and P2 into LCs to create LC materials that respond to two stimuli. Overall, our results validate a simple and generalizable approach to the rational design of polymers that can be used to program stimuli-responsiveness into LC materials.

Manipulating molecular order in nematic liquid crystal capillary bridgesviasurfactant adsorption: guiding principles from dissipative particle dynamics simulations

Effect of surfactant tail length on the orientation of liquid crystals is investigated with dissipative particle dynamics (DPD) simulations.

Thermotropic Liquid Crystal-Assisted Chemical and Biological Sensors

In this review article, we analyze recent progress in the application of liquid crystal-assisted advanced functional materials for sensing biological and chemical analytes. Multiple research groups demonstrate substantial interest in liquid crystal (LC) sensing platforms, generating an increasing number of scientific articles. We review trends in implementing LC sensing techniques and identify common problems related to the stability and reliability of the sensing materials as well as to experimental set-ups. Finally, we suggest possible means of bridging scientific findings to viable and attractive LC sensor platforms.