Dislocations and Disclinations in Smectic Systems

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.

Correlation between dielectric properties and phase transitions of 8CB/Sn2P2S6 liquid crystal nanocolloids

The ordering of synthetic liquid crystals near surfaces is known to be dependent on the nanoscopic structure and chemical functionality of surfaces. In this letter, we report that the orientational ordering of synthetic liquid crystals on surfaces decorated with viruses is also dependent on the structures of the viruses. Each of the four virions investigated had diameters of approximately 100 nm, but three of the viruses (influenza virus, La Crosse virus, and vesicular stomatitis virus) were enveloped in a lipid bilayer, whereas one virus (adenovirus) was not. We observed that lipid bilayer-enveloped viruses induce homeotropic (perpendicular) ordering of a nematic liquid crystal upon contact with the liquid crystal. In contrast, nonenveloped virus (adenovirus)-treated surfaces caused a near-planar orientation of the liquid crystal. We conclude that the homeotropic ordering of liquid crystals is a signature of the presence of enveloped viruses present on surfaces. These results suggest new approaches to the design of nanostructured materials that incorporate viruses as well as suggest methods that can be used to amplify the presence of nanoscopic virions into micrometer-sized domains of liquid crystal that can be optically probed.

A series of spacer-free ionic azobenzene-containing stoichiometric complexes was prepared from monosulfonated azo dyes and cationic polyelectrolytes [methylated poly(4-vinylpyridine) (PVP) and poly(dimethylaminoethyl methacrylate) (PDM)]. Their thermal and structural characteristics and optical responses, particularly photoinduced birefringence (PIB) and surface relief grating (SRG) inscription, were investigated as a function of selected molecular parameters. All of the complexes have high apparent Tg values, 180−210 °C, and show liquid crystal (LC) order of the single-layer SmA type from ambient to very high temperature, usually to degradation. The range of LC order is greater in the complexes of chromophores with longer alkyl tails and shorter in the complex of the chromophore with an OH end group and the complex of methyl orange (MO) with PDM. PIB, SRG quality, and diffraction efficiency were all shown to depend in a similar way on molecular structural features: the more rigid the molecular structure,...

This brief perspective focuses on recent advances in the design of functional soft materials that are based on confinement of low molecular weight liquid crystals (LCs) within micrometer-sized droplets. While the ordering of LCs within micrometer-sized domains has been explored extensively in polymer-dispersed LC materials, recent studies performed with LC domains with precisely defined size and interfacial chemistry have unmasked observations of confinement-induced ordering of LCs that do not follow previously reported theoretical predictions. These new findings, which are enabled in part by advances in the preparation of LCs encapsulated in polymeric shells, are opening up new opportunities for the design of soft responsive materials based on surface-induced ordering transitions. These materials are also providing new insights into the self-assembly of biomolecular and colloidal species at defects formed by LCs confined to micrometer-sized domains. The studies presented in this perspective serve additionally to highlight gaps in knowledge regarding the ordering of LCs in confined systems.

Liquid crystal (LC) ordering and phase transition behavior under confined conditions have attracted extensive attention and enabled many applications. However, the ordering and phase transition behavior of LCs in submicrometer capsules have seldom been studied, primarily due to the lack of proper capsulizing and visualization approaches to such small LC microcapsules. Herein, we achieve submicrometer LC capsules with the sizes down to 100 nm by using emulsion-based interfacial sol-gel reaction. The behavior of LCs under the submicrometer confinement conditions is investigated while the sizes and chemical composition of the microcapsule shell surface are tuned in a controllable way. The phase transition temperatures of LCs in the submicrometer capsules shift from those of bulk LCs due to the surface-induced ordering of LCs under the strong confinement conditions, which causes formation of topological defects and alters the order parameter. Using nonlinear optical imaging technology, we explore the structures of director field of LCs that arise as a result of the competition between the surface boundary conditions and LC elasticity. The results show that the nanoscale encapsulation can significantly influence the structural configurations of the director and phase transitions of LCs under various confinement conditions.

In this chapter, applications of new NMR techniques to the study of liquid crystals are described in a cursory way. Two-dimensional (2D) 13C NMR of liquid crystals was mentioned at the end of Chapter 3. The carbon-proton dipolar couplings were determined by combining near magic-angle spinning of the sample and the separated local field spectroscopy [9.1]. A 2D 19F spin echo experiment on a sample rotating near the magic-angle has also been reported [9.2]. The sample contains a chiral solute dissolved in a liquid crystal. Deuteron NMR is by far the most extensively used probe of internal motion and molecular ordering in liquid crystals. One common problem encountered in 2H NMR of liquid crystals is the assignment of deuterium resonances. This can be overcome by synthesis of singly labeled compounds, but this is often time-consuming and expensive. The assignment problem is even more difficult when two deuterated chains are present in a mesogen. The deuterium 2D autocorrelation experiment [9.3] can be used to assign the deuteron signals of perdeuterated liquid crystals. Deuteron 2D exchange spectroscopy [9.4, 9.5] has been used to study slow motions in glass-forming discotic liquid crystals [9.6, 9.7] and in liquid-crystalline side group polymers [9.8]. Some of these 2D NMR experiments on liquid crystals will be surveyed in Section 9.1.KeywordsLiquid CrystalQuadrupolar SplittingDipolar CouplingOrientational DistributionBiphenyl MoietyThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Active colloids and liquid crystals are capable of locally converting the macroscopically supplied energy into directional motion and promise a host of new applications, ranging from drug delivery to cargo transport at the mesoscale. Here we uncover how topological solitons in liquid crystals can locally transform electric energy to translational motion and allow for the transport of cargo along directions dependent on frequency of the applied electric field. By combining polarized optical video microscopy and numerical modeling that reproduces both the equilibrium structures of solitons and their temporal evolution in applied fields, we uncover the physical underpinnings behind this reconfigurable motion and study how it depends on the structure and topology of solitons. We show that, unexpectedly, the directional motion of solitons with and without the cargo arises mainly from the asymmetry in rotational dynamics of molecular ordering in liquid crystal rather than from the asymmetry of fluid flows, as in conventional active soft matter systems.

Alignment of liquid crystals (LCs) is critical for actuation of LC polymer‐based devices. A common strategy to induce surface deformation in LC polymer coatings involves engineering the polymers into multidomain configurations. The application of an electric field serves as an efficient means to achieve this goal. While LCs are partially ordered fluids characterized by intrinsic dynamic disorder and order fluctuations, these fluctuations can be suppressed by electric fields, a phenomenon known as electrical quenching. However, the impact of quenching LC order fluctuations on the molecular actuation behavior of LC polymers remains insufficiently understood. Herein, the translation of the electric quenching effect on LC order from the molecular scale to the macroscopic level, with a focus on regulating the topographical deformation of polymerized LC polymer coatings, is demonstrated. The findings reveal that the suppression of monomeric LC order fluctuations through electric quenching significantly reduces the topographical deformation of the polymerized network. This advancement offers new opportunities for fabricating tunable surfaces with complex patterns, broadening the potential applications of LC‐based dynamic surfaces.

All-aromatic calamitic liquid crystals are an unconventional family of rigid linear mesogens that represents the closest embodiment of the idealised rod-like molecule central to liquid crystal theories. 2,6-biphenyl naphthalene (PPNPP), a prototypical all-aromatic nematogen, has recently been the focus of scientific interest for both fundamental and technological purposes. While it provides a valuable benchmark for classical theories of nematic order, its experimental study presents challenges given the high temperature of the nematic phase. Herein, molecular dynamics (MD) simulations are contrasted with X-ray diffraction (XRD) data to resolve the thermotropic phase behaviour of PPNPP and the main structural features of its liquid crystal order. The observed trend of the molecular conformation with temperature points to an unexpected and significant distortion of the molecular framework in the nematic phase, which demonstrates the substantial non-linearity of the PPNPP molecule. The simulated mesomorphic behaviour and related thermodynamic parameters are in close agreement with the experimental data. The different phases are described in terms of their molecular organisation, pair distribution function, as well as the orientational and positional order parameters. We compare the classical and extended Maier-Saupe nematic theories with the computationally (MD) and experimentally (XRD) determined orientational order parameters. The observed deviation from the theoretical models indicates substantial inadequacies of the classical theories to describe the nematic-isotropic phase transition for this class of compounds. We suggest that modifications of theory should include the effects of short-range positional order, i.e., cybotaxis.

Graphene oxide (GO) forms well ordered liquid crystal (LC) phases in polar solvents. Here, we map the lyotropic phase diagram of GO as a function of the lateral dimensions of the GO flakes, their concentration, geometrical confinement configuration and solvent polarity. GO flakes were prepared in water and transferred into other polar solvents. Polarising optical microscopy (POM) was used to determine the phase evolution through the isotropic-biphasic-nematic transitions of the GO LC. We report that the confinement volume and geometry relative to the particle size is critical for the observation of the lyotropic phase, specifically, this determines the low-end concentration limit for the detection of the GO LC. Additionally, a solvent with higher polarisability stabilises the LC phases at lower concentrations and smaller flake sizes. GO LCs have been proposed for a range of applications from display technologies to conductive fibres, and the behaviour of LC phase formation under confinement imposes a limit on miniaturisation of the dimensions of such GO LC systems which could significantly impact on their potential applications.

Liquid crystals are considered as gases of long barlike molecules. Variables describing the position and orientation of individual molecules are introduced. These are used to arrive at a microscopic definition of the order in liquid crystals in terms of a symmetric traceless tensor. The Frank and Leslie-Ericksen directors are shown to be equivalent to the unit eigenvector associated with the largest eigenvalue of this symmetric traceless tensor. Attention is then restricted to nematics. It is shown that the Frank free energy gives a valid description of equilibrium properties of nematics to order $\frac{{\ensuremath{\chi}}^{a}{H}^{2}}{\ensuremath{\rho}\ensuremath{\kappa}T{m}^{\ensuremath{-}1}}$, where ${\ensuremath{\chi}}^{a}$ is the anisotropic magnetic susceptibility, $H$ the external magnetic field, $\ensuremath{\rho}$ the mass density, $T$ the temperature, $k$ the Boltzmann constant, and $m$ the molecular mass. Microscopic justification is given for the linearized Leslie-Ericksen hydrodynamic theory for nematics. A microscopic response-function description of nematics is then presented, and contact is made between the general frequency- and wave-number-dependent response functions and those calculated from the phenomenological hydrodynamic theory. Applications to light scattering and nuclear magnetic resonance are considered. $\frac{1}{{T}_{1}}$ calculated here differs from that calculated from the Pincus theory by a factor of ${S}^{2}$, where $S$ is the parameter measuring the degree of nematic order.

The growing demand for energy-efficient displays, adaptive photonic elements, and low-power optical technologies has build up interest in carbon nanomaterials dispersed liquid crystal (LC) systems. These nanomaterials offer unique electrical, optical and interfacial properties that can significantly make advancement for faster, more responsive and more sustainable electro-optical (EO) devices. This review provides a comprehensive overview of how various carbon nanomaterials such as carbon nanotubes (CNTs), graphene oxide (GO), carbon dots (CDs) and fullerenes interact with LCs to influence alignment behavior, dielectric response, interfacial anchoring and device performance. The manuscript presents trends, compares advantages and limitations of each nanomaterial category as well as explains the underlying mechanisms governing their influence on LC order and switching characteristics. Key challenges related to dispersion stability, interface control and long-term reliability are highlighted, along with future opportunities for developing next-generation LC-based photonic and optoelectronic systems. This perspective is intended to guide material selection and inspire further innovation in LC–nanomaterial hybrid technologies.

Ferroelectric ordering, the electroclinic effect, and chiral smectic C(SmC*)-smectic A phase transitions in thin planar ferroelectric liquid crystal (FLC) cells are studied by means of linear electro-optic and second harmonic generation (SHG) techniques. The ferroelectric switching is detected in biased FLC cells by measuring azimuthal dependences of linear and nonlinear responses. The applied dc electric field rotates the FLC symmetry axis with initial and final orientations in the cell plane. Comparative studies of the SHG switching behavior in reflection and transmission geometries allows one to distinguish the contributions from the bulk and the subsurface layers of the cell. The analysis of SHG temperature dependences shows the existence of a strong surface coupling. The temperature-dependent nonlinear polarization shows a critical behavior with the exponent approximately 0.3 in SmC* phase.

Liquid crystal surfaces can undergo topographical morphing in response to external cues. These shape-shifting coatings promise a revolution in various applications, from haptic feedback in soft robotics or displays to self-cleaning solar panels. The changes in surface topography can be controlled by tailoring the molecular architecture and mechanics of the liquid crystal network. However, the nanoscopic mechanisms that drive morphological transitions remain unclear. Here, we introduce a frequency-resolved nanostrain imaging method to elucidate the emergent dynamics underlying field-induced shape-shifting. We show how surface morphing occurs in three distinct stages: (i) the molecular dipoles oscillate with the alternating field (10–100 ms), (ii) this leads to collective plasticization of the glassy network (~1 s), (iii) culminating in actuation of the topography (10–100 s). The first stage appears universal and governed by dielectric coupling. By contrast, yielding and deformation rely on a delicate balance between liquid crystal order, field properties and network viscoelasticity.