Solid Liquid Crystal Research Articles

Pattern formation, structure and functionalities of wrinkled liquid crystal surfaces: A soft matter biomimicry platform

This review presents an integrated theoretical and computational characterization and analysis of surface pattern formation in chiral and achiral liquid crystal self-assembly and the mechanical/optical/tribological/tissue engineering surface functionalities that emerge from various wrinkling processes. Strategies to target surface patterns include linear, non-linear, multidirectional and multiscale wrinkling phenomena. The focus of the review is to show the unique surface structure-functionalities that emerge from anisotropic liquid crystal soft matter, eliminating or reducing the need of aggressive solvents, extreme pressure/temperature conditions, erosion and other surface morphing approaches. The surface pattern formation theoretical-modelling- computational results are then connected and validated with actual biological surfaces that are considered solid liquid crystal analogues, such as exocuticles of insects, fish scales, and flowers. A unique feature of thein silicosurface pattern formation platform used throughout this review is the generalized liquid crystal shape equation that includes surface anchoring elasticity, membrane elasticity, and stress loads from liquid crystals orientation gradients. Clear characterization of surface shapes, curvatures, roughness, that are behind surface functionalities are introduced and applied to strengthen validation of predictions with actual nature’s surfaces. Wrinkling scaling laws, and the dependence of material properties on morphing mechanisms are elucidated. The predictions capture very well the two-scale wrinkling patterns in tulips, wrinkling gradients that display water sensor capabilities, egg carton shapes in rose petals and their potential for cell alignment, and the ability to create surface roughness with targeted kurtosis and skewness to control and optimize friction and tribological functionalities. The results are summarized in terms of surface geometry (open or closed) mechanisms and phenomena (anchoring, membrane elasticity), material properties (anchoring coefficients, membrane bending modulus, Frank elasticity), wrinkling scales and scaling laws (amplitude, wave-lengths, skewness, kurtosis) and functionalities (optical iridescence, friction, wettability, structural color, curvature-driven cell alignment and differentiation). Taken together, the range of surface geometries and surface functionalities captured by the liquid crystal biomimeticin silicoplatform provides a foundation for future experimental green manufacturing pathways based on anisotropic soft matter.

Printed actuators in a flap

Solid liquid crystals couple orientational order and mechanical strain, enabling fundamentally new mechanisms of actuation. Depositing the materials using inkjet printing allows precise control of their shapes and composition, producing devices with new microfluidic applications.

Nonlinear elasticity, fluctuations and heterogeneity of nematic elastomers

Liquid crystal elastomers realize a fascinating new form of soft matter that is a composite of a conventional crosslinked polymer gel (rubber) and a liquid crystal. These solid liquid crystal amalgams, quite similarly to their (conventional, fluid) liquid crystal counterparts, can spontaneously partially break translational and/or orientational symmetries, accompanied by novel soft Goldstone modes. As a consequence, these materials can exhibit unconventional elasticity characterized by symmetry-enforced vanishing of some elastic moduli. Thus, a proper description of such solids requires an essential modification of the classical elasticity theory. In this work, we develop a rotationally invariant, nonlinear theory of elasticity for the nematic phase of ideal liquid crystal elastomers. We show that it is characterized by soft modes, corresponding to a combination of long wavelength shear deformations of the solid network and rotations of the nematic director field. We study thermal fluctuations of these soft modes in the presence of network heterogeneities and show that they lead to a large variety of anomalous elastic properties, such as singular length-scale dependent shear elastic moduli, a divergent elastic constant for splay distortion of the nematic director, long-scale incompressibility, universal Poisson ratios and a nonlinear stress–strain relation for arbitrary small strains. These long-scale elastic properties are universal, controlled by a nontrivial zero-temperature fixed point and constitute a qualitative breakdown of the classical elasticity theory in nematic elastomers. Thus, nematic elastomers realize a stable “critical phase”, characterized by universal power-law correlations, akin to a critical point of a continuous phase transition, but extending over an entire phase.

Study of liquid crystalline N -[4-(4- n -alkoxybenzoyloxy)-2-hydroxybenzylidene]methylanilines

New homologous series of N-[4-(4-n-alkoxybenzoyloxy)-2-hydroxybenzylidene]methylanilines [nAHmM(n=1-8/10; m=2: ortho, m=3: meta, m=4: para)] were synthesized. They exhibited a nematic phase except for 1AH3M. The temperature dependence of their Raman spectra was observed in the spectral range of 900–1700 cm-1. In one group of nAHmM compounds, the Raman band at about 1360 cm-1 abruptly decreased in intensity and wavenumber when the crystalline solid-liquid crystal phase transition was approached. In another group, the corresponding band increased through the phase transition. The bands have been assigned to the coupling mode between the in-plane CCH deformational vibration and the ring-N stretching vibration. Such a behaviour can be explained by the molecular conformation with different twist angles of the aniline ring in relation to the Schiff's base plane of the molecule. Some nAHmMs exhibited photochromism.

Raman Spectroscopic Study of Liquid Crystalline N-[4-(4-n-Alkoxybenzoyloxy)-2-Hydroxybenzylidene]-Chloroanilines

N-[4-(4-n-Alkoxybenzoyloxy)-2-hydroxy-benzylidene]-chloroanilines[nAHmC(n; n=1-8, m; m=2: ortho, m=3: meta, m=4: para)] were synthesized. They exhibited a nematic phase. In addition, 8AH4C exhibited the smectic C and A phases. The temperature dependence of their Raman spectra has been observed in the spectral range of 20–1800 cm−1. In a group of nAHmC, the Raman band at about 980 cm−1 decreased in intensity and wavenumber through the solid-liquid crystal phase transition. In the other group, the corresponding band increased through the phase transition. The bands have been assigned to the CH out-of-plane vibration(980 cm−1) of the benzene ring. Such a behavior can be explained by the molecular conformations with different twist angles of the aniline ring to the salicylaldimino part of the molecule in conjunction with information gained in the photochromic studies of N-2-hydroxy-benzylidenanilines. The temperature dependence of the 1190 cm−1 band ascribed to the ring-N bond stretching vibration gave support to the result of the CH deformation vibration.

Discovery of a new stable polymorph of 4-(3-ethynylphenylamino)-6,7-bis(2-methoxyethoxy)quinazolinium methanesulfonate using near-infrared spectroscopy to monitor form change kinetics †

Polymorphism is an important property of crystalline organic molecules, particularly when used to develop medicines. Discovery of all the polymorphs in a series is often difficult. This paper highlights the use of near-infrared spectroscopy to monitor the kinetics of form changes of polymorphs and solvates (hydrates). In the case of mesylate salt 5, this led to the discovery of a new preferred form. Identification and confirmation of unique polymorph crystal states are determined using X-ray powder diffraction patterns. This complements and confirms the kinetic change observed in the near-infrared. The technique is generally applicable to the study of two-phase solid–liquid crystal slurries under isothermal conditions.

Intrinsically Biaxial Systems: A Variational Theory for Elastomers

Elastomeric networks of polymer liquid crystals are a classical example of “solid liquid crystals”. They are organized in aggregates of ellipsoidal shape, which can be represented by a symmetric tensor field L , which is closely related to the nematic order parameter Q . We describe a continuum theory, which takes into account both the mechanical forces and the nematic properties of the system.

Orientation of nematic elastomers and gels by electric fields

Solid liquid crystals, formed by crosslinking polymeric nematics into elastomers, display novel and complex elasticity. The internal nematic direction experiences a barrier to its rotation which couples it to the deformations of standard elasticity. Electric fields acting on anisotropic chains induce orientational torques, which compete with rubber elastic effects. Outcome structures crucially depend on the mechanical constraints applied to the sample. In setups with no or few constraints, an electric field rotates the nematic director without resistance, inducing also a spontaneous shape change of a rubber or gel matrix. When certain strains are prevented in the sample by external constraints, the magnitude of the elastic barrier is much higher than the electric contribution and a very high electric field is required to create an observable director rotation. In weakly anisotropic elastomers, for instance conventional rubbers which have been strained during crosslinking, the characteristic field will be considerably lower. Experimental observations on nematic gels support our predictions

“Soft elasticity” — deformation without resistance in liquid crystal elastomers

Polymeric nematic liquid crystals crosslinked into elastomers (solid liquid crystals) are shown to display novel and complex elasticity. The internal (nematic) direction can experience a barrier to its rotation which couples to standard elasticity. We predict a new phenomena unique to anisotropic rubber - a «soft elastic response»; uniaxial strain is developed without resistance below a critical deformation λ * due to the relaxation of related shear strains and reorientation of the nematic director. We discuss possible experiments to verify this prediction and interpret the existing experimental observation in terms of the concept of «soft elasticity»

Deformation–induced orientational transitions in liquid crystals elastomer

Solid liquid crystals, formed by crosslinking polymeric nematics into elastomers are shown to display novel and complex elasticity. The internal (nematic) direction can experience a barrier to its rotation which couples to standard elasticity. We investigate this elasticity by considering imposed strains and demonstrate several new orientational phase transitions, caused by the interaction between applied stress fields and bulk barriers to rotation.

Reflection and refraction of an obliquely incident shear wave at a solid-smectic liquid crystal interface

This paper provides an analysis of a possible experiment to measure viscous coefficients in a recently proposed theory of smectic liquid crystals. The experiment initially used to measure viscosities of a nematic liquid crystal employs a transverse shear wave in an isotropic elastic solid incident upon a plane solidliquid crystal interface where it is both reflected and refracted. The calculation presented shows that as for nematic theory, measurement of properties of the reflected wave in the solid can provide useful information concerning certain viscous coefficients appearing in the smectic theory.

Evidence of pronounced positional order at the graphite–liquid crystal interface of a bulk nematic material

We present scanning tunneling microscopy images of a bulk nematic liquid crystal, viz., 4′-n-hexyl-4-cyanobiphenyl, on the surface of graphite. These images clearly show the existence of pronounced positional order at the solid–liquid crystal interface for a material with no positional order in the bulk. A feature of the molecular arrangement is the pronounced interdigitation of the aromatic cores. The unit cell consists of two opposing molecules, the aromatic core of one of them lying flat on the graphite surface, while the core of the adjacent molecule lies on its side. The packing arrangement also features large open spaces of uncovered substrate between the alkyl tails. The molecular arrangement is compared with that of the smectic alkylcyanobiphenyls.

The interfacial properties of a model of a nematic liquid crystal

We have used the theory developed in the previous paper to study the induced orientational order at simple models of a solid-liquid crystal (LC) interface. The LC is modelled by a Maier-Saupe type of anisotropic pairwise potential while the solid is modelled by an impenetrable wall which exerts an anisotropic external potential on the fluid molecules. Our theory treats both the solid-fluid and fluid-fluid potentials on a microscopic level which allows us to study the role played by these two types of interactions in the alignment of the fluid molecules at the solid surface. In particular, we study in detail the nature of the wetting of the solid-isotropic liquid interface by the nematic phase. We illustrate this by presenting the results of numerical calculations for the density and orientational order parameter profiles at the wetting transition for two different models of the solid substrate. We find that due to the weakness of the first order N/I bulk transition, the surface phase transition for z -3 substrate potentials is also weakly first order, in contrast with the results for isotropic systems. For exponential solid substrates the transition is found to depend on the range of the solid-fluid interaction; for short-ranged potentials the transition is continuous whereas for longer-ranged potentials the transition becomes more strongly first-order as the range of the interaction increases. This type of behaviour is in agreement with the results for isotropic solid-fluid interfaces. We calculate the pre-wetting lines for the two solid-fluid models. We find that the structure of the co-existing films as well as the surface critical point depend rather strongly on the model for the solid-fluid interaction. Finally, we compare the results of our theory with the results obtained from a phenomenological one-order parameter Landau-de Gennes theory and with some recent experimental results. We assess their validity and possible range of application.

Synthetic bilayer membranes prepared from dialkyl amphiphiles with nonionic and zwitterionic head groups

Dialkyl amphiphiles with nonionic and zwitterionic head groups were prepared. The nonionic group is the polyoxyethylene chain and the zwitterionic group is composed of the combinations of ammonium sulfonate, ammonium phosphate, and ammonium amidate. These dialkyl amphiphiles, when dissolved in water, produced very large aggregates as determined by small-angle light scattering: 1–50 million daltons. The critical micelle concentration was 10−5– 10−6 M. The aggregate morphology was examined by electron microscopy. In the case of the nonionic amphiphile, the chain lengths of both the polyoxyethylene and dialkyl groups were shown to affect the morphology. When the alkyl chain length was C12, C14, C16, and C18 (but not C8), the bilayer formation was observed, and vesicles, lamellae, and disks were found depending on the amphiphile structure. The zwitterionic amphiphiles similarly produced vesicles and lamellae. Most of these aggregates showed the phase transition phenomenon (solid-liquid crystal). The transition temperature rose with increasing alkyl chain lengths in general agreement with that of lecithin.