Continuous nematic anchoring transition due to surface-induced smectic order

Macroscopic Nematic Orientation Dictated by an Orientationally Frustrated Random-Field Surface: Equilibrium Structure and Kinetics

Liquid crystals subjected to frustrated surfaces with mixed anchoring conditions demonstrate a rich variety of orientational patterns. Particularly, it would trigger either continuous or discontinuous variation of the bulk orientation, i.e., a phenomenon known as the anchoring or orientational transition. Despite its prime importance in developing novel optoelectronic devices, how the surface anchoring patterns dedicate the energy landscape of a system, thus the equilibrium state, still needs to be understood. Here, we designed a simulation to model boundary substrates with two randomly mixed anchoring domains in space, which exhibit planar and homeotropic preferences. We numerically obtain general bulk orientational state diagrams under various surface and electric field conditions, which reveal the roles of each domain's size and surface fraction and anchoring strength on the bulk orientational state. Furthermore, we examine how the external electric field modifies the orientational state diagram and uncovers a field-assisted anchoring transition. We discuss the observed bistability and compare it to experimental evidence.

Mesoscopic simulations of temperature-dependent anchoring and wetting behavior at aqueous-liquid crystal interfaces in the presence of a rod-coil amphiphilic monolayer

Dissipative particle dynamics simulations have been applied to study the temperature dependent anchoring and wetting behavior of thermotropic liquid crystals (LCs) in the presence of a rod-coil amphiphilic monolayer at the aqueous-LC interface. Upon cooling in the nematic phase, a thermally-induced anchoring transition from homeotropic through tilted to planar has been observed. The growth and propagation of smectic order from the interfaces to the bulk nematic LCs are demonstrated to be mainly responsible for this novel transition sequence. In particular, when a complete smectic layer in the amphiphile monolayer is induced around the bulk transition of nematic-smectic-A, the propagation of homeotropic alignment fails instantly and a unique planar anchoring configuration is formed instead. While heating towards the isotropic phase, simulation results show that the nematic-isotropic transition of confined LCs is slightly shifted to a higher temperature, and a nematic wetting layer with homeotropic alignment appears in the rod block monolayer when the bulk LCs is isotropic. Our systematic simulations throughout the whole phase regimes of LCs provide important molecular-level insight into how the coupling between the ordering of LCs and adsorbents and their temperature dependencies affect the anchoring behavior in this complex system, which should be instrumental in the rational design and application of advanced LC-based biosensors with optimal operating temperature range.

Transition from Spin Dewetting to continuous film in spin coating of Liquid Crystal 5CB

Spin dewetting refers to spontaneous rupture of the dispensed solution layer during spin coating, resulting in isolated but periodic, regular sized domains of the solute and is pre-dominant when the solute concentration (C_n) is very low. In this article we report how the morphology of liquid crystal (LC) 5CB thin films coated on flat and patterned PMMA substrate transform from spin dewetted droplets to continuous films with increase in C_n. We further show that within the spin dewetted regime, with gradual increase in the solute concentration, periodicity of the isotropic droplets (λ_D) as well as their mean diameter (d_D), gradually decreases, till the film becomes continuous at a critical concentration (C_n*). Interestingly, the trend that λ_D reduces with increase in C_n is exact opposite to what is observed in thermal/solvent vapor induced dewetting of a thin film. The spin dewetted droplets exhibit transient Radial texture, in contrast to Schlieren texture observed in elongated threads and continuous films of 5CB, which remains in the Nematic phase at room temperature. Finally we show that by casting the film on a grating patterned substrate it becomes possible to align the spin dewetted droplets along the contours substrate patterns.

Local structural ordering in surface-confined liquid crystals

The effect of the interplay between attractive nonlocal surface interactions and attractive pair long-range intermolecular couplings on molecular structures of liquid crystals confined in thin cells with flat solid surfaces has been studied. Extending the McMillan mean field theory to include finite systems, it has been shown that confining surfaces can induce complex orientational and translational ordering of molecules. Typically, local smectic A, nematic, and isotropic phases have been shown to coexist in certain temperature ranges, provided that confining cells are sufficiently thick, albeit finite. Due to the nonlocality of surface interactions, the spatial arrangement of these local phases can display, in general, an unexpected complexity along the surface normal direction. In particular, molecules located in the vicinity of surfaces can still be organized in smectic layers, even though nematic and/or isotropic order can simultaneously appear in the interior of cells. The resulting surface freezing of smectic layers has been confirmed to occur even for rather weak surface interactions. The surface interactions cannot, however, prevent smectic layers from melting relatively close to system boundaries, even when molecules are still arranged in layers within the central region of the system. The internal interfaces, separating individual liquid-crystal phases, are demonstrated here to form fronts of local finite-size transitions that move across cells under temperature changes. Although the complex molecular ordering in surface confined liquid-crystal systems can essentially be controlled by temperature variations, specific thermal properties of these systems, especially the nature of the local transitions, are argued to be strongly conditioned to the degree of molecular packing.

Cholesterol-Based Grafted Polymer Brushes as Alignment Coating with Temperature-Tuned Anchoring for Nematic Liquid Crystals

Novel alignment coating with temperature-tuned anchoring for nematic liquid crystals (NLCs) was successfully fabricated in three step process, involving polymerization of poly(cholesteryl methacrylate) (PChMa) from oligoproxide grafted to the glass surface premodified with 3-aminopropyltriethoxysilane. Molecular composition, thickness, wettability of the PChMa coating and its alignment action for a NLC were examined with time of flight-secondary ion mass spectrometry, ellipsometry, contact angle measurements, polarization optical microscopy and commercially produced PolScope technique allowing for mapping of the optic axis and optical retardance within the microscope field view. We find that the PChMa coating provides a specific monotonous increase (decrease) in the tilt angle of the NLC director with respect to the substrates normal upon heating (cooling) referred to as anchoring tuning.

Structure-Specific Liquid Crystal Anchoring Induced by the Molecular Combing of Short Oligonucleotides

Surface-immobilized oligonucleotides were "combed" by meniscus motion and exposed to a nematic liquid crystal (LC). Although the oligonucleotides were as short as 16 bases, they were apparently oriented by this process and, in turn, successfully biased the orientation of the adjacent LC material. Single-stranded DNA (ssDNA) induced LC orientation in the combing direction, while hybridized double-stranded DNA (dsDNA) rotated the azimuthal LC orientation by ∼30° from the combing direction. The sensitivity of the chiral response to mixed ssDNA/dsDNA surfaces was characterized by employing complementary DNA that was longer than the immobilized DNA, resulting in single-stranded overhangs of various lengths. A rotated LC orientation was observed even when more than 70% of the DNA was single-stranded, and the transition from the rotated to nonrotated response was apparently discontinuous as a function of ssDNA surface coverage. These phenomena represent a sensitive DNA hybridization detection strategy that can potentially comprise a multiplexed assay.

Discontinuous thermal diffusivity change due to the anchoring transition of a liquid crystal on a perfluoropolymer surface

Thermal diffusivity of a liquid crystal, 4'-butyl-4-heptyl-bicyclohexyl-4-carbonitrile, was measured using a temperature wave method. The liquid crystal was sandwiched by two glass substrates, which were treated with three different surface agents for providing distinct molecular orientations. Here, we demonstrate that: 1) a large thermal diffusivity anisotropy arising from different orientations, that is, planar and homeotropic states, was found in the nematic and smectic A phases; 2) when substrates were coated with a perfluoropolymer, abrupt changes of the thermal diffusivity were observed in the nematic phase both on cooling and heating due to the discontinuous anchoring transition between planar and homeotropic states. The temperature dependence of the thermal diffusivity anisotropy was well described by a power law, with an exponent of 0.27 according to the mean-field theory.

Anchoring transition induced by gelation in a liquid crystal system

We report a novel type of anchoring transition (ANT) in a liquid crystal driven by the physical gelation of the system. The ANT manifests as anomaly in the thermal behaviour of the dielectric permittivity. Data from X-ray studies suggest that ANT is caused by the ability of the gel fibres to compete with the substrate-driven orientation conditions. It is further found that the molecular reorientation is possible only in cases where the gel is weak, the difference between weak and strong gels being established by rheological measurements.

Evolution of the discontinuous anchoring transition under an electric field

Anchoring transition (ATr), resulting in the orientational change of liquid crystal (LC) molecules at surfaces with temperature variation, is shown to be essentially affected by the application of an electric field (E field). By combining defocusing polarizing microscopy with retardation measurement, a detailed investigation of the molecular orientational change in the vicinity of ATr during a cooling process under an E field was performed. Two important conclusions can be made from our observations: (i) the ATr temperature exhibits a large downshift in the presence of an E field; (ii) the orientational change is always discontinuous from a planar (P) state to a homeotropic (H) state. If the applied E field is below the Freedericksz transition, P and H states are uniform, whereas the surface H state is influenced by the tilt in the bulk due to negative dielectric anisotropy, resulting in a bent-splay deformed state.

Critical behavior in an electric-field-induced anchoring transition in a liquid crystal

We report an anchoring transition of a liquid crystal under the influence of an electric field (E field) in the vicinity of a first-order anchoring transition (ATr) temperature showing bistable homeotropic (H) and planar (P) states. By means of polarizing microscopy combined with retardation and switching and dielectric measurements, three important observations were made: (1) The anchoring transition from H to P driven by an E field is first order with a threshold E-field strength, which decreases with increasing temperature; (2) the thickness of the H layer that is to be converted to the P orientation increases toward ATr temperature T(ATr); and (3) the temperature dependence of both parameters is well described by a power law.

Formation and ordering of topological defect arrays produced by dilatational strain and shear flow in smectic-A liquid crystals

A microscale shear cell is used to study the formation of parabolic focal conic defects in the thermotropic smectic-A liquid crystal 8CB (4-octyl-4'-cyanobiphenyl). Defects are produced by four distinct methods: by the application of dilatational strain alone, by shear flow alone, by dilatational strain and subsequent shear flow, and by the simultaneous application of dilatational strain and shear flow. We confirm that defects originate within the bulk, consistent with the previously suggested undulation instability mechanism. In the presence of a shear flow, we observe that defect formation requires micrometer-level dilatations, whose magnitude depends on the sample thickness. The size and ordering of both disordered and ordered defect arrays is quantified using a pair distribution function. Deviations from the predictions of linear stability theory are observed that have not been reported previously. For example, defects form a square array with greater ordering in the principal flow direction. Ordering due to shear flow does not change the average defect size. It has been shown previously that the principal defect sizes of ordered defects scale differently with sample thickness than the wavelength of the small amplitude undulations. We find that disordered defects show a similar deviation from this predicted wavelength.

Polar-horizontal versus polar-vertical reverse-tilt-domain walls: influence of a pretilt angle below the nematic-isotropic phase transition

On cooling through the isotropic-to-nematic phase transition in a cell whose substrates induce a large pretilt angle theta0 from the vertical direction, but with no preferential azimuthal orientation, tilt domains appear. The boundary walls between reverse tilt domains are found to be bendlike and twistlike when theta0(T=TNI) is sufficiently large just below the isotropic-nematic phase transition temperature TNI--i.e., for a nearly planar orientation. Here the director becomes planar approximately midway through the wall, and we refer to this type of wall as "polar horizontal," which is topologically stable. However, if theta0(T=TNI) is sufficiently small just below TNI--i.e., closer to vertical orientation--a splay like and twistlike domain wall obtains, where the director is vertically oriented approximately midway through the wall; we refer to this type of wall as "polar vertical," whose stability depends on the anchoring. On cooling through the nematic phase, the pretilt angle theta0 decreases, with the director aligning closer to the vertical orientation. Nevertheless, the structures of both types of domain walls remain unchanged on variation of theta0 with temperature owing to topological constraints and also are unchanged after the application and removal of a large electric field. We examine the structure of domain walls for the liquid crystal ZLI-4330 (Merck) as a function of pretilt angle theta0(T=TNI) and calculate a critical value theta0c(T=TNI) of the pretilt angle just below TNI for which the predominance of domain walls crosses over from polar horizontal to polar vertical.

Liquid-crystal anchoring transitions on aligning substrates processed by a plasma beam

We have studied a sequence of anchoring transitions observed in nematic liquid crystals (NLCs) sandwiched between hydrophobic polyimide substrates treated with a plasma beam. There is a pronounced continuous transition from a homeotropic to a slightly tilted (nearly planar) alignment with the easy axis parallel to the incidence plane of the plasma beam (the zenithal transition) which takes place as the exposure dose increases. In NLCs with positive dielectric anisotropy, a further increase in the exposure dose results in in-plane reorientation of the easy axis by 90 degrees (the azimuthal transition). This transition occurs through the twofold degenerate alignment characteristic of second-order anchoring transitions. In contrast to the critical behavior of anchoring, the contact angle of the NLC and water on the treated substrates declines monotonically with increasing exposure dose. It follows that the surface concentration of hydrophobic chains decreases continuously. The anchoring transitions under consideration are qualitatively interpreted by using a simple phenomenological model of competing easy axes which is studied by analyzing anchoring diagrams of generalized polar and nonpolar anchoring models.

Naturally occurring reverse tilt domains in a high-pretilt alignment nematic liquid crystal

A cell whose substrates were coated with the polyamic acid SE1211 (Nissan Chemical Industries) and baked at high temperatures was filled with a nematic liquid crystal in the isotropic phase. On cooling into the nematic phase, naturally occurring and temporally and thermally robust reverse tilt domains separated by thin filamentlike walls were observed. The properties of these structures are reported.

Instabilities Across the Isotropic Conductivity Point in a Nematic Phenyl Benzoate under AC Driving

We characterize the sequence of bifurcations generated by ac fields in a nematic layer held between unidirectionally rubbed ITO electrodes. The material, which possesses a negative dielectric anisotropy epsilona and an inversion temperature for electrical conductivity anisotropy sigmaa, exhibits a monostable tilted alignment near TIN, the isotropic-nematic point. On cooling, an anchoring transition to the homeotropic configuration occurs close to the underlying smectic phase. The field experiments are performed for (i) negative sigmaa and homeotropic alignment, and (ii) weakly positive sigmaa and nearly homeotropic alignment. Under ac driving, the Freedericksz transition is followed by bifurcation into various patterned states. Among them are the striped states that seem to belong to the dielectric regime and localized hybrid instabilities. Very significantly, the patterned instabilities are not excited by dc fields, indicating their possible gradient flexoelectric origin. The Carr-Helfrich mechanism-based theories that take account of flexoelectric terms can explain the observed electroconvective effects only in part.

Anchoring of a nematic liquid crystal on a wettability gradient

We have studied the anchoring of the nematic liquid crystal 5CB (4'-n-pentyl-4-cyanobiphenyl) as a function of the surface wettability, thickness of the liquid crystal layer, and temperature by measuring the birefringence of a hybrid aligned nematic cell where the nematic material was confined between octadecyltriethoxysilane-treated glass surfaces, with one surface linearly varying in its hydrophobicity. A homeotropic-to-tilted anchoring transition was observed as a function of the lateral distance along the hydrophobicity gradient, typically in a region corresponding to a water contact angle of approximately 64 degrees. The effect of the nematic layer thickness was measured simultaneously by preparing a wedge cell where the thickness varied along the direction perpendicular to the wettability. The detailed behavior of the onset of birefringence was found to be consistent with a dual-easy-axis model that predicts a discontinuous anchoring transition from homeotropic to planar. The anchoring was independent of temperature, except within 1 degrees C of the nematic-to-isotropic transition temperature (T(NI)). As the temperature approached T(NI), the tendency for planar anchoring gradually increased relative to that for homeotropic anchoring.

Microscopic and macroscopic description of anchoring at the nematic-solid substrate interface

A simple microscopic mean-field model for a flat nematic liquid crystal (NLC) sample in a contact to the solid substrate surface is offered. An interaction between NLC molecules is simulated by the well known McMillan model potential, and an orienting action of the solid substrate surface on NLC molecules is modeled by a short-range external field which acts directly only on molecules within the first molecular layer of the nematic sample adjacent to the substrate surface. For an undistorted NLC sample, the model allows the calculation of local order parameter profiles for different values of strength of this external orienting field and temperature of the sample. These profiles are used in a description of a director field distortion caused by a certain external action and in the calculation of the anchoring energy coefficient used in a macroscopic description of the anchoring at the nematic-solid substrate interface. Dependence of this coefficient on the strength of the short-range orienting field is obtained, and an unequivocal relation between the magnitude of and the orientational order parameter profile near the substrate surface is established. The temperature dependence of the coefficient W calculated from the offered microscopic model is in good agreement with the experimental data on NLC MBBA.

Quasidivergent nematic surface electroclinic effect

A polyimide coated substrate is treated so that vertical liquid crystal alignment (theta=0) obtains over the temperature range T(NA)< T < T(a), where T(NA) is the nematic-smectic-A transition temperature. When the cell is filled with a chiral liquid crystal whose helical pitch is unwound (surface stabilized), application of an in-plane electric field for T(NA)< T < T(a) induces a nonzero polar tilt theta proportional to E of the liquid crystal director at the surface, where the tilted orientation propagates elastically into the bulk. On heating toward T(a), this surface electroclinic response becomes large, corresponding to the onset of a surface tilt transition at T(a) from theta=0 to nonzero theta.

Smectic tilt susceptibility: anharmonic behavior in surface-induced smectic layers above the nematic--smectic-a transition temperature

Fréedericksz transition measurements were performed on the Merck liquid crystal SCE12R. The results were used to determine the quartic contribution to the free energy associated with molecular tilt relative to the layer normal in the surface-induced smectic layers above the nematic-smectic-A transition temperature T(NA) . Both the quadratic and quartic coefficients are consistent with the scaling relation (T- T(NA))(-3nu) , where nu is the correlation length critical exponent, and their ratio was approximately constant with T . The dielectric constants, the refractive indices, and the bend elastic constant for SCE12R also are reported.

Polar anchoring strength of a tilted nematic: confirmation of the dual easy axis model

The polar anchoring strength coefficient W and polar pretilt angle theta0 were measured simultaneously for the liquid crystal pentylcyanobiphenyl at a rubbed polyimide alignment layer that is ordinarily used for vertical alignment. It was found that W proportional theta(2)0 over the range 0 degrees < or =theta0 less or similar to 35 degrees . The results provide a confirmation of the dual easy axis model, wherein the liquid crystal director adopts an equilibrium orientation theta0 at the substrate that is determined by competition between a pair of preferred orientation directions.

Surface-induced molecular tilt above the smectic-A-smectic-C phase transition in a nonchiral liquid crystal

A polyimide-coated substrate was rubbed in such a way as to possess two competing easy axes for liquid crystal alignment. On cooling a homeotropically aligned liquid crystal through the smectic-A phase toward the smectic-C phase transition, an increasing tilt of the molecules relative to the layer normal was observed. The tilt was localized to within a smectic-C correlation length of the interface, and was found to increase monotonically with the rubbing strength associated with the preparation of the polyimide surface. The results are discussed in light of the dual easy axis model [T. Shioda et al., Phys. Rev. E 67, 041706 (2003)], and suggest that the two easy axes are not mutually orthogonal.

Transverse surface-induced polarization at the interface between a chiral nematic liquid crystal and a substrate

A chiral nematic liquid crystal that is tilted by an angle theta(i) with respect to a substrate is subjected to an ac electric field at frequency omega applied parallel to the substrate. The nematic director is found to oscillate azimuthally about the normal to the liquid crystal-substrate interface at frequency omega, indicating that a nonzero polarization perpendicular to the molecular tilt plane exists at the interface. The interfacial polarization, anchoring strength coefficient, and bulk viscosity are obtained by measurements of the oscillation amplitude as a function of omega.