Crystal-liquid Interface Research Articles

Anchoring transitions and periodic deformations in nematic slabs

Elastic energy terms linear in the deformation tensor can appear in an ultrathin layer at the interface of a nematic liquid crystal with a substrate. These Lifchitz invariant like terms can destroy an initially uniform alignment. In particular, they can induce modulated structures. The instability towards a periodic deformation of the nematic director can occur only if the anchoring energy is lower, and the elastic constant of the Lifchitz term is larger than well-defined critical values. The present phenomenological model accounts for experimental observations where a homeotropic slab undergoes, upon cooling, a tilt transition followed by a structural transition towards a stripe texture.

Optical switch based on the electrically controlled liquid crystal interface.

The peculiarities of the linearly polarized light beam reflection at the interface within the bulk of a nematic liquid crystal (NLC) cell with different orientations of the director are analyzed. Two methods to create the interface are considered. Combination of the planar and homeotropic orientations of the NLC director is realized by means of a spatially structured electrode under the applied voltage. In-plane patterned azimuthal alignment of the NLC director is created by the patterned rubbing alignment technique. All possible orthogonal orientations of the LC director are considered; the configurations for realization of total internal reflection are determined. The revealed relationship between the propagation of optical beams in a liquid crystal material and polarization of laser radiation has enabled realization of the spatial separation for the orthogonally polarized light beams at the interface between two regions of NLC with different director orientations (domains). Owing to variations in the applied voltage and, hence, in the refractive index gradient, the light beam propagation directions may be controlled electrically.

A cationic surfactant-decorated liquid crystal sensing platform for simple and sensitive detection of acetylcholinesterase and its inhibitor

In this paper, construction of the liquid crystal (LC)-based sensing platform for simple and sensitive detection of acetylcholinesterase (AChE) and its inhibitor using a cationic surfactant-decorated LC interface was demonstrated. A change of the optical images of LCs from bright to dark appearance was observed when the cationic surfactant, myristoylcholine chloride (Myr), was transferred onto the aqueous/LC interface, due to the formation of a stable surfactant monolayer at the interface. A dark-to-bright change of the optical appearance was then observed when AChE was transferred onto the Myr-decorated LC interface. The sensitivity of this new type of LC-based sensor is 3 orders of magnitude higher in the serum albumin solution than that only in the buffer solution. Noteworthy is that the AChE LC sensor shows a very high sensitivity for the detection of the enzyme inhibitor, which is around 1 fM. The constructed low-cost LC-based sensor is quite simple and convenient, showing high promise for label-free detection of AChE and its inhibitors.

Nanoparticle self-assembly at the interface of liquid crystal droplets

Nanoparticles adsorbed at the interface of nematic liquid crystals are known to form ordered structures whose morphology depends on the orientation of the underlying nematic field. The origin of such structures is believed to result from an interplay between the liquid crystal orientation at the particles' surface, the orientation at the liquid crystal's air interface, and the bulk elasticity of the underlying liquid crystal. In this work, we consider nanoparticle assembly at the interface of nematic droplets. We present a systematic study of the free energy of nanoparticle-laden droplets in terms of experiments and a Landau-de Gennes formalism. The results of that study indicate that, even for conditions under which particles interact only weakly at flat interfaces, particles aggregate at the poles of bipolar droplets and assemble into robust, quantized arrangements that can be mapped onto hexagonal lattices. The contributions of elasticity and interfacial energy corresponding to different arrangements are used to explain the resulting morphologies, and the predictions of the model are shown to be consistent with experimental observations. The findings presented here suggest that particle-laden liquid crystal droplets could provide a unique and versatile route toward building blocks for hierarchical materials assembly.

Crystal-liquid interfacial free energy of hard spheres via a thermodynamic integration scheme.

The hard-sphere crystal-liquid interfacial free energy γcl is determined from molecular dynamics simulations using a thermodynamic integration (TI) scheme. The advantage of this TI scheme compared to previous methods is to successfully circumvent hysteresis effects due to the movement of the crystal-liquid interface. This is accomplished by the use of extremely-short-range and impenetrable Gaussian flat walls that prevent the drift of the interface while imposing a negligible free-energy penalty. We find that it is crucial to analyze finite-size effects in order to obtain reliable estimates of γcl in the thermodynamic limit.

Adsorption of rationally designed "surf-tides" to a liquid-crystal interface.

The interfacial adsorption of proteins in surfactant laden systems occurs both in nature and industrial processing, yet much of the fundamental behavior behind these systems is still not well understood. We report the development of a system that monitors optical transitions of a liquid-crystalline/aqueous interface to examine the dynamics of adsorption of two rationally designed model peptide molecules. The two molecules synthesized in this study were both designed to become surface-active upon folding and contain the same net charge of +3, but one of the peptides, K-2.5, has its three charges separated by 2.5 amino acids as compared to K-6.0, which has its three charges separated by 6 amino acids. Our study examines the roles that surfactant adsorption, peptide charge distribution and secondary structure have on the relative adsorption dynamics of these two models peptides onto a fluid/fluid interface. Using the optical detection of molecular adsorption and image analysis of these events, we obtain quantitative information about the dynamics as a function of the charge spacing and initial peptide concentration. We show that both peptides initially follow a diffusion-limited adsorption model onto the interface. Additionally, our results suggest that the K-6.0 peptides demonstrate enhanced adsorption kinetics, where the enhanced rates are a consequence of the well-folded adsorbed state and spatial distribution on the surface. These findings provide further insights into the role that charge spacing has on secondary structure and subsequently the dynamics of adsorption, while developing a versatile system capable of extracting quantitative information from a simple inexpensive optical system.

Crystal growth in (GeS2)x(Sb2S3)1 − x thin films

The isothermal crystal growth kinetics of Sb2S3 in (GeS2)x(Sb2S3)1−x thin films (x=0.1, 0.2 and 0.3) has been investigated by static and high speed optical microscopy in a wide temperature range of 480–625K allowing measurement of growth rates over four orders of magnitude. The formed crystals developed linearly with time, which is associated with crystal growth controlled by liquid–crystal interface kinetics. The appropriate growth model was derived from the dependence of reduced crystal growth rate on undercooling of the system, indicating that the 2D surface nucleated growth model is operative in this particular case. Thermodynamic and kinetic aspects are discussed.

Structure of the cholesteric-isotropic interface.

The interface of a cholesteric liquid crystal with an isotropic fluid can display a range of unusual properties, such as a layer of topological defects close to an undulated interface. These properties have been known for a long time and have been explored for technological applications as a tunable substrate for colloidal self-assembly. However, from a fundamental point of view, the cholesteric-isotropic interface remains poorly understood and even basic properties, such as the dependence of the surface tension on the attributes of the liquid crystal, remain unknown. Here, we present a systematic calculation of the structure and surface tension of the cholesteric-isotropic interface and how these vary with the properties of the liquid crystal. We find that the anchoring at the interface depends on the pitch of the cholesteric and the interface undulations scale with the square root of the pitch. Our results also suggest the intriguing possibility of wetting of this interface by a blue phase.

Effect of an organic liquid on the surface energy of scandium and titanium

The dependence of the energy at a metallic crystal face-organic liquid interface on the dielectric permeability of the liquid is determined using a modified version of the electron statistical theory. The results from calculating the orientational dependence of the energy at the interfaces between α-Sc and α-Ti polymorphic phases and hexane, nonane, benzene, and o-xylene are described.

Crystal-liquid interfacial free energy via thermodynamic integration.

A novel thermodynamic integration (TI) scheme is presented to compute the crystal-liquid interfacial free energy (γcl) from molecular dynamics simulation. The scheme is applied to a Lennard-Jones system. By using extremely short-ranged and impenetrable Gaussian flat walls to confine the liquid and crystal phases, we overcome hysteresis problems of previous TI schemes that stem from the translational movement of the crystal-liquid interface. Our technique is applied to compute γcl for the (100), (110), and (111) orientation of the crystalline phase at three temperatures under coexistence conditions. For one case, namely, the (100) interface at the temperature T = 1.0 (in reduced units), we demonstrate that finite-size scaling in the framework of capillary wave theory can be used to estimate γcl in the thermodynamic limit. Thereby, we show that our TI scheme is not associated with the suppression of capillary wave fluctuations.

A method for estimating interfacial tension of liquid crystal embedded in polymer matrix forming PDLC

ABSTRACTA simple and convenient method based on sessile drop technique for measuring surface tensions of polymer and nematic liquid crystal (LC) is described. Contact angles formed by drops of probe liquids and a nematic LC on a photocurable polymer were measured. The surface energies were evaluated using the Fowkes method, Neumann's equation, and new equations developed based on Neumann's approach. The values of surface tensions were used to evaluate the interfacial interaction in term of work of adhesion between the LC and polymer. Further, the effect of dichroic dye on the extent of interaction and work of adhesion was examined by measuring contact angle in consequence of dye addition. A difference in work of adhesion between the lower and higher dye‐doped LC droplets gave an indication of affinity relationship between polymer and LC molecules. A change in work of adhesion resulted in variability of nematic director configurations inside phase separated LC droplets embedded in polymer matrix; when viewed under polarizing optical microscope. Thus, our approach of estimating surface energy of polymer and LC has found to be useful in determining interaction at polymer–LC interface. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41137.

Using Liquid Crystals to Reveal How Mechanical Anisotropy Changes Interfacial Behaviors of Motile Bacteria

Bacteria often inhabit and exhibit distinct dynamical behaviors at interfaces, but the physical mechanisms by which interfaces cue bacteria are still poorly understood. In this work, we use interfaces formed between coexisting isotropic and liquid crystal (LC) phases to provide insight into how mechanical anisotropy and defects in LC ordering influence fundamental bacterial behaviors. Specifically, we measure the anisotropic elasticity of the LC to change fundamental behaviors of motile, rod-shaped Proteus mirabilis cells (3 μm in length) adsorbed to the LC interface, including the orientation, speed, and direction of motion of the cells (the cells follow the director of the LC at the interface), transient multicellular self-association, and dynamical escape from the interface. In this latter context, we measure motile bacteria to escape from the interfaces preferentially into the isotropic phase, consistent with the predicted effects of an elastic penalty associated with strain of the LC about the bacteria when escape occurs into the nematic phase. We also observe boojums (surface topological defects) present at the interfaces of droplets of nematic LC (tactoids) to play a central role in mediating the escape of motile bacteria from the LC interface. Whereas the bacteria escape the interface of nematic droplets via a mechanism that involved nematic director-guided motion through one of the two boojums, for isotropic droplets in a continuous nematic phase, the elasticity of the LC generally prevented single bacteria from escaping. Instead, assemblies of bacteria piled up at boojums and escape occurred through a cooperative, multicellular phenomenon. Overall, our studies show that the dynamical behaviors of motile bacteria at anisotropic LC interfaces can be understood within a conceptual framework that reflects the interplay of LC elasticity, surface-induced order, and topological defects.

Fast and slow crystal growth kinetics in glass-forming melts.

Published values of crystal growth rates are compared for supercooled glass-forming liquids undergoing congruent freezing at a planar crystal-liquid interface. For the purposes of comparison pure metals are considered to be glass-forming systems, using data from molecular-dynamics simulations. For each system, the growth rate has a maximum value U(max) at a temperature T(max) that lies between the glass-transition temperature T(g) and the melting temperature T(m). A classification is suggested, based on the lability (specifically, the propensity for fast crystallization), of the liquid. High-lability systems show "fast" growth characterized by a high U(max), a low T(max)/T(m), and a very broad peak in U vs. T/T(m). In contrast, systems showing "slow" growth have a low U(max), a high T(max)/T(m), and a sharp peak in U vs. T/T(m). Despite the difference of more than 11 orders of magnitude in U(max) seen in pure metals and in silica, the range of glass-forming systems surveyed fit into a common pattern in which the lability increases with lower reduced glass-transition temperature (T(g)/T(m)) and higher fragility of the liquid. A single parameter, a linear combination of T(g)/T(m) and fragility, can show a good correlation with U(max). For all the systems, growth at U(max) is coupled to the atomic/molecular mobility in the liquid. It is found that, across the diversity of glass-forming systems, T(max)/T(g) = 1.48 ± 0.15.

Surface plasmon based sensor with order-of-magnitude higher sensitivity to electric field induced changes in dielectric environment at metal/nematic liquid-crystal interface

A highly sensitive surface plasmon based sensor is developed for monitoring changes in the dielectric environment at metal/dielectric interfaces. It consists of high-index prism/30–40nm gold/nematic liquid crystal (E44)/indium-tin-oxide/glass structure, which enables measurements of surface plasmon resonance (SPR) curves by using the Kretschmann configuration. The baseline sensitivity of the device to changes in the refractive index is evaluated by using Au/air, Au/water, and Au/nematic liquid crystal (E44); the measured baseline sensitivity is in excellent agreement with theoretically predicted optimal sensitivity. The sensitivity of the device to electric field induced changes in the refractive index of the material at the interface, however, is 28-times higher than previously known value for a similar sensor. Effects related to the periodic alignment of the liquid crystal on gold surface and surface plasmon mediated diffraction are investigated.

GEOCHEMISTRY, MINERALOGY, AND EVOLUTION OF Li-Al MICAS AND FELDSPARS FROM THE MOUNT MICA PEGMATITE, MAINE, USA

Mount Mica is a poorly zoned, sodic LCT-type pegmatite consisting dominantly of quartz, albite, and muscovite in the outer portion of the pegmatite. Micas are the principal K minerals throughout the pegmatite. Potassium feldspar and rare-element minerals such as lepidolite, elbaite, Cs-rich beryl, fluorapatite, and pollucite are restricted to the core zone of the pegmatite. The dearth of rare element (Li, F, Cs, and Rb) substitution in minerals of the outer portion of the pegmatite and the relatively low K/Rb ratio of K-feldspar in the core zone indicate that the overall degree of fractionation of the pegmatite is moderate. Aluminum-micas belong to the muscovite–polylithionite chemical trend, with a compositional gap between muscovite and trilithionite. Lithium is incorporated into the micas mainly via the Li 3 Al −1 □ −2 substitution mechanism. Lithium-Al micas in the wall zone are chemically homogeneous, but abruptly evolve into higher Cs + Rb bearing Li-rich muscovite and lepidolite in the core zone. An abrupt change in grain size, texture, and chemical composition is evident in the micas from the core zone. Pods of fine- to coarse-grained lepidolite occur sporadically throughout the core zone. Muscovite crystals located adjacent to lepidolite pods or adjacent to or within miarolitic cavities may be overgrown by a coarse-grained mosaic of lepidolite crystals. Some of the overgrown muscovite crystals display interlayering on a microscopic μm scale. The interlayering suggests late-stage rapid crystallization along a diffusion-controlled crystal-liquid interface. The abrupt increase of the Cs, Rb, and F in K-feldspar, muscovite, and lepidolite and the occurrence of such highly evolved species as F-rich lepidolite, pollucite pods, elbaite tourmaline, Cs-rich beryl, and spodumene in miarolitic cavities in the core zone suggest that incompatible elements were retained in the residual fluid until their concentration was high enough to initiate crystallization of incompatible-rich mineral phases. The relatively low abundance of incompatible elements in the hanging-wall zone (HWZ) and footwall zone (FWZ) suggest that the fractionation process was very efficient in sweeping the incompatibles into the core zone of the pegmatite, producing proportionally small volumes inside the pegmatite with very high enrichment in incompatible elements.

Crystallization behavior in Se90Te10 and Se80Te20 thin films

Isothermal crystal growth kinetics in Se90Te10 and Se80Te20 thin films was studied by microscopy and in situ X-ray diffraction (XRD) measurements. The spherulite-like crystals grew linearly with time. In a narrow temperature range of between 65 and 85 °C, crystal growth rates exhibit simple exponential behavior with activation energies EG = 193 ± 4 kJ mol−1 for Se90Te10 and EG = 195 ± 4 kJ mol−1 for Se80Te20. The crystal growth in both compositions is controlled by liquid-crystal interface kinetics and can be described by a screw dislocation growth model. From the XRD data, the crystallization fraction was estimated. The crystallization data were described by Johnson-Mehl-Avrami (JMA) model with Avrami exponents m = 1.4 ± 0.3 for Se90Te10 and m = 1.6 ± 0.4 for Se80Te20. Activation energies were estimated from the temperature dependence of rate constant evaluated from the JMA model. The activation energies of nucleation-growth process were found to be Ec = 184 ± 21 kJ mol−1 for Se90Te10 and Ec = 179 ± 7 kJ mol−1 for Se80Te20, and are comparable with activation energies of crystal growth.

DNA Hybridization-Mediated Liposome Fusion at the Aqueous Liquid Crystal Interface.

The prominence of receptor-mediated bilayer fusion in cellular biology motivates development of biomimetic strategies for studying fusogenic mechanisms. An approach is reported here for monitoring receptor-mediated fusion that exploits the unique physical and optical properties of liquid crystals (LC). PEG-functionalized lipids are used to create an interfacial environment capable of inhibiting spontaneous liposome fusion with an aqueous/LC interface. Then, DNA hybridization between oligonucleotides within bulk phase liposomes and a PEG-lipid monolayer at an aqueous/LC interface is exploited to induce receptor-mediated liposome fusion. These hybridization events induce strain within the liposome bilayer, promote lipid mixing with the LC interface, and consequently create an interfacial environment favoring re-orientation of the LC to a homeotropic (perpendicular) state. Furthermore, the bi-functionality of aptamers is exploited to modulate DNA hybridization-mediated liposome fusion by regulating the availability of the appropriate ligand (i.e., thrombin). Here, a LC-based approach for monitoring receptor (i.e., DNA hybridization)-mediated liposome fusion is demonstrated, liposome properties that dictate fusion dynamics are explored, and an example of how this approach may be used in a biosensing scheme is provided.

Surface free energy of the crystal-liquid interface on the metastable extension of the melting curve

The surface free energy γ, excess surface energy e, and excess surface stress τ at the crystal-liquid interface of a Lennard-Jones system have been determined by molecular dynamics simulation. The calculations have been performed for temperatures both above and below the triple-point temperature for the region where each of the coexisting phases is metastable and is at a negative pressure. The asymptotic behavior of γ, e, and τ has been analyzed near the endpoint of the melting curve, which is a point of the contact of the metastable extension of the melting curve and the spinodal of the stretched liquid [V.G. Baidakov and S.P. Protsenko, Phys. Rev. Lett. 95, 015701 (2005)]. It has been found that γ, e, and τ at this point are finite and the excess surface entropy is zero.

Solid phase properties and crystallization in simple model systems

We review theoretical and simulational approaches to the description of equilibrium bulk crystal and interface properties as well as to the nonequilibrium processes of homogeneous and heterogeneous crystal nucleation for the simple model systems of hard spheres and Lennard-Jones particles. For the equilibrium properties of bulk and interfaces, density functional theories employing fundamental measure functionals prove to be a precise and versatile tool, as exemplified with a closer analysis of the hard sphere crystalliquid interface. A detailed understanding of the dynamic process of nucleation in these model systems nevertheless still relies on simulational approaches. We review bulk nucleation and nucleation at structured walls and examine in closer detail the influence of walls with variable strength on nucleation in the Lennard-Jones fluid. We find that a planar crystalline substrate induces the growth of a crystalline film for a large range of lattice spacings and interaction potentials. Only a strongly incommensurate substrate and a very weakly attractive substrate potential lead to crystal growth with a non-zero contact angle.

The investigation of the two-dimensional surface relief grating on dye-doped polymer film

Two-dimensional surface relief grating (2D-SRG) on azo-dye doped polymer film (DDPF) was fabricated using the two-step of holographic writing technique. The groove structure of the 2D-SRG was dependent on the rotational angle of the two-step of writing, and the depth of the groove could be enhanced about 2~3 times by using nematic liquid crystal as the interface. The surface modulation of groove on DDPF with or without the interface of nematic liquid crystal depending on the polarization of the writing beams is also discussed. As the rotational angle between the two steps of writing was increased, the depth of the groove was gradually decreased for the writing beams with the S-polarization in the second step of writing, while increasing for the writing beams with the P-polarization.