Surface-induced discrete smectic order in the isotropic phase of 12 CB in cylindrical pores

Surface induced smectic order in ionic liquids - an X-ray reflectivity study of [C22C1im]+[NTf2]. Phys Chem Chem Phys 2018;19:26651-26661. [PMID: 28960006 DOI: 10.1039/c7cp04852a]

Surface induced smectic order was found for the ionic liquid 1-methyl-3-docosylimidazolium bis(trifluoromethlysulfonyl)imide by X-ray reflectivity and grazing incidence scattering experiments. Near the free liquid surface, an ordered structure of alternating layers composed of polar and non-polar moieties is observed. This leads to an oscillatory interfacial profile perpendicular to the liquid surface with a periodicity of 3.7 nm. Small angle X-ray scattering and polarized light microscopy measurements suggest that the observed surface structure is related to fluctuations into a metastable liquid crystalline SmA₂ phase that was found by supercooling the bulk liquid. The observed surface ordering persists up to 157 °C, i.e. more than 88 K above the bulk melting temperature of 68.1 °C. Close to the bulk melting point, we find a thickness of the ordered layer of L = 30 nm. The dependency of L(τ) = Λ ln(τ/τ₁) vs. reduced temperature τ follows a logarithmic growth law. In agreement with theory, the pre-factor Λ is governed by the correlation length of the isotropic bulk phase.

Paranematic-to-nematic ordering of a binary mixture of rodlike liquid crystals confined in cylindrical nanochannels

We explore the optical birefringence of the nematic binary mixtures 6CB_{1-x}7CB_{x} (0 ≤ x ≤ 1) embedded into parallel-aligned nanochannels of mesoporous alumina and silica membranes for channel radii of 3.4 ≤ R ≤ 21.0 nm. The results are compared with the bulk behavior and analyzed with a Landau-de Gennes model. Depending on the channel radius the nematic ordering in the cylindrical nanochannels evolves either discontinuously (subcritical regime, nematic ordering field σ<1/2) or continuously (overcritical regime, σ>1/2), but in both cases with a characteristic paranematic precursor behavior. The strength of the ordering field, imposed by the channel walls, and the magnitude of quenched disorder varies linearly with the mole fraction x and scales inversely proportionally with R for channel radii larger than 4 nm. The critical pore radius, R_{c}, separating a continuous from a discontinuous paranematic-to-nematic evolution varies linearly with x and differs negligibly between the silica and alumina membranes. We find no hints of preferred adsorption of one species at the channels walls. By contrast, a linear variation of the nematic-to-paranematic transition point T_{PN} and of the nematic ordering field σ versus x suggests that the binary mixtures of cyanobiphenyls 6CB and 7CB keep their homogeneous bulk stoichiometry also in nanoconfinement, at least for channel diameters larger than ∼7 nm.

Dispersion of γ-Alumina Nano-Sized Spherical Particles in a Calamitic Liquid Crystal. Study and Optimization of the Confinement Effects

We report an experimental study on confined systems formed by butyloxybenzylidene octylaniline liquid crystal (4O.8) + γ-alumina nanoparticles. The effects of the confinement in the thermal and dielectric properties of the liquid crystal under different densities of nanoparticles is analyzed by means of high resolution Modulated Differential Scanning Calorimetry (MDSC) and broadband dielectric spectroscopy. First, a drastic depression of the N-I and SmA-N transition temperatures is observed with confinement, the more concentration of nanoparticles the deeper this depression is, driving the nematic range closer to the room temperature. An interesting experimental law is found for both transition temperatures. Second, the change in shape of the heat capacity peaks is quantified by means of the full width half maximum (FWHM). Third, the confinement does not noticeably affect the molecular dynamics. Finally, the combination of nanoparticles and the external applied electric field tends to favor the alignment of the molecules in metallic cells. All these results indicate that the confinement of liquid crystals by means of γ-alumina nanoparticles could be optimum for liquid crystal-based electrooptic devices.

Influence of nanoconfinement on the nematic behavior of liquid crystals

We explore the nematic ordering of the rodlike liquid crystals 5CB and 6CB, embedded into parallel-aligned nanochannels in mesoporous silicon and silica membranes as a function of mean channel radius (4.7 ≤ R ≤ .3 nm), and, thus, geometrical confinement strength, by optical birefringence measurements in the infrared region. The orientational order inside the nanochannels results in an excess birefringence, which is proportional to the nematic order parameter. It evolves continuously on cooling with a precursor behavior, typical of a paranematic state at high temperatures. These observations are compared with the bulk behavior and analyzed within a phenomenological model. Such an approach indicates that the strength of the nematic ordering fields σ is beyond a critical threshold σ(c) = 1/2 that separates discontinuous from continuous paranematic-to-nematic behavior. In agreement with the predictions of the phenomenological approach, a linear dependency of σ on the inverse channel radius is found and we can infer therefrom the critical channel radii, R(c) separating continuous from discontinuous paranematic-to-isotropic behavior, for 5CB (12.1 nm) and 6CB (14.0 nm). Our analysis suggests that the tangential anchoring at the channel walls is of similar strength in mesoporous silicon and mesoporous silica membranes. A comparison with the bulk phase behavior reveals that the nematic order in nanoconfinement is significantly affected by channel wall roughness, leading to a reduction of the effective nematic ordering.

Study of the isotropic to smectic-A phase transition in liquid crystal and acetone binary mixtures

The first-order transition from the isotropic (I) to smectic-A (Sm A) phase in the liquid crystal 4-cyano-4(')-decylbiphenyl (10CB) doped with the polar solvent acetone (ace) has been studied as a function of solvent concentration by high-resolution ac-calorimetry. Heating and cooling scans were performed for miscible 10CB+ace samples having acetone mole fractions from x(ace)=0.05 (1 wt %) to 0.36 (10%) over a wide temperature range from 310 to 327 K. Two distinct first-order phase transition features are observed in the mixture whereas there is only one transition (I-Sm A) in the pure 10CB for that particular temperature range. Both calorimetric features reproduce on repeated heating and cooling scans and evolve with increasing x(ace) with the high-temperature feature relatively stable in temperature but reduced in size while the low-temperature feature shifts dramatically to lower temperature and exhibits increased dispersion. The coexistence region increases for the low-temperature feature but remains fairly constant for the high-temperature feature as a function of x(ace). Polarizing optical microscopy supports the identification of a smectic phase below the high-temperature heat capacity signature indicating that the low-temperature feature represents an injected smectic-smectic phase transition. These effects may be the consequence of screening the intermolecular potential of the liquid crystals by the solvent that stabilizes a weak smectic phase intermediate of the isotropic and pure smectic-A.

Collective molecular reorientation of a calamitic liquid crystal (12CB) confined in alumina nanochannels

We study the smectic director structure of the rodlike liquid crystal 4-n-dodecyl-4'-cyanobiphenyl (12CB) confined in cylindrical cavities of 200 nm diameter in porous alumina templates by means of combined broadband dielectric spectroscopy, optical birefringence, and neutron scattering measurements. We show that the collective molecular orientation differs between entering the smectic A phase upon cooling from the isotropic state and entering the same phase upon heating while melting the confined crystal. We discuss this collective molecular realignment in terms of a competition between weak planar anchoring at the p-Al2O3/12CB interface and a preferred texture typical of the crystallization of rodlike molecules in nanochannels (Bridgman growth).

Cylindrical sub-micrometer confinement results for the odd-symmetric dimer alpha,omega-bis[(4-cyanobiphenyl)-4'-yloxy]undecane (BCB.O11)

Broadband dielectric spectroscopy (10(2) Hz to 1.9 x 10(9) Hz) and specific heat measurements have been performed on the odd-symmetric dimer alpha,omega-bis[(4-cyanobiphenyl)-4'-yloxy]undecane (BCB.O11) in the isotropic (I) and nematic (N) phases confined to 200 nm diameter parallel cylindrical pores of Anopore membranes. Unlike previous studies on liquid crystal monomers, untreated and hexadecyltrimethylammonium bromide-treated membranes give rise to radial and axial confinements, respectively. An attempt is made to explain these unexpected results by means of a qualitative model for the dimer arrangement on alumina substrates. The model suggests that the population of conformers, which follow the bulk-like dynamics, is modified by confinement. Such a fact seems to be consistent with other distinctive features attributed to confinement, as for example, the increasing of the entropy change at the N-to-I phase transition for both axial and radial confinements. Specific-heat measurements have shown how confinement affects the N-to-I phase transition by a downward shift in transition temperature as well as by broadened and rounded specific-heat peaks. However, these modifications are revealed to be substantially different from what has been found previously in similar studies on liquid crystal monomers. Dynamic dielectric measurements have probed the different molecular motions in both confinements and how these motions are developed in a way similar to the bulk-dimer. Dielectric results have also proved that the surface-pinned molecular layer (where molecular motions are very restricted) adjacent to the pore-wall is temperature-dependent as already found previously for liquid crystal monomers.

Surface triple points and multiple-layer transitions observed by tuning the surface field at smectic liquid-crystal-water interfaces

We present an ellipsometric study of the interface between a smectic liquid crystal and water in the presence of a nonionic surfactant. The surfactant concentration serves as a handle to tune the surface field. For sufficiently large surfactant concentrations, a smectic phase is present at the interface in the temperature range above the smectic-A-isotropic bulk transition; when the bulk transition is approached, the thickness of this surface phase grows via a series of layer-by-layer transitions at which single smectic layers are formed. At lower surfactant concentrations, transitions appear at which the thickness of the surface phase jumps by multiple smectic layers, thereby implying the existence of triple points at which surface phases with different smectic layer numbers coexist. This is the first experimental demonstration of such surface triple points which are predicted by theoretical models.

Crossover in the wetting behavior at surfactant-laden liquid-crystal-water interfaces: experiment and theory

The behavior of a nematic liquid crystal at a surfactant-laden interface to an aqueous phase is studied under the condition of homeotropic anchoring. It is shown that with decreasing surfactant concentration the system shifts from surface-enhanced to surface-decreased order, i.e., the behavior changes from complete nematic wetting when the nematic-isotropic phase transition is approached from above to a different wetting behavior below the transition, characterized by a considerably decreased Maier-Saupe order parameter at the interface. The experimental behavior is analyzed within the framework of the Landau-de Gennes theory supplemented by a surface free energy, in which the wetting behavior is controlled by the magnitude of the anchoring strength and the preferred surface order parameter in comparison to the bulk order parameter. The theoretical modeling is able to account for all experimental observations.

Self-Assembly of Chemically Linked Rod−Disc Mesogenic Liquid Crystals

A series of new molecular discs (RDn, here n is the number of carbon atoms between the rod and disc mesogens) was synthesized via the chemical attachment of six cyanobiphenyl calamitic (rod) mesogens (R) linked to the triphenyl discotic (disc) mesogen (D) with a series of six alkyl chain linkages (n = 6-12). In this study, phase structures, transitions, and liquid crystalline (LC) behavior of the RD12 compound with 12 carbon atoms in each alkyl chain linkage between the rod and disc mesogens were investigated. Differential scanning calorimetry, polarized light microscopy, wide-angle X-ray diffraction (WAXD), and selected area electron diffraction (SAED) allowed us to identify three ordered phases below the isotropization temperature: nematic (N) LC and K1 and K2 crystalline phases. On the basis of the structural results obtained via 2D WAXD experiments on oriented samples and SAED experiments on single crystals, the K1 crystalline unit cell was determined to be triclinic with the dimensions of a = 1.36 nm, b = 1.45 nm, c = 2.11 nm, alpha = 85 degrees, beta = 100 degrees, and gamma = 50 degrees. The K2 phase was metastable with respect to the K1 phase. It also possessed a triclinic unit cell with a = 1.40 nm, b = 1.51 nm, c = 1.92 nm, alpha = 87 degrees, beta = 117 degrees, and gamma = 62 degrees. Molecular packing models for the crystalline phases were proposed on the basis of the diffraction results. In the whole range of ordered structures, it was found that RD12 molecular discs are intercalated. Both triphenyl discotic mesogens and cyanobiphenyl calamitic mesogens are completely interdigitated.

Thermodynamic and dielectric studies concerning the influence of cylindrical submicrometer confinement on heptyloxycyanobiphenyl

Measurements of the specific heat and the static dielectric permittivity of heptyloxycyanobiphenyl (7OCB) confined to the 0.2 microm diameter parallel cylindrical pores of Anopore membranes in the isotropic phase and nematic mesophase, are presented. A comparison between the bulk and the confined 7OCB in treated and untreated pore wall surfaces using a chemical surfactant (HTBA) is performed. Both the treated and untreated membrane confinements seem to affect the nematic-to-isotropic phase transition by a downshift in transition temperature and some rounding at the specific-heat maximum, in a way similar to that which was earlier published for other liquid crystals confined in the same geometry. The static dielectric measurements clearly point out that untreated membrane confinement is axial, with the nematic director aligned parallel to the pore axis being homeotropic bulklike, i.e., with the nematic director aligned perpendicular to the electrode cell surfaces. After chemical surfactant treatment, the nematic director is constrained in a radial alignment being perpendicular to the pore walls. The dielectric measurements are revealed to be specially sensible to analyze the surface-induced nematic order due to the pore wall. The tricritical nature of the nematic-to-isotropic phase transition in bulk 7OCB as well as in treated and untreated Anopore confined geometries is discussed through both the specific heat and the static dielectric data.

Comparative x-ray measurements of a de Vries smectic- material in bulk and confined geometries

X-ray investigations have been carried out on a material exhibiting the de Vries smectic-A (Sm-A) phase in bulk and confined geometries. In the confined geometry realized by confining the material in an Anopore membrane, while the minimal layer shrinkage feature characteristic of the de Vries materials is retained, the overall change of the layer spacing from the Sm-A to the Sm-C phase is much less compared to the bulk geometry and also the transition gets smeared out. A "crossover" behavior, which has been recently ascribed to different molecular packing arrangements within the Sm-A phase, is seen in both geometries. With the support from data on another non-de Vries type of material we suggest that the associated feature may not be due to a phase transition, but competition between the thermally driven stretching and molecular tilt.

Interaction of surfaces in smectic membranes and their instability near thinning transitions

We report measurements of the interaction between surfaces of the presmectic membrane above the temperature of transition to the phase without layer ordering. Investigations were performed employing cholesteric droplets embedded in the membrane in the temperature range of thinning transitions. Upon heating, the difference between the membrane tension and surface tension of the bulk sample decreases sufficiently, which leads to membrane instability. After the thinning transition, the membrane returns to a stable state with a larger value of surface interaction.

Influence of finite size and wetting on nematic and smectic phase behavior of liquid crystal confined to controlled-pore matrices

The high-resolution calorimetric study was carried out on octylcyanobiphenyl liquid crystal (LC) confined to various controlled-pore glass (CPG) matrices with silane-treated surface. The diameter of the voids cross section ranged between 23.7 and 395 nm. The results are compared to those obtained previously on CPG voids nontreated with silane. We found a striking similarity between the shifts in the isotropic to nematic and nematic to smectic-A phase transition temperatures as a function of the void radius in which order parameter variations at the LC-void interface play the dominant role. Weaker temperature shifts are observed in silane-treated samples, where surface ordering tendency is larger. In nontreated samples, a finite-size scaling law in the maximum value of the heat capacity at the nematic to smectic-A transition was observed for void diameters larger than 20 nm. In silane-treated samples, this behavior is considerably changed by surface wetting interactions.

X-ray and dielectric measurements of the smectic- A -hexatic- B transitionin bulk and confined geometries

We report x-ray and dielectric measurements on a liquid crystal exhibiting a smectic- A -hexatic- B (Sm- A -Hex- B ) transition in bulk and confined geometries. The confined geometry experiments have been performed by filling the compound, n -butyl 4(') - n -hexyloxybiphenyl-4-carboxylate, into Anopore membranes with 20 nm pore size. The surprising result seen in the x-ray studies on both the bulk and the Anopore samples is the presence of unusually strong second harmonic reflection arising from the smectic layers in both the Sm- A and Hex- B phases. In fact, the third harmonic reflection is also observed in the entire temperature range of the Hex- B phase and close to the transition in the Sm- A phase of the bulk sample, with the ratio of I(2qo) / I(qo), and I(3qo) / I(qo) being approximately 1/40 and 1/470, respectively (here I(qo), I(2qo), and I(3qo) correspond to the peak intensities of the first, second, and third harmonic reflections). These features point to the fact that the layering is much better defined in this system, unlike the usual description of pure sinusoidal mass density wave with a typical value of 10(-4) for I(2qo) / I(qo). Another notable feature observed is that whereas the extent of positional correlations of the molecules within the smectic layer, is comparable in the Sm- A phase of the bulk and Anopore samples, the correlations are reduced in the Hex- B phase. In the bulk as well as the Anopore case, the critical behavior of the positional correlations is found to be well described by a power-law expression with the exponent alpha being slightly larger for the Anopore sample. It is also seen that the weak first order nature of the transition seen in the bulk, gets weakened further, perhaps becoming second order. Despite such significant differences between the bulk and Anopore samples, the dynamics associated with the director fluctuations is only slightly influenced by confinement.

Structural Forces near Phase Transitions of Liquid Crystals

The structure of the fragile liquid-crystalline phases has a strong impact on the forces between bodies immersed in a liquid crystal (LC). We have equipped an atomic force microscope with a precise temperature control and measured various liquid-crystalline structural forces at temperatures close to the phase transitions. The observed forces agree well with predictions of Landau--de Gennes phenomenological theory of LCs, even at a nanoscale length. In addition to this, we have observed a molecular layer, adsorbed on the surfactant-covered glass surface, and determined its thickness and elastic properties.

Calorimetric study of octylcyanobiphenyl liquid crystal confined to a controlled-pore glass

We present a calorimetric study of the phase behavior of octylcyanobiphenyl (8CB) liquid crystal confined to a controlled-pore glass (CPG). We used CPG matrices with characteristic void diameters ranging from 400 to 20 nm. In bulk we obtain weakly first-order isotropic to nematic (I-N) phase transition and nearly continuous character of the nematic to smectic-A (N-SmA) phase transition. In all CPG matrices the I-N transition remains weakly first order, while the N-SmA one becomes progressively suppressed with decreasing CPG pore radius. With decreased pore diameters both phase transition temperatures monotonously decrease following similar trends, but increasing the stability range of the N phase. The heat-capacity response at the weakly first order I-N and continuous N-SmA phase transitions gradually approaches the tricritical-like and three-dimensional XY behavior, respectively. The main observed features were explained using a bicomponent single pore type phenomenological model.

Surface ordering transitions at a liquid crystal-solid interface above the isotropic smectic-A transition

The degree of orientational order induced by confining cylindrical surfaces is monitored via deuteron nuclear magnetic resonance linesplitting and linewidth above the smectic-A to isotropic phase transition. The orientational order strongly depends on the length of the surfactant coupling molecule, on the surface coverage, and on the liquid crystal. Continuous and stepwise growth of orientational order and surface-induced orientational order transitions found in the isotropic phase are explained in terms of a simplified model of surface-induced layering and molecular self-diffusion.

Comparative x-ray and dielectric measurements of smectic A-smectic-C* transition in bulk and confined geometries

Comparative x-ray and dielectric measurements have been made on a liquid crystal exhibiting a smectic-A-chiral-smectic-C (smectic-C*) transition in bulk and confined geometries. It is observed that confining the material in Anopore membranes having 200-nm pore size leads to the following features: (1) the temperature dependence of the x-ray layer spacing shows a qualitatively different behavior, (2) in the smectic-A phase the soft mode relaxation frequency increases by a factor of 2.5, and (3) in the smectic-C* phase the relaxation frequency of the Goldstone mode increases dramatically by as much as 400 times, perhaps owing to a partial unwinding of the helix by the surface induced field.

Nonspontaneous surface-induced nematic phase

By means of a Landau-de Gennes mean field model, we predict the existence of a nonspontaneous surface nematic phase in a smectogenic compound in contact with a suitable solid substrate. In the bulk the system does not show any nematic phase, the latter being solely induced by the substrate-liquid crystal interaction. Depending on the strength of the surface potential, a prewetting line, terminating at a critical point, may appear. For strong enough coupling, a new surface smectic phase can be induced, accompanied by a reentrant behavior. Our analysis might explain some recent experimental results [T. Moses, Phys. Rev. E 64, 010702(R) (2001)]: to validate it we suggest possible further experimental investigations.

Molecular dynamics study of the isotropic-nematic quench

Effects of cylindrical and spherical confinement on the kinetics of the isotropic-nematic quench is studied numerically. The nematic liquid crystal structure was modeled by a modified induced-dipole--induced-dipole interaction. Molecules were allowed to wander around points of a hexagonal lattice. Brownian molecular dynamics was used in order to access macroscopic time scales. In the bulk we distinguish between the early, domain, and late stage regime. The early regime is characterized by the exponential growth of the nematic uniaxial order parameter. In the domain regime domains are clearly visible and the average nematic domain size xi(d) obeys the dynamical scaling law xi(d)-t(gamma). The late stage evolution is dominated by dynamics of individual defects. In a confined system the qualitative change of the scaling behavior appears when xi(d) becomes comparable to a typical linear dimension R of the confinement. In the confining regime (xi(d)>or=R) the scaling coefficient gamma depends on the details of the confinement and also the final equilibrium nematic structure. The domain growth is well described with the Kibble-Zurek mechanism.

Surface-induced nematic and smectic order at a liquid-crystal-silanated-glass interface observed by atomic force spectroscopy and Brewster angle ellipsometry

We have investigated nematic and smectic surface-induced ordering in the isotropic phase of the 4-cyano-4'-n-octylbiphenyl liquid crystal on silanated BK7 and LaSF glass using atomic force spectroscopy and Brewster angle ellipsometry. We have observed complete wetting of the silanated glass surfaces with the nematic phase when approaching the isotropic-nematic phase transition from above. Using the atomic force spectroscopy at the same interfaces, we have detected a significant presmectic ordering that is enhanced at the nematic-isotropic transition. We have observed the first, strongly adsorbed layer of liquid-crystalline molecules underneath this presmectic ordering. This first molecular layer is laterally inhomogeneous with typically 100 nm voids and covers approximately 70% of the surface. It is stable far above the clearing point and is responsible for the surface-memory effect. The results have been analyzed using Landau-de Gennes theory. The surface coupling energies of the nematic and smectic order have been determined, as well as the coupling energy between the nematic and smectic order.

Surface ordering above the isotropic-smectic-A transition at a silane-treated substrate

Surface ordering in a homologous series of alkyl cyanobiphenyl (nCB) liquid crystals having a direct isotropic-smectic-A (I-A) transition was investigated using evanescent-wave ellipsometry. The liquid crystal was bounded by a solid substrate treated with a silane surfactant which induced homeotropic (perpendicular) ordering of the liquid crystal molecules in the smectic-A phase. In the isotropic phase, one of the liquid crystals (10CB) partially wet the interface with an orientationally ordered, homeotropically aligned layer. The ordered interfacial layer grew without layering transitions but remained finite in thickness as the bulk I-A transition was approached. The interfacial layer has significantly lower orientational order than is observed in the smectic phase, indicating the possibility that the surface region of 10CB may be in a surface-induced, nonspontaneous nematic phase. The other liquid crystals (11 and 12CB) showed no surface ordering behavior whatsoever. Models describing the ordered surface layer of 10CB are presented. The results can be interpreted as a sharp transition in the surface ordering behavior as the chain length of the liquid crystal is varied, at the I-A transition of a liquid crystal.

Atomic force microscope evidence for the existence of smecticlike surface layers in the isotropic phase of a nematic liquid crystal

Using a temperature controlled atomic force microscope we have observed presmectic layering in the isotropic phase of 4-cyano-4(')-n-octylbiphenyl (8CB) on silanated glass. The first molecular layer shows a smecticlike compressibility modulus of B approximately 10(7) N/m(2) and is stable more than 20 K beyond the bulk clearing point. It is followed by a presmectic modulation that increases when cooling towards the isotropic-nematic transition. In the bulk isotropic phase, the layers cover approximately 70% of the glass surface, indicating a clusterlike organization.

Influence of surface treatment on the smectic ordering within porous glass

The influence of the surface treatment on the Sm-A-N phase transition of the 8CB (octylcyanobiphenyl) liquid crystal confined to controlled pore glass (CPG) matrices is studied. The characteristic linear size of voids in the chosen CPG matrix is 0.2 &mgr;m. The voids' surface was either nontreated or silane treated enforcing tangential or homeotropic anchoring, respectively. In both cases the x-ray measurements reveal a qualitative change of the temperature dependence of the smectic order-parameter correlation length in comparison to the bulk sample. In addition, the apparent smectic pretransitional ordering is observed for the silane-treated sample. A theoretical description based on the Landau-de Gennes type approach is developed to explain the experimental data. The surface positional anchoring strength of the silane-treated sample is estimated to be of the order of 10(-4) J/m(2) and at least 100 times weaker for the nontreated case.

Interaction of spherical particles dispersed in a liquid crystal above the nematic-isotropic phase transition

The effective interaction of micron-size particles immersed in a nematic liquid crystal at temperatures above the nematic-isotropic phase transition is examined. Using the Landau-de Gennes theory we describe the surface-induced ordering of the liquid crystal. Modeling a spherical particle by a sequence of conical surfaces enables us to draw an analogy with the partial ordering in a thin nematic film and to obtain an analytical expression for the two-particle interaction. Special attention is paid to the dependence of the effective interaction on temperature and on the interfacial coupling parameters, which are chosen in agreement with recent experiments. We show that the interaction as a function of the interparticle distance exhibits a potential barrier of the order of kT. The height of the barrier decreases exponentially with increasing temperature while it grows linearly with the interfacial coupling parameters.

Influence of substrate microrelief on the Fréedericksz transition in a thin nematic cell

An effect of substrate surface regular wavy microrelief on the Freedericksz transition in a thin homeotropically aligned nematic cell near the smectic-A-nematic second-order transition temperature is theoretically investigated. It is shown that because of the suppression of an interfacial smectic-A structure, the Fréedericksz critical field for the cell with substrates having a sufficiently sharp microrelief should be significantly lower than that for the cell of the same thickness, but with perfectly flat substrates.