Isotropic Droplets Research Articles

Isotropic Droplets in Thin Free Standing Smectic Films

Isotropic liquid droplets are formed by different mechanisms near the smectic−isotropic phase transition in smectic freely suspended films. Because of the layered structure of the smectic films, the interplay of tensions that determines the droplet geometry is essentially different from that of liquid droplets on solid or isotropic liquid surfaces. By means of interferometric measurements, the three-dimensional droplet geometry is determined. A model is proposed that describes the droplet shapes, and the relevant parameters are discussed on the basis of experimental data. We derive surface and interface tensions of low molecular mass and polymeric mesogens and provide a molecular interpretation. In particular, the small difference between surface tensions in the smectic and isotropic phases is accessible with the method; it can be related to the structure of the isotropic liquid to air interface of smectogens. Capillary forces causing a lateral motion of droplets in inhomogeneously thick films are discuss...

Two-Dimensional Liquid Crystal Dispersions in Free Standing Films of Tilted Smectics

We report on the appearance of small isotropic droplets or bubbles, in free standing films of a non-chiral symmetric mixture of MHTAC, which exhibits a SmCA to isotropic phase transition. The isotropic droplets are initiated by applying a dc electric field tangentially to the film plane. Then, when increasing temperature towards the isotropic phase transition, the droplets are observed to grow. They then organize to form chainlike and Y-shaped structures. The formation of these structures evidences the existence of anisotropic interactions of the dipolar type between the droplets, with a long range attraction and a short range repulsion. The short range repulsion is reduced when the droplets become large, so that they eventually collapse with one another before being swallowed by the meniscus. Hence, the appearance of such droplets may also be viewed as a precursor mark of a thinning process of the free standing film.

Orientation and anchoring effects in stretched polymer dispersed nematic liquid crystals

The induction of liquid crystal orientation through mechanical stretching was investigated for polymer dispersed liquid crystals (PDLCs) by means of infrared dichroism. Using a nematic liquid crystal BL006 and polyacrylic acid as the polymer matrix, it was possible to stretch the PDLC films with BL006 in either the isotropic or the nematic phase. After cooling the films under strain to room temperature, the molecular orientation of BL006 was found to be much higher for films that contained isotropic liquid droplets of BL006 at the time of stretching than for films that had nematic droplets. Stretching PDLC films with isotropic droplets results in no molecular orientation, but the orientation is induced during the subsequent cooling when BL006 goes through the isotropic-to-nematic phase transition. Interestingly for PAA/BL006, the nematic director orients along the long axes of the elongated droplets despite liquid crystal anchoring perpendicular to the polymer interface.

Studying Polymer-Dispersed Liquid-Crystal Formation by FTIR Spectroscopy. 2. Phase Separation and Ordering

Polymerization-induced phase separation to form polymer-dispersed liquid crystals (PDLCs) is a complex process involving simultaneous curing and phase separation to form nematic microdomains. Real-time FTIR spectroscopy has been used to simultaneously observe and quantify the curing process using a model systema fast photocurable matrix (NOA65) and a liquid crystal (E7). While the role of FTIR spectroscopy in monitoring chemical changes is well recognized and reinforced for PDLCs, it is demonstrated in this paper that it is also a powerful tool to monitor physical changes (phase separation and nematic ordering). Phase separation was detected during the curing process by a scattering-induced change in the absorbance spectrum of the sample. Nematic ordering could be observed and quantified based on a change in a characteristic band of the liquid crystal. Moreover, the phase separation and the onset of nematic ordering are temporally resolved. The conversion at phase separation decreases strongly with an increase in liquid-crystal content, while the conversion required for phase separation increases with increasing temperature. Isotropic droplets are formed followed by nematic ordering in the domains at lower E7 concentrations, while the processes are simultaneous for higher concentrations. Temperatures close to and above the LC transition temperature also lead to isotropic domains, which form nematic domains upon cooling. The fraction of liquid crystal present as nematic droplets and total fraction of nematic domains in the PDLC are quantified based on changes in the vibrational spectrum of E7. On the basis of mass balances applied to the closed process, the phase diagram of the system could be determined as a function of curing temperature. Solubility limits obtained by this method agree well with results obtained by other researchers. The concept of a spectroscopic composite plot that completely describes the formation process in a PDLC is proposed.

PDLC-like patterns at the isotropic to cholesteric transition entrapped by in situ photopolymerization

When investigated by optical microscopy between crossed polarizers, the isotropic to cholesteric transition may appear like fingerprint-patterned droplets embedded in a black isotropic matrix. In the present work, such PDLC-like (polymer dispersed liquid crystal) patterns, only occurring over 0.7 C, have been entrapped and stored at ambient temperature in a polymer film. We used a UV polymerization process with different sequences in which illumination time and UV power progressively vary. From a conceptual viewpoint, these PDLC-like patterns come solely from liquid crystalline material, whereas all the conventional PDLCs are binary mixtures of a macromolecular compound or 'prepolymer' with a conventional low molecular mass liquid crystal. The fact that isotropic matrix and cholesteric droplets differ only from the viewpoint of molecular order and not in their chemical nature, permits comparisons with the usual case for which the choice of polymer-forming material is crucial and the polymer/liquid crystal interface is an important factor for controlling PDLC electro-optic properties. The present system gives an opportunity to investigate by scanning electron microscopy (SEM) the droplet microstructure (isotropic-cholesteric interface, fingerprint patterns or defects), whereas previous SEM studies were focused on the shape and size of empty cavities, since the fluid liquid crystal was inevitably removed from the PDLC system.

Periodic Structures in Induced Films of Tilted Smectics

Surface freezing commonly induces smectic films at the free surface of isotropic droplets close to the isotropic to smectic transition. In the case of films of tilted smectics they generally exhibit stable stripes between crossed polarizers. We show that these stripes correspond to an in-plane continuous rotation of the director, equivalent to a set of parallel disclination walls and which is different from the frequently reported 1D array of defects. On cooling down the isotropic droplet, the disclination walls are observed to form as early as the first smectic layer, one after the other one, from the edge of the film. Then, on decreasing temperature again, the number of the smectic layers increases in the film. During this process the number of disclination walls is kept constant due to anchoring effects onto the borders of the film. We perform such observations on two different non-chiral compounds leading respectively to smectic CA (or smectic O) and smectic C films. This allows us to exclude chirality effects in the physical mechanism for the disclination walls formation.

Cholesteryl ester hydrolysis in J774 macrophages occurs in the cytoplasm and lysosomes

The relationship of cholesteryl ester hydrolysis to the physical state of the cholesteryl ester in J774 murine macrophages was explored in cells induced to store cholesteryl esters either in anisotropic (ordered) inclusions or isotropic (liquid) inclusions. In contrast to other cell systems, the rate of cholesteryl ester hydrolysis was faster in cells containing anisotropic inclusions than in cells containing isotropic inclusions. Two contributing factors were identified. Kinetic analyses of the rates of hydrolysis are consistent with a substrate competition by co-deposited triglyceride in cells with isotropic inclusions. In addition, hydrolysis of cholesteryl esters in cells with anisotropic droplets is mediated by both cytoplasmic and lysosomal lipolytic enzymes, as shown by using the lysosomotropic agent, chloroquine, and an inhibitor of neutral cholesteryl ester hydrolase, umbelliferyl diethylphosphate. In cells containing anisotropic inclusions, hydrolysis was partially inhibited by incubation in media containing either chloroquine or umbelliferyl diethylphosphate. Together, chloroquine and umbelliferyl diethylphosphate completely inhibited hydrolysis. However, when cells containing isotropic inclusions were incubated with umbelliferyl diethylphosphate, cholesteryl ester hydrolysis was completely inhibited, but chloroquine had no effect. Transmission electron microscopy demonstrated a primarily lysosomal location for lipid droplets in cells with anisotropic droplets and both non-lysosomal and lysosomal populations of lipid droplets in cells with isotropic droplets. These results support the conclusion that there is a lysosomal component to the hydrolysis of stored cholesteryl esters in foam cells.—Avart, S. J., D. W. Bernard, W. G. Jerome, and J. M. Glick. Cholesteryl ester hydrolysis in J774 macrophages occurs in the cytoplasm and lysosomes. J. Lipid Res. 1999. 40: 405–414.

Light scattering from acrylate-based polymer dispersed liquid crystals: theoretical considerations and experimental examples

A light scattering (LS) study made using acrylate-based polymer dispersed liquid crystals (PDLCs) is presented. The polarized component IVV is measured for blends of a polyacrylate and the liquid crystal (LC) E7 at several compositions. Only the off-state configuration of the droplets with no external fields is considered here. These composites consist commonly of micron-sized nematic LC droplets dispersed in a solid polymer matrix. Theoretical expressions for the scattered intensities in the case of isotropic and anisotropic spherical droplets are given both in the Rayleigh-Gans approximation (RGA) and in the anomalous diffraction approximation (ADA). Series of VV and VH components of the scattering intensities are calculated using the models of Meeten, Stein and coworkers. The model calculations are compared with the light scattering data. This comparison enables us to extract information on the size and the shape of droplets assuming that the size distribution is uniform and that the scattering is due to single droplets, neglecting inter-particle correlation and multiple scattering effects. This paper demonstrates that the LS technique is a useful tool for studying the morphology of PDLC samples and estimating the average size of nematic droplets.

Electro-optic control of non-linear optical effects in twisted nematic liquid crystal cells

We study the consequences of the application of an electric field on the non-linear transparency effects given by the light-induced creation of an isotropic droplet in a twisted nematic liquid crystal cell bounded by two dichroic polarizers. The possibility of voltage control is shown. A simple mathematical model is presented for the description of the phenomenon. Experimental results are presented and discussed by comparison with theoretical predictions.

Anomalous anisotropy of the elastic constants in the induced smectic O film

Abstract Surface tension is known to induce smectic O (SO) films at the free surface of isotropic droplets of 1-methylheptyl terephthalylidene-bis-4-aminocinnamate (MHTAC). The SO film has its molecules disposed in a herringbone fashion, with the layers parallel to the free surface. It constitutes a model of a 2D polar nematic liquid crystal without any contact to solid substrates. The film is oriented uniformly when applying a uniform electric field, except along disclination walls. By measuring the width of the disclination walls in the two configurations with the electric field parallel and perpendicular, we determine the anisotropy of the elastic constants in the induced So films. We find the ratio of the 2D bend to splay elastic constants K b/K s to be ≃ 0.25 which is anomalously small when compared to the corresponding 3D ratio (K 2 + K 3)/2K 1 usually found for the bulk nematic phase. This experimental result is comparable to measurements performed with 2D suspended Sc films with, however, a different physics.

Anchoring strength onto simple surface edge dislocations in an induced smectic-O film.

Surface freezing is able to produce thick smectic-O films at the free surface of isotropic droplets of 1-(methyl)-heptyl-terephtalidene-bis-amino cinnamate which are naturally free from solid contacts. This particular situation allows us to realize a microbalance for determining the anchoring conditions of the director onto the surface edge dislocations. The director of the film is thus found to be anchored at normal angle to the simple edge dislocations with an anchoring constant per unit length of dislocation independent of the film thickness: ${\mathit{k}}_{\mathit{a}}$\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}14}$ N. This value corresponds to a maximum anchoring energy \ensuremath{\sim}1/25${\mathit{k}}_{\mathit{B}}$T per molecule of the dislocation line.

Modulation of the physical state of cellular cholesteryl esters by 4,4'-(isopropylidenedithio)bis(2,6-di-t-butylphenol) (probucol).

The effect of 4,4'-(isopropylidenedithio)bis(2,6-di-t-butylphenol) (probucol) on cholesteryl ester physical state was examined in dry mixtures, phospholipid-containing dispersions, and cells. Probucol has little effect on the solid to isotropic transition of cholesteryl oleate, but broadens and decreases the enthalpy of the liquid-crystalline transitions at concentrations as low as 1-2 mol %. A probucol transition is only observed at concentrations greater than 20 mol %. The mesomorphic phases of the cholesteryl oleate/probucol mixtures were identified by visual inspection and polarized light microscopy. Mixtures are liquid at probucol concentrations in excess of 5 mol % at 37 degrees C. Probucol also dramatically reduces the enthalpy of the liquid-crystalline transitions of the cholesteryl oleate core of dispersions of the ester with phospholipids at a concentration of 10 mol %, reducing the enthalpy by greater than 80% and the transition temperatures by approximately 2 degrees C. The phase state of cholesteryl esters in Fu5AH rat hepatoma cells was examined after incubation with cholesterol/phospholipid dispersions that caused the accumulation of anisotropic cholesteryl ester droplets. Differential scanning calorimetry scans of cells incubated with cholesterol-rich phospholipid dispersions indicated a phase transition near 48 degrees C, which was abolished when the cells were co-incubated with 50-100 micrograms/ml of probucol in the loading medium. Subsequent to the formation of isotropic cholesteryl ester droplets in the presence of probucol, the rate of efflux of cholesterol from the cells to phosphatidylcholine-containing acceptors in the medium was increased. These data show that probucol is relatively soluble in cholesteryl esters and that probucol changes the phase state of cholesteryl ester droplets in cells to a more fluid phase in which the cholesteryl esters are more readily mobilized.

Antiferroelectric chiral smectic-O* liquid crystal.

A macroscopic study of chiral smectic-${\mathit{O}}^{\mathrm{*}}$ films, which float at the free surface of isotropic droplets of 1(methyl)-heptyl-terephtalidene-bis-amino cinnamate, is presented. It allows us to directly prove antiferroelectricity with in-plane polarization in the smectic-${\mathit{O}}^{\mathrm{*}}$ phase, and to confirm the herringbone structure of this original phase. The bulk antiferroelectric polarization is measured to be about 0.1 D per molecule.

Nucleation in liquid crystals and liquid crystal polymers

A nematic mixture that undergoes a first-order phase transition to an ordered state is considered. The dynamics of the phase separation in terms of a theory based on a time-dependent Landau–Ginzburg equation is described. The laws of growth and dissolution for isotropic and anisotropic droplets are discussed. An example is given for a bidisperse and a polydisperse (liquid crystal polymer) sample. The possibility of spinodal decomposition in these systems is also explored.

Self-transparency effect in a twisted nematic liquid crystal cell

In a twisted nematic liquid crystal cell we report and study the self-transparency effect induced by the creation of an isotropic droplet. A simple theoretical model is presented and good agreement with experimental data is found. In a two beams configuration transistor-like behavior is observed and theoretically discussed.

Study of thermally induced optical bistability in a twisted nematic liquid crystal

We report the observation and study of optical bistability in the power transmitted by a twisted nematic liquid crystal bounded by crossed dichroic polarizers. A detailed study of the dependence of the phenomenon on the sample initial temperature has been performed. The effect is a good test for a simple theoretical model which describes the isotropic droplet formation in a nematic sample under laser illumination. A good agreement is found between experimental results and theoretical predictions. Applications of the effect to realize temperature sensors are envisaged.

Solubility of heat-resisting polymers in mesophase pitches at their spinning temperatures

Solubilities of two thermoresisting polymers (polyethylene telephthalatep-hydroxybenzoate, a liquid crystal polymer and polyethylene naphthalate) in mesophase pitches (MP) derived from coal tar (C1) and petroleum residues (P1 and P2) were examined to prepare blended fibre as the better precursor for the high-performance carbon fibre. MP-C1 and MP-P1 of high aromaticity dissolved 5 wt% of both polymers on mixing at 360 °C for 3 h, maintaining 100 vol% anisotropy which the parent mesophase pitches exhibited, although only MP-C1 did so at 340 °C. In a marked contrast, a number of isotropic droplets from both polymers dispersed in MP-P2 after the blending under the same conditions. The blends of MP-C1 with polymers were spinnable at around 370 °C into fibres of 10 to 20 μm diameter although their spinnability was slightly inferior to that of the pitch alone. The blended fibre exhibited slightly higher stabilization reactivity at 270 to 300 °C than that of the parent mesophase pitch fibre. Structural factors of the mesophase pitch which influence their dissolving ability are discussed. Thermal stability of the polymers is also briefly examined.

Metabolism of cholesteryl ester lipid droplets in a J774 macrophage foam cell model

J774 macrophages rapidly incorporated [ 3H]cholesteryl oleate droplets by a non-saturable phagocytic process. In less than 2 h, foam cell morphology was acquired. The extent of loading obtained after 2 h was a linear function of the mass of cholesteryl oleate provided to the cells. The cholesteryl oleate incorporated was hydrolyzed in the cells at a linear rate over 24 h and the fractional hydrolysis was constant over a wide range of cellular esterified cholesterol contents. The rate of hydrolysis was influenced by the physical state of the cholesteryl ester; cholesteryl oleate in isotropic droplets was hydrolyzed 2–3-fold more rapidly than cholesteryl oleate in anisotropic droplets. The hydrolysis of both types of droplets was inhibited by lysosomotropic agents, indicating that hydrolysis occurred in the lysosomes. Only a small fraction (less than 10% after 24 h) of the free [ 3H]cholesterol generated in the lysosomes was esterified by ACAT resulting in a doubling of the cell free cholesterol content. Electron microscopy of cells treated with digitonin revealed the accumulation of free cholesterol in lipid-laden lysosomes. ACAT was active as endogenous free [ 14C]cholesterol was esterified in a linear manner over 24 h and was responsive to the presence of lysosomally-derived cholesterol, as the extent of esterification of the endogenous pool was directly proportional to the mass of [ 3H]cholesterol generated in the lysosomes.

Surface frustration and texture instability in smectic-A liquid crystals.

Uniform smectic-A plates, floating freely on top of isotropic droplets, show static instabilities. Above a thickness threshold, the stable state of the smectic plate is an hexagonal network of focal conics. This instability is quantitatively explained by the anisotropy of surfaces tensions, which favors the alignment of smetic layers parallel to the air interface and perpendicular to the isotropic phase. This intrinsic frustration seems general for most smectic-A\ensuremath{\rightarrow}isotropic first-order transitions.

The Interfacial Free Energy of Nematogen Droplets in an Isotropic Matrix: Determination of its Temperature Dependence from Coalescence Kinetics

Abstract We have determined the surface free energy, [sgrave], at the interface between a nematogen and an isotropic liquid by measurement of the time constant for coalescence of nematogen droplets imbedded in the matrix. The basis for this technique is an analysis developed by Frenkel in 1945. The major conclusions of the present work are four-fold: 1) the magnitude of the interfacial free energy is on the order of 10–2 to 10–1 dyne/cm; 2) the temperature dependence of [sgrave] for nematic droplets shows a greater negative slope than for isotropic droplets; 3) the accuracy of the method is insufficient to detect a discontinuity in [sgrave] at the nematic to isotropic transition temperature; and 4) the interfacial free energy decreases toward zero as the temperature increases toward that at which the droplets dissolve in the matrix. This last behavior was expected by analogy with the Eotvos expression for ordinary liquid/vapor surface free energies. The droplet coalescence technique shows promise as a means for determining temperature-dependent interfacial free energies for other liquid/liquid systems.