We have studied the electro-optical properties of the 4-pentyl-4-biphenylcarbonitrile nematic liquid crystal also known as 5CB doped by 0.5%wt of pure and cobalt-incorporated TiO(2 )nanoparticles. The obtained results show that the effect of the NPs on the electrooptical properties is mainly due to the modification of the dielectric and the viscoelastic properties of the nematic in the presence of the NPs. Furthermore, the nematic isotropic transition temperature of all doped samples remained almost unchanged. This indicates that the order parameter was almost unchanged too. A significant decrease in the dielectric anisotropy and the splay elastic constant was observed. The rotational viscosity has been deduced from response time data; it was found that rotational viscosity shows a significant increase for cobalt-incorporated TiO2 . However, the rotational viscosity was decreased for pure TiO2 nanoparticles compared to pure 5CB.

A new series of hydrogen-bonded liquid crystals was prepared by mixing 4-n-alkoxy-2.3-difluorobenzoic acid and 4-pentyl-4-biphenylcarbonitrile (5CB). Thermal and liquid crystal behaviour was established using differential scanning calorimetry and polarizing optical microscopy. The influence of terminal acid donors on mesogenic behaviour was discussed in detail. It was found that all complexes with a (1:1) molar ratio exhibit the smectic B phase. In addition, the binary phase diagram that has been established revealed that the mixtures with lower concentration exhibit a nematic phase at room temperature with large stability. Dielectric, electro-optic and viscoelastic properties of 5CB/9OBAFF with molar fraction x = 0.1 were discussed.

ABSTRACT Nematic phase at room temperature with wide stability and high birefringence is particularly desirable for functional devices. Two thermotropic liquid crystals (tolane and the 4-pentyl-4-biphenylcarbonitrile nematic also known as 5CB) are mixed by varying the weight fraction (wt%) in order to investigate the phase sequences and their physical properties using differential scanning calorimetry (DSC), polarised optical microscopy (POM) and dielectric spectroscopy techniques. Phase coexistence was observed for a concentrations 20 ≤ C ≤ 60 wt% of tolane in 5CB and the phase diagram was established. In addition, the nematic-isotropic temperature TNI increases with increasing the concentration of tolane LCs. A eutectic mixture at room temperature with large nematic range and high birefringence is obtained for C = 10 wt%. The increase in the nematic range is due to an enhancement of the nematic order induced by the LCs. It is found that the electro optical parameters like the threshold voltage, the response time were relatively increased. This increment can be attributed to an increase in the viscoelastic properties of the mixture. Graphical abstract

In the present investigation, we have undertaken photoluminescence, UV-Vis absorbance, transmission and polarised light microscopy of the composites of a cholesteric liquid crystal (CLC) doped with quantum dots (QDs). The locally formed QDs clusters may distort the CLC's helix to induce an imperfect planar texture and cause broadening of CLC's photonic band gap. This broadening is accompanied by a change of the wavelength positions of the central and both the long and short band-edges of the photonic band gap as the concentration of the doped QDs increases. A remarkable enhancement in the photoluminescence intensity of CLC materials is observed by doping with QDs. A decrement in the FWHM parameter also supports this increased photoluminescence intensity. The UV absorbance increases for the QDs dispersed CLC material and is followed by a slight red shift when compared to the pure CLC. The optical band gap obtained by the Tauc plot method suggests that the observed optical band gap narrows after the dispersion of QDs into the CLC. The outcome of these investigations proves that the dispersion of QDs in CLC is beneficial and may be useful in liquid crystal displays and other opto-electronic devices, which require less band gap materials.

Organizing nanoparticles in a controlled way allows us to monitor their optical properties. It is particularly interesting to organize them on top of liquid crystal films to take advantage, in a second step, of the easy actuation of liquid crystals with external parameters such as temperature, electric fields, and so forth. We show that despite their fluidity, nematic and smectic films allow the formation of well-ordered hexagonal domains of gold spherical nanoparticles (AuNPs) at their surface, but we also show that both nematic films and AuNP domains impact each other. Using optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), and spectrophotometry, we compare nematic, polymer-stabilized nematic, and smectic films with AuNP domains made of NPs of diameter 6 nm. On the liquid crystal films, depressions are revealed below the AuNP domains, whereas the AuNP domains appear well-organized but with a hexagonal period shortened with respect to AuNP monolayers formed on hard substrates. We interpret these features by the anchoring tilt imposed by the AuNP domains on the liquid crystal molecules. The smectic-A layers characteristic of the nematic surface transform into smecticC layers, which induce the formation of depression. The energy penalty associated with the local smectic-A/smectic-C transition induces the shortening of the AuNP domain period in order to decrease the AuNP domain surface. The observed large depth of the polymer-stabilized nematic depressions below AuNP domains may be explained either by an increased size of the polymer-stabilized smectic layers close to the surface or by an increased number of polymer-stabilized smectic liquid crystal smectic layers close to the surface with respect to pure nematic films.

We investigated the impact of two types of nanoparticles, magnetic (gamma Fe2O3) and nonmagnetic one (CeO2) dispersed in nematic 5CB matrix. Dielectric spectroscopy was used to investigate the dielectric behavior of the nematic 5CB doped by both kinds of nanoparticles. By fitting a suitable equivalent electric circuit model to the experimental data we show the change of electro-physical parameters of the nematic in the bulk and at the nematic-electrode interface. It is found that the impedance modulus decreases with increasing the concentration of nanoparticles. This increase is attributed to an increase of the conductivity, which is related to the mobile charge carriers accumulating near the electrodes. For nonmagnetic nanoparticles, both the diffusion constant D and the mobility mu were increased with increasing concentration. Surprisingly, for magnetic nanoparticles, the increase of D and mu is followed by a considerable decrease at concentration higher than 3%.

In this article we show how spherical nanoparticles (NPs) imposing planar anchoring can strongly impact the viscoelastic, dielectric, and electro-optical properties of a nematic liquid crystal when they are not aggregated. We also demonstrate that when the NPs are magnetic, most nematic properties are more impacted than when they are nonmagnetic. With magnetic NPs a molecular disorder is induced that decreases the nematic order parameter, this decrease impacting the values of elastic constants, viscosity, and response time. The impact on 5CB liquid crystal (LC) has been investigated with spherical nanoparticles (NPs) of identical size around 6 nm, magnetic (γFe_{2}O_{3}), and nonmagnetic (CeO_{2}) ones that are both surface functionalized by poly(aminopropylmethylsiloxane-b-dimethylsiloxane) (PAPMS-b-PDMS) block copolymer ligands to promote planar anchoring. In the presence of nonmagnetic NPs, despite an almost constant nematic order parameter, a significant decrease of elastic constants (25.4%), viscosity (22%), and response time (23%) is measured. It suggests a dilution effect for the intermolecular interactions in the presence of NPs. This hypothesis is supported by the observation of an enhanced decrease of the same nematic parameters in the presence of magnetic NPs that can be fully explained by the corresponding order parameter decrease. This finally leads to a remarkable decrease of the splay elastic constant by 51% in the presence of magnetic NPs. The decrease of the nematic order parameter by 18% in the presence of magnetic NPs demonstrates that the NP magnetic moments are only weakly coupled to the nematic director and consequently only induce a disorder in the composite system. A significant influence of the expected large LC structural modifications in the presence of magnetic NPs is, however, shown by a particularly large increase of the diffusion coefficient 43% and large decrease of the dielectric anisotropy (43%). We believe that the observed impact of NPs with planar anchoring on nematic properties could be extended to most spherical NPs if their aggregation can be avoided. In particular, the difference between magnetic and nonmagnetic NPs could be extended to ferroelectric and nonferroelectric NPs.

We investigated dielectric and viscoelastic properties of the 4-pentyl-4-biphenylcarbonitrile nematic liquid crystal also known as 5CB doped by 0.5%wt of pure and copper-incorporated TiO(2)nanoparticles. Optical microscopy observations showed uniform copper-incorporated TiO(2)nanoparticles dispersion into the nematic 5CB while we have seen nanoparticles aggregations for pure TiO2. Further, except the pure TiO(2)nanoparticles, the nematic isotropic transition temperature of all doped samples slightly increased by about 1.4-2.4 degrees C as compared to the pure 5CB. This indicates that the order parameter was increased. Significant decrease in the dielectric anisotropy, response time, splay elastic constant and threshold voltage was observed for all samples. The rotational viscosity has been deduced from response time data, it was found that rotational viscosity shows a slight increase with respect to pure 5CB for copper-incorporated TiO(2)and a clear decreasing for pure TiO(2)nanoparticles.

A long time ago, Brochard and de Gennes predicted the possibility of significantly decreasing the critical magnetic field of the Fredericksz transition (the magnetic Fredericksz threshold) in a mixture of nematic liquid crystals and ferromagnetic particles, the so-called ferronematics. This phenomenon is rarely measured to be large, due to soft homeotropic anchoring induced at the nanoparticle surface. Here we present an optical study of the magnetic Fredericksz transition combined with a light scattering study of the classical nematic liquid crystal: the pentylcyanobiphenyl (5CB), doped with 6 nm diameter magnetic and nonmagnetic nanoparticles. Surprisingly, for both nanoparticles, we observe at room temperature a net decrease of the threshold field of the Fredericksz transition at low nanoparticle concentrations, which appears associated with a coating of the nanoparticles by a brush of polydimethylsiloxane copolymer chains inducing planar anchoring of the director on the nanoparticle surface. Moreover, the magnetic Fredericksz threshold exhibits nonmonotonic behavior as a function of the nanoparticle concentration for both types of nanoparticles, first decreasing down to a value from 23% to 31% below that of pure 5CB, then increasing with a further increase of nanoparticle concentration. This is interpreted as an aggregation starting at around 0.02 weight fraction that consumes more isolated nanoparticles than those introduced when the concentration is increased above c=0.05 weight fraction (volume fraction 3.5×10^{-2}). This shows the larger effect of isolated nanoparticles on the threshold with respect to aggregates. From dynamic light scattering measurements we deduced that, if the decrease of the magnetic threshold when the nanoparticle concentration increases is similar for both kinds of nanoparticles, the origin of this decrease is different for magnetic and nonmagnetic nanoparticles. For nonmagnetic nanoparticles, the behavior may be associated with a decrease of the elastic constant due to weak planar anchoring. For magnetic nanoparticles there are non-negligible local magnetic interactions between liquid crystal molecules and magnetic nanoparticles, leading to an increase of the average order parameter. This magnetic interaction thus favors an easier liquid crystal director rotation in the presence of external magnetic field, able to reorient the magnetic moments of the nanoparticles along with the molecules.

Long time ago, Brochard and de Gennes predicted the possibility of significantly decreasing the critical magnetic field of the Fredericksz transition (the magnetic Fredericksz threshold) in a mixture of nematic liquid crystals and ferromagnetic particles, the so-called ferronematics. This phenomenon has rarely been measured, usually due to soft homeotropic anchoring induced at the nanoparticle surface. Here we present an optical study of the magnetic Fredericksz transition combined with a light scattering study of the classical nematic liquid crystal, 5CB, doped with 6 nm diameter magnetic and non-magnetic nanoparticles. Surprisingly, for both nanoparticles, we observe at room temperature a net decrease of the threshold field of the Fredericksz transition at low nanoparticle concentrations, which appears associated with a coating of the nanoparticles by a brush of polydimethylsiloxane copolymer chains inducing planar anchoring of the director on the nanoparticle surface. Moreover the magnetic Fredericksz threshold exhibits non-monotonic behaviour as a function of the nanoparticle concentration for both types of nanoparticles, first decreasing down to a value from 23% to 31% below that of pure 5CB, then increasing with a further increase of nanoparticle concentration. This is interpreted as an aggregation starting at around 0.02 weight fraction that consumes more isolated nanoparticles than those introduced when the concentration is increased above c = 0.05 weight fraction (volume fraction 3.5 × 10−2). This shows the larger effect of isolated nanoparticles on the threshold with respect to aggregates. From dynamic light scattering measurements we deduced that, if the decrease of the magnetic threshold when the nanoparticle concentration increases is similar for both kinds of nanoparticles, the origin of this decrease is different for magnetic and non-magnetic nanoparticles. For non-magnetic nanoparticles, the behavior may be associated with a decrease of the elastic constant due to weak planar anchoring. For magnetic nanoparticles there are non-negligible local magnetic interactions between liquid crystal molecules and magnetic nanoparticles, leading to an increase of the average order parameter. This magnetic interaction thus favors an easier liquid crystal director rotation in the presence of external magnetic field, able to reorient the magnetic moments of the nanoparticles along with the molecules.

1 CNRS, UMR7588, INSP, UPMC Univ Paris 06 PARIS, France 2 Dept. of Physics, Case Western Reserve University, Cleveland, Ohio 44106 USA 3 CNRS, CRPP, avenue du docteur Albert Schweitzer, 33600 Pessac, France 4 ITODYS, Université Denis Diderot-Paris 7, 15, Rue Jean de Baïf, 75205 PARIS cedex 13, France 5 CNR-IPCF, Liquid Crystal Laboratory, Università della Calabria, Rende, Italy 6 IPCMS, Université de Strasbourg, Strasbourg, France

We show that the study of gold nanoparticle self-assemblies induced by a liquid crystal matrix reveals the intimate distorted structure of the liquid crystal existing prior to nanoparticles' incorporation. We also show how this intimate structure controls the spacing between nanoparticles in the self-assemblies. We have created hybrid films of cholesteric liquid crystal (CLC) and gold nanoparticles, the CLC being deformed by competing anchorings at its two interfaces. Whereas previous results have evidenced formation of only slightly anisotropic clusters for large nanoparticles (diameter 20 nm), we now demonstrate for smaller nanoparticles (diameter 4.2 nm) formation of long needles of lengths larger than 50 nanoparticles and widths smaller than 5 nanoparticles, on average oriented perpendicular to the anchoring direction. The difference between the two kinds of nanoparticle aggregations is interpreted by a modification of the balance between aggregation between nanoparticles and trapping by the defects, favoured by the disorder induced by the alkylthiol molecules grafted around the nanoparticles. This leads to a well-defined, anisotropic Localized Surface Plasmonic Resonance (LSPR) of the 4.2 nm embedded nanoparticles. Interpretation of these optical properties using generalized Mie theory allows for a comparison between CLC/gold nanoparticles and the same nanoparticles trapped within smectic topological defects or deposited on the same substrate without a liquid crystal. A smaller spacing between nanoparticles is demonstrated in the CLC system with an attraction between nanoparticles induced by the CLC matrix, related to the additional disorder associated with the nanoparticles' presence. The experimental observations allow us to estimate the disordered size of the liquid crystal shell around the nanoparticles in the CLC to be of some nanometers. They also suggest that the CLC distorted by competing anchorings is characterized by the presence of arrays of defects with topological cores of width smaller than 5 nm that act as efficient anisotropic traps for the nanoparticles.

In the presence of oriented smectic liquid crystal defects, hybrid systems of nanoparticles/liquid crystals form straight chains of nanoparticles of length longer than tens of micrometers and width equal to one single nanoparticle. The interparticle distance in a chain can be varied between a few micrometers and 1.5 nm, highlighting the control of optical absorption by light polarization monitored by gold nanoparticle concentration.

By combining experimental and calculated optical extinction spectra, scanning electron microscopy and optical microscopy, we investigate the optical properties of a hybrid system consisting of a cholesteric liquid crystal and gold nanoparticles. Close to the air-exposed surface, the liquid crystal film exhibits a structural modulation. In this article, we explore how to use this modulation to drive the formation of gold nanoparticle assemblies and thus to control their localized surface plasmon properties. We found that the penetration of the gold nanoparticles within the liquid crystal modulation depends on the initial concentration of nanoparticles in the colloidal solution. Two distinct regimes are pointed out: (i) at low concentration the nanoparticles are weakly interacting and embedded in a homeotropic environment at the film surface and (ii) at high concentration the penetration of the NPs occurs within the modulation, and their localized surface plasmon resonance is strongly red-shifted due to nanoparticles packing and electromagnetic interactions between the nanoparticles. This surface plasmon resonance shows sensitivity to the light polarization attributed to the formation of anisotropic aggregates oriented by the structural modulation of the liquid crystal.

Thermotropic nematic materials relax strong distortions by lowering the nematic order: the uniaxial symmetry is broken and is locally replaced by biaxial domains. We investigated the dynamics of the nematic order near a boundary surface of an asymmetric π-cell submitted to an external electric field, close to the electric order reconstruction threshold. An unexpected phenomenon is observed close, but below the threshold: the biaxial order spreads on the surface inducing a consequent bulk topological behaviour equivalent to the splay-bend fast transition allowed by order reconstruction at higher voltage.

Large-area ordered arrays can be created by the self-assembly of linear defects in thin smectic-A films deposited in air on crystalline substrates. Such structures could find applications in nanoparticle assembly and soft lithography. The smectic layers are bent in concentric cylinders under the effect of conflicting strong anchoring conditions at the substrate surface and free interface, and neighboring cylindrical domains are separated by curvature walls. We have studied the internal structure of such domains near the surface of muscovite mica using synchrotron X-ray diffraction. Our findings show that the domains are centered on virtual singularities, running below the substrate plane, and rest upon a surface region of submicrometric thickness where layers are flat and vertical, satisfying the planar anchoring condition imposed by the substrate.

Investigations on π-cell, a sandwich cell with the director rotating by 180°, demonstrate the possibility to obtain nematic transitions between two textures with different topologies, for instance between an untwisted state and a π-twisted one. These fast textural changes can be obtained by bulk order reconstruction, which allows the director reorientation between two perpendicular directions without macroscopic rotations of the director itself, or by anchoring breaking, which transforms a weak planar anchoring in a homeotropic surface state. Now, we demonstrate that order reconstruction close to a boundary surface with strong or infinite anchoring conditions provides transitions equivalent to anchoring breaking.

Objective. - Compare three ventilatory strategies during the immediate postoperative transfer of cardiac Surgical patient.Study design. - Prospective. comparative and observational Study.Patients and methods. - After approval by our local ethical committee, 330 patients undergoing on-pump cardiac surgery were consecutively included. Patients suffering from chronic obstructive pulmonary disease, exhibiting intraoperative hypoxemia or requiring nitric oxide were excluded. The ventilatory mode was left at the discretion of the anesthesiologist and included: controlled mechanical ventilation (FiO(2) = 1, N = 124) or (FiO(2) = 0.6, N = 106), and Manual ventilation using rebreathing bag (N = 100). A blood gas analysis was performed immediately prior to connecting patient at ventilator at the arrival in ICU.Results. - The mean duration of transfer was 3.9 +/- 1.4 min. Invasive pressure monitoring was used in all patients. The pulse oxymetry and electrocardiogram were respectively used in 78% and 24% of patients. PaO2 values less than 100 mmHg and those more than 300 mmHg were more frequently found in patients ventilated by rebreathing bag (42%) and mechanical ventilation FiO(2) 1 (52%), respectively. No significant difference was found between groups regarding PaCO2 values.Conclusion. - When rebreathing bag is used for transfer in ICU, severe decrease in PaO2 may be observed. In absence of intraoperative hypoxemia. a mechanical ventilation with FiO(2) 0.6 seems to be the most suitable ventilatory strategy for such short immediate postoperative transfer. (C) 2009 Elsevier Masson SAS. All rights reserved.

In this article, we present a precise modeling translating the influence of the ground inertia in the thermal behavior of a greenhouse without vegetation. This work takes into account all the real mechanisms of exchanges (solar conduction, convection, radiations, thermal inertia) between the various elements of the system (cover, interior air, ground), but does not take into account the mass transfers (diffusions of moisture in the ground, evapotranspiration). We sought here to define a model constituting a core of procedure on which new extensions will be based. We show via the Green functions theory that the differential equations of the model are reduced to a system of integral equations on the ground surface. These equations implicitly take into account the heat propagation in the ground. This model carefully describes in detail the exchanges between the ground and the interior of the greenhouse. It also aims at defining the evolution of the greenhouse internal air temperature as well as that of the superficial temperature of its ground according to the following external data (power, exterior temperature). The mathematical study is completed by a digital simulation on an isolated greenhouse.

A two-dimensional model within the Q-tensor description of liquid crystals is used to describe the inhomogeneous order reconstruction in a nematic cell driven by tony modulation in the anchoring conditions. Homogeneous and inhomogeneous reconstruction are contrasted: the former is defectless, the latter is defect mediated. While the transition thresholds are comparable in both cases and in good agreement with experimental data, the biaxial wall breaking is considerably slower in the inhomogeneous transition than in the homogeneous one. The shape of the signal given by the electric current flowing through the cell allows us to distinguish the actual path followed by the transition.