P. Barois

Colloidal ordering from phase separation in a liquid-crystalline continuous phase

Some binary mixtures exist as a single phase at high temperatures and as two phases at lower temperatures; rapid cooling therefore induces phase separation that proceeds through the initial formation of small particles and subsequent growth and coarsening. In solid and liquid media, this process leads to growing particles with a range of sizes, which eventually separate to form a macroscopically distinct phase. Such behaviour is of particular interest in systems composed of an isotropic fluid and a liquid crystal, where the random distribution of liquid-crystal droplets in an isotropic polymer matrix may give rise to interesting electro-optical properties. Here we report that a binary mixture consisting of an isotropic fluid and a liquid crystal forming the continuous phase does not fully separate into two phases, but self-organizes into highly ordered arrays of monodisperse colloidal droplet chains. We find that the size and spatial organization of the droplets are controlled by the orientational elasticity of the liquid-crystal phase and the defects caused by droplets exceeding a critical size. We expect that our approach to forming monodisperse, spatially ordered droplets in liquid crystals will allow the controlled design of ordered composites that may have useful rheological and optical properties.

Mesomorphic and Electro-Optical Properties of New Achiral Banana-Shaped Molecules

Abstract Two new series of achiral banana-shaped molecules have been prepared. Their mesomorphic, electrooptical and structural properties have been investigated. The first series of hydrogenated molecules shows the nematic phase with short chains (n=8–10), N and SmC phases with intermediate chain lengths (n = 11–13) and SmC plus an unknown smectic phase with long chains. The second series of fluorinated molecules is more peculiar, exhibiting four disordered smectic phases. Two of these have broken fan-shaped or schlieren optical textures like the SC phase, whereas the two others show slightly different textures with more or less mosaic domains. These phases display various switching characteristics, including but not limited to the “antiferroelectric” one reported for the Niori series. The relation between molecular structure and mesomorphic properties is discussed.

Blue phases and twist grain boundary phases (TGBA and TGBC) in a series of fluoro-substituted chiral tolane derivatives

A series of fluoro-substituted tolane derivatives: (R)-1-methylheptyl 3-fluoro-4-(3-fluoro-4- n alkoxybenzoyloxy)tolane-4-carboxylates is reported. Some members of this series exhibit the phase sequence: Cr-SmC*-TGBC-TGBA-BPI-BPII-BPIII-I. The blue phases, the TGBA and TGBC phases and the SmC* phase were characterized in detail by microscopic observation, differential scanning calorimetry, helical pitch measurements, X-ray structural analysis and electro-optical study. The blue phases directly next to the TGBA phase were shown to be a new type of blue phase exhibiting smectic ordering. A commensurate TGBC phase with constant number of slabs per pitch was observed.

Topological Darkness in Self‐Assembled Plasmonic Metamaterials

Self-assembled plasmonic metamaterials are fabricated from silver nanoparticles covered with a silica shell. These metamaterials demonstrate topological darkness or selective suppression of reflection connected to global properties of the Fresnel coefficients. The optical properties of the studied structures are in good agreement with effective medium theory. The results suggest a practical way of achieving high phase sensitivity in plasmonic metamaterials.

Synthesis, characterization and crystal structures of two polyphilic mesogenic compounds

Abstract Three homologous series of semi-perfluorinated liquid crystals: 4-(2,2,3,3,4,4,4-heptafluoro-butyloxycarbonyl)phenyl, 4-(2,2,3,3,4,4,5,5-octafluoropentyloxycarbonyl)phenyl and 4-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyloxycarbonyl)phenyl 4-[(n-perfluoroalkyl)alkyloxy]benzoates have been synthesized. Their mesomorphic properties were studied by DSC, microscopic observation and X-ray diffraction. Their layer spacings are smaller than the molecular length (L). The ratio d SA/L is about 0.7. The crystal structure of two derivatives of the first family have also been investigated. In both structures the molecules pack in smectic C-like sheets where neighbouring molecules are antiparallel, interacting through dipolar and van der Waals forces. The interactions between contiguous sheets, through the ends of perfluorinated chains are very weak. The X-ray diffraction results on the mesophases and on the crystalline structures of two compounds of the first family are compared. From this, we propose a mo...

Smectic A twist grain boundary phase in three new series with chiral (L) lactic acid derivatives

Abstract Three new series with a tolane core and a chiral L-lactic acid moiety have been synthesized. The theoretically predicted SA-TGBA-N∗ phase sequence is found in several compounds with short chains. The smectic A twist grain boundary phase (TGBA) obtained with a cholesteric texture, is characterized by different techniques. Helical pitch measurements were performed using Grandjean-Cano wedges or homeotropic drops. All pitch values are lower than those found in previously reported series exhibiting the TGBA phase. Ferroelectric smectic C (S∗C) phases appear with long chains and have striated fan-shaped or pseudo-homeotropic textures. Electro-optical properties of the S∗C phase were studied with the classical SSFLC geometry. We observed high polarization, slow response time and a saturated tilt angle of 20°. These results are comparable with previous studies.

New Chiral Tolane Series with Antiferroelectric Properties

Abstract A new series of chiral tolane with aliphatic end chains ranging from heptyloxy to dodecyloxy has been synthesized and characterized. The mesomorphic properties have been analyzed by optical microscopy on pure compounds and mixtures, D.S.C, electro-optical and pitch measurements. This series displays a very rich polymesomorphism including SA, SC*α, SC*, SC*FI, SC*A, SI*A, and SJ* phases with reference to the compound MHPOBC through miscibility test. One of the characteristic of this sene is the two SC*FI phases (SC*FI1 and SC*FI2) between SC* and SC*A phases. The electro-optical measurements were carried out with the classical SSFLC geometry. The threshold field is measured in each tilted mesophase; this one is peculiarly high in SJ*, SI*A and SC*A phase (respectively more than 6; 4 and 1V/μm). In the SC*FI phase one can note a first plateau between 0.3V/μm an 0.7V/μm, and the saturation is reached at 1V/μm. Helical pitch measurement shows a strong variation of the pitch at the SC*A-SC*FI and SC*F...

New Achiral Non Symmetric Banana-Shaped Mesogens: Mesomorphic and Electro-Optical Properties

Abstract The first series of achiral non symmetric banana-shaped mesogens has been synthesized. The mesophases observed for these compounds depend on the whole number of carbons in the alkyl chains (N). They were investigated by optical microscopy, calorimetry, X-ray diffraction experiments and electro-optical measurements. For intermediate and long chains (N ≥ 20) the liquid crystal phase is smectic (Sm?) and for shorter chains (N < 20) the mesophase appears to be columnar ordered (Col?). Yet the structure of neither one is fully resolved. Both phases exhibit a ferroelectric behaviour.

SC∗A and S[Ctilde]∗ phases in chiral non-symmetric dimesogens

Abstract We have synthesized two separate homologous series of chiral non-symmetric dimesogens: these have a two phenyl ring moiety bearing a terminal alkoxy chain linked via a methylenic spacer to a chiral three phenyl ring mesogen which itself displays antiferroelectric properties. Depending on the parity of the number of carbons constituting the spacer, two kinds of mesomorphic behaviour have been found. When the spacer is even, various Sc∗ phases (Sc∗A, Sc∗Fi, Sc∗ Sc∗α) have been identified without doubt, whereas when the spacer is odd, a tilted modulated phase (S[ctilde]∗) has been observed and characterized. In each case, we study the effect of the length of the alkoxy chain at the end of the shorter moiety. The alternation of the mesomorphic behaviour with the spacer parity has been interpreted in terms of molecular shape, but we have also discussed the influence of some chemical parameters on the stability of the Sc∗A phase.

Bottom-up fabrication and optical characterization of dense films of meta-atoms made of core-shell plasmonic nanoparticles.

In an attempt to fabricate low index metamaterials by a bottom-up approach, meta-atoms constituted of silica-coated silver nanoparticles are assembled by a Langmuir-Schaefer technique into thin films of large area and well-controlled thickness. The silica shells ensure a constant distance between the silver cores, hence providing a constant coupling of the localized surface plasmon resonance (LSPR) of the nanoparticles in the assembled composite material. The optical response is studied by normal angle spectral reflectance measurements and by variable angle spectroscopic ellipsometry. The normal incidence data are described well in the framework of a single effective Lorentz oscillator model. The resonance of the assembled material is blue-shifted and shows no significant broadening with respect to the absorption band of the individual nanoparticles. The observation of these two effects is enabled by the core-shell structure of the meta-atoms that prevents aggregation of the metallic cores. The ellipsometry study confirms the general behavior and reveals the natural birefringence of the few-layer materials. The amplitude of the observed resonance is weaker than expected from the Maxwell-Garnett mixing rule. This well-characterized system may constitute a good model for numerical simulations.