60 research outputs found
An experimental study of adsorption interference in binary mixtures flowing through activated carbon
The isothermal transmission through activated carbon adsorber beds at 25 C of acetaldehyde-propane and acetylene-ethane mixtures in a helium carrier gas was measured. The inlet concentration of each component was in the range between 10 ppm and 500 ppm. The constant inlet volumetric flow rate was controlled at 200 cc (STP)/min in the acetaldehyde-propane experiments and at 50 cc (STP)/min in the acetaldehyde-ethane experiments. Comparison of experimental results with the corresponding single-component experiments under similar conditions reveals interference phenomena between the components of the mixtures as evidenced by changes in both the adsorption capacity and the dispersion number. Propane was found to displace acetaldehyde from the adsorbed state. The outlet concentration profiles of propane in the binary mixtures tend to become more diffuse than the corresponding concentration profiles of the one-component experiments. Similar features were observed with mixtures of acetylene and ethane; however, the displacement of acetylene by ethane is less pronounced
Molecular simulation of hierarchical structures in bent-core nematics
The structure of nematic liquid crystals formed by bent-core mesogens (BCMs)
is studied in the context of Monte Carlo simulations of a simple molecular
model that captures the symmetry, shape and flexibility of achiral BCMs. The
results indicate the formation of (i) clusters exhibiting local smectic order,
orthogonal or tilted, with strong in-layer polar correlations and
anti-ferroelectric juxtaposition of successive layers and (ii) large homochiral
domains through the helical arrangement of the tilted smectic clusters, whilst
the orthogonal clusters produce achiral (untwisted) nematic states.Comment: 14 pages, 2 figure
Tilt order parameters, polarity and inversion phenomena in smectic liquid crystals
The order parameters for the phenomenological description of the smectic-{\it
A} to smectic-{\it C} phase transition are formulated on the basis of molecular
symmetry and structure. It is shown that, unless the long molecular axis is an
axis of two-fold or higher rotational symmetry, the ordering of the molecules
in the smectic-{\it C} phase gives rise to more than one tilt order parameter
and to one or more polar order parameters. The latter describe the indigenous
polarity of the smectic-{\it C} phase, which is not related to molecular
chirality but underlies the appearance of spontaneous polarisation in chiral
smectics. A phenomenological theory of the phase transition is formulated by
means of a Landau expansion in two tilt order parameters (primary and
secondary) and an indigenous polarity order parameter. The coupling among these
order parameters determines the possibility of sign inversions in the
temperature dependence of the spontaneous polarisation and of the helical pitch
observed experimentally for some chiral smectic-{\it } materials. The
molecular interpretation of the inversion phenomena is examined in the light of
the new formulation.Comment: 12 pages, 5 figures, RevTe
Symmetries and alignment of biaxial nematic liquid crystals
The possible symmetries of the biaxial nematic phase are examined against the
implications of the presently available experimental results. Contrary to the
widespread notion that biaxial nematics have orthorhombic symmetry, our study
shows that a monoclinic () symmetry is more likely to be the case for
the recently observed phase biaxiality in thermotropic bent-core and calamitc
tetrapode nematic systems. The methodology for differentiating between the
possible symmetries of the biaxial nematic phase by NMR and by IR spectroscopy
measurements is presented in detail. The manifestations of the different
symmetries on the alignment of the biaxial phase are identified and their
implications on the measurement and quantification of biaxiality as well as on
the potential use of biaxial nematic liquid crystals in electro-optic
applications are discussed.Comment: 24 pages, 4 figure
Flexoelectricity and piezoelectricity - reason for rich variety of phases in antiferroelectric liquid crystals
The free energy of antiferroelectric liquid crystal which takes into account
polar order explicitly is presented. Steric, van der Waals, piezoelectric and
flexoelectric interactions to the nearest layers and dipolar electrostatic
interactions to the nearest and to the next nearest layers induce indirect tilt
interactions with chiral and achiral properties, which extend to the third and
to the fourth nearest layers. Chiral indirect interactions between tilts can be
large and induce helicoidal modulations even in systems with negligible chiral
van der Waals interactions. If indirect chiral interactions compete with chiral
van der Waals interactions, the helix unwinding is possible. Although strength
of microscopic interactions change monotonically with decreasing temperature,
effective interlayer interactions change nonmonotonically and give rise to
nonmonotouous change of modulation period through various phases. Increased
enatiomeric excess i.e. increased chirality changes the phase sequence.Comment: 4 pages, 1 figur
Resistivity Measurements on Aligned Amphiphilic Liquid Crystalline States
[[abstract]]We measured the resistivity of a ternary liquid mixture consisting of decylammoniumchloride, ammoniumchloride and water (45:5:50 wt%). The mixture forms a nematic phase between 41°C and 61°C; below 41°C a neat soap, and above 61°C an isotropic micellar solution. The resistivity decreases with increasing temperature. In the nematic and in the neat soap phases the resistivity is anisotropic, and it is higher for currents parallel to the director. The anisotropy increases strongly with decreasing temperature, due to a much stronger temperature dependence of the resistivity parallel to the director. The changes accompanying the phase transitions are small. The surprisingly small change of the resistivity parallel to the director at the nematic to neat soap transition indicates that the lamellae in the neat soap contain a large number of defects.[[notice]]補正完畢[[journaltype]]國外[[booktype]]紙本[[countrycodes]]US
AN EXTENSION OF THE LENNARD-JONES AND DEVONSHIRE MODEL TO LIQUID CRYSTALLINE PHASES
Une théorie à pression constante sur la fusion est présentée. Cette théorie prédit une phase caractérisée par un ordre translationnel dans une dimension. Des théories semblables ont été déjà développées par un réseau incompressible. Ce modèle, qui est une extension de celui présenté par Lennard-Jones et Devonshire, utilise un double réseau associé à deux réseaux conjugués, ce qui donne deux modes de fusion. Le premier correspond à la fusion dans les plans, le second à la fusion smectique. La distinction entre les deux modes est attribuée à l'ordre orientationnel à large distance. Le dernier n'est pas inclus directement dans le modèle statistique. Un terme quadratique représentant l'ordre orientationnel est ajouté à l'expression de l'énergie libre. Les valeurs du parametre d'ordre nématique sont obtenues à partir d'une équation de self-consistance. En utilisant les facteurs de compressibilité tabulés on obtient les isothermes pour discuter les transitions de phase cristal-nématique et smectique-nématique.A constant pressure theory of melting is presented which predicts a phase characterized by translational order in one direction. Similar theories have been already developed for an incompressible lattice. The present model, which is an extension of the one developed by Lennard-Jones and Devonshire, uses a twin lattice, along with the conjugate lattices, and hence allows for two modes of melting. One mode corresponds to the melting within the planes, the second represents the smectic melting. The distinction between the two modes is attributed to the long-range orientational order. The latter is not included directly in the statistical model ; instead, a quadratic term representing the orientational order is added to the expression for the free energy. The values of the nematic order parameter are derived from a separate self-consistency equation. Using tabulated compressibility factors we obtain the isotherms, which are used to discuss the crystal to smectic and smectic to nematic phase transitions
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