Ferrielectricity in smectic-C

Recent experiments probing a new ferroelectric liquid crystal (CLF08) confined in cells with planar alignment have shown dielectric and optic anomalies suggesting the onset of ferrielectric ordering within the surface lattice of dechiralization lines. We present a phenomenological theory describing the corresponding phase transition sequence SmA→SmC^{*}→Ferri. Phase diagrams and thermodynamic, dielectric, and optic properties are worked out and compared with experiments. The anomalies are related to the predicted tristability of the experimental cells under applied electric field. The order parameters of Landau theory are reinterpreted in terms of line positions, allowing description of the entrance and exit line behavior, and yielding the prediction and identification of new limit phases within a nonconventional Landau approach.

Blood group typing in five Afghan populations in the North Hindu-Kush region: implications for blood transfusion practice.

BACKGROUND AND OBJECTIVES: Blood incompatibility arises from individual and ethnic differences in red blood cell (RBC) antigen profiles. This underlines the importance of documenting RBC antigen variability in various ethnic groups. Central Asia is an area with a long and complex migratory history. The purpose of this article is to describe key antigen frequencies of Afghan ethnic groups in the Hindu-Kush region of Afghanistan as a basis for improving blood transfusion practices in that area. MATERIALS AND METHODS: The key ABO, Rh and Kell antigens were investigated in five Afghan populations. In order to depict accurately the blood group gene diversity in the area, DNA from eight additional Pakistani populations were included, and the entire sample set screened using two multiplex polymerase chain reactions sensitive for 17 alleles in 10 blood group genetic systems (MNS, Kell, Duffy, Kidd, Cartwright, Dombrock, Indian, Colton, Diego and Landsteiner-Wiener). RESULTS: Phenotype and allele frequencies fell within the ranges observed in Western European and East Asian populations. Occurrence of DI*01, IN*01, LW*07 and FY*02N.01 and prevalence of ABO*B were consistent with migratory history as well as with putative environmental adaptation in the subtropical environment Hindu-Kush region. CONCLUSION: These findings expand the current knowledge about key antigen frequencies. Regarding occurrence of viral markers, further blood transfusion in the region requires rigorous typing.

Cell adhesion: the effect of a surprising cohesive force.

When an experimentalist or a biological mechanism applies an external force onto a cell chemically sticking to its substrate, a reacting "suction" force, due to the slow penetration of the surrounding fluid between the cell and the substrate, opposes to the dissociation. This force can overcome other known adhesive forces when the process is sufficiently violent (typically 10(5) pN ). Its maximal contribution to the total adhesive energy of the cell can then be estimated to 2 x 10(-3) J/m(2). The physical origin of this effect is quite simple and it may be compared to that leaning a "suction cup" against a bathroom wall. We address the consequences of this effect on (i) the separation energy, (ii) the motion of the fluid surrounding the cell, and more especially on the pumping of the fluid by moving cells, and (iii) the inhibition of cell motion.

Theory of the non-planar surface phase in tilted smectic films under electric field.

We present a phenomenological theory of the non-planar tilted surface phase recently observed in freely suspended smectic films. We show that the three phases --Synclinic, Anticlinic, and non-planar (Axial)- stable in non-chiral systems, merge into a single phase in chiral systems. We discuss some experimental consequences of this specific chiral feature. We predict the effects of electric field application parallel to the smectic layers on the stability of the various zero-field surface phases, and determine the conditions for the observation of field-induced chiral Anticlinic/Synclinic crossovers. In thin films they depend only on the ratio between the bulk electroclinic coupling energy and the surface electric-field-tilt coupling.

Theory of the anticlinic pretilted surface phase in tilted chiral smectic films.

In freely suspended chiral smectic films, the molecules tilt at the surfaces above the bulk ferroelectric transition temperature. The tilt angle at the two surfaces can be either equal (synclinic surface phase) or opposite (anticlinic surface phase) on each surface of the film. We propose a realistic theoretical model accounting for the stability and properties of these two structures, based on competing surface interactions. The anticlinic phase is stable for sufficiently thin samples and high temperatures. We predict a reentrant SmA phase merging with the surface phases at a three-phase point.

Prediction of apparent extinction for optical transmission through rain.

At optical wavelengths, geometrical optics holds that the extinction efficiency of raindrops is equal to two. This approximation yields a wavelength-independent extinction coefficient that, however, can hardly be used to predict accurately rain extinction measured in optical transmissions. Actually, in addition to the extinct direct incoming light, a significant part of the power scattered by the rain particles reaches the receiver. This leads to a reduced apparent extinction that depends on both rain characteristics and link parameters. A simple method is proposed to evaluate this apparent extinction. It accounts for the additional scattered power that enters the receiver when one considers the forward-scattering pattern of the raindrops as well as the multiple-scattering effects using, respectively, the Fraunhofer diffraction and Twersky theory. It results in a direct analytical formula that enables a quick and accurate estimation of the rain apparent extinction and highlights the influence of the link parameters. Predictions of apparent extinction through rain are found in excellent agreement with measurements in the visible and IR regions.