T. Beica

Bio: T. Beica is an academic researcher. The author has contributed to research in topics: Liquid crystal & Homeotropic alignment. The author has an hindex of 4, co-authored 7 publications receiving 72 citations.

Surface tension of some liquid crystals in the cyanobiphenyl series

Abstract: The temperature dependence of the surface tension was measured by the pendant drop method for four compounds from the homologous series of alkylcyanobiphenyls (nCB), in the nematic liquid crystal and isotropic phases. For 8CB (octylcyanobiphenyl) the temperature dependence was also measured in the smectic range. Not very close to the isotropic transition temperature, and with the exception of 8CB, the surface tension decreases with increase in temperature in the nematic range. A downward jump at the transition temperature was observed for all liquid crystals studied. The shape of the drop in the smectic A phase of 8CB gives indications of stratification in a system of terraces.

Orientational properties of PVA coated SiO films

Abstract: A new method for producing tilt angle in liquid crystal cells is proposed. The method requires a SiO vacuum deposition layer which is subsequently covered with a polyvinyl alcohol (PVA) layer, obtained by dipping in a solution. It was possible to obtain tilt angles between 7 and 30° by changing the concentration of PVA in the solution.

Periodic structures in a twisted nematic liquid-crystal cell under d.c. excitation

Abstract: Some new results concerning the appearance of a periodic structure in a π/2-twist cell filled with MBBA under d.c. excitation are reported. This structure consists of rectilinear domains whose orientation varies with the applied voltage around the normal to the molecular orientation in the middle of the cell and this variation covers a large range of angles even for fixed polarity of the excitation. When the polarity is changed, the sense of the domain orientation is also changed.

On some transition metal complexes having orientational properties

Abstract: The orientation properties of some complexes of chromium (III) or cobalt (II) with oxygen-containing ligands are presented. The orientation obtained is mostly homeotropic. The possibility of anchoring by coordination of liquid crystal (LC) molecules to the transition metal ion within the alignment layer is discussed on the basis of spectroscopic arguments.

Temperature induced surface transitions in liquid crystal tilted homogeneous cells

Abstract: Temperature induced surface transitions in liquid crystal tilted homogeneous cells are experimentally studied. The results are theoretically explained by an elastic theory of surface transitions and the resulting values for the effective splay-bend elastic constant k 13 for each liquid crystal material are compared with the results from experience with other types of surface anchoring.

Ordered droplet structures at the liquid crystal surface and elastic-capillary colloidal interactions.

Abstract: We demonstrate a variety of ordered patterns, including hexagonal structures and chains, formed by colloidal particles (droplets) at the free surface of a nematic liquid crystal (LC). The surface placement introduces a new type of particle interaction as compared to particles entirely in the LC bulk. Namely, director deformations caused by the particles lead to distortions of the interface and thus to capillary attraction. The elastic-capillary coupling is strong enough to remain relevant even at the micron-scale when its buoyancy-capillary counterpart becomes irrelevant.

Convection instabilities in nematic liquid crystals

Abstract: Pattern formation in hydrodynamic instabilities has been studied intensely over the past few decades (Manneville 1990, Cross & Hohenberg 1993). Rayleigh-Benard convection (RBC) in simple fluids has been the prime example. Our goal in this review is to draw attention to the rich variety of scenarios found in nematic liquid crystals (LCs). Liquid crystals are materials made up of highly anisotropic organic molecules in a phase that reflects this anisotropy. The class of nematic LCs (nematics) is fully liquid without long-range translational, but with long-range uniaxial orienta tiona I ordering of the molecules. Thus in the well-established hydrodynamic description (Ericksen 1 976, Leslie 1 979, de Gennes 1974, Stephen & Straley 1974, Chandrasekhar 1977) the director n characterizing the preferred axis has to be included as an additional variable. One then has six shear viscosities lXI' • . • , 1X6 with 1X6 -1X5 = 1X3 + IXz (1X4/2 corresponds to the isotropic viscosity) in the momentum balance equation (generalized Navier-Stokes equation). In the director equation, which can be associated loosely with a balance of torque, there are two rotational viscosities, YI and Yz, which are expressible in terms of the shear viscosities. For the low-molecular-weight materials discussed here the vis­ cosities relevant in the following are of order 10-1 kg m -I S-I. One needs three orientational elastic modules, kl), k22' k33' to describe the three basic

Optical gratings formed in thin smectic films frustrated on a single crystalline substrate.

Abstract: Through the combination of three different, complementary techniques (optical microscopy, x-ray diffraction and atomic force microscopy), we reveal the deformations inside thin smectic films frustrated between two interfaces imposing antagonistic anchorings. We study the model system, 4-n-octyl-4'-cyanobiphenyl (8CB) between MoS2 and air, which is characterized by the competition between homeotropic anchoring at air and planar unidirectional anchoring on the substrate, with thicknesses varying around 0.3 microm. Optical microscopy and x-ray diffraction demonstrate the continuous topology of smectic layers between the interfaces, which are stacked into periodic flattened hemicylinders. These latter are one-dimensional (1D) focal conic domains which form an optical grating in the smectic film, of a period ranging from 1 to 2.5 microm. The interpretation of our results through an energetic model, associated with the atomic force microscopy (AFM) measurements, shows the presence below a critical thickness of a new type of curvature wall between neighboring hemicylinders.

Sharp-Interface Nematic–Isotropic Phase Transitions without Flow

Abstract: We derive a supplemental evolution equation for an interface between the nematic and isotropic phases of a liquid crystal when flow is neglected Our approach is based on the notion of configurational force As an application, we study the behavior of a spherical isotropic drop surrounded by a radially oriented nematic phase: our supplemental evolution equation then reduces to a simple ordinary differential equation admitting a closed-form solution In addition to describing many features of isotropic-to-nematic phase transitions, this simplified model yields insight concerning the occurrence and stability of isotropic cores for hedgehog defects in liquid crystals