R. Moldovan

Bio: R. Moldovan is an academic researcher from Romanian Academy. The author has contributed to research in topic(s): Liquid crystal & Surface tension. The author has an hindex of 7, co-authored 25 publication(s) receiving 171 citation(s).

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.

Measurements of optical anisotropy of a calamitic lyotropic liquid crystal

Abstract: Optical birefringence measurements for a calamitic lyotropic liquid crystal containing sodium lauryl sulphate/decanol/water are presented. A value of the order of 10-3 was obtained, much smaller than that of thermotropic liquid crystals. The order parameter temperature dependence is evaluated using an extrapolating method and taking into accounts the influence of the micelle shape. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of the density of some liquid crystals in the alkyl cyanobiphenyl series

Abstract: Experimental results for the temperature dependence of the density of propyl – cyanobiphenyl (3CB), butyl – cyanobiphenyl (4CB) and hexyl – cyanobiphenyl (6CB) are presented. The results are compared with previous results for temperature dependence of other members of the alkyl cyanobiphenyl series (nCB). The deviation of the density from linear temperature dependence is discussed in terms of the recently discovered corresponding rule for nematic liquid crystals. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Self-assembly of amphiphiles, polymers and proteins at interfaces between thermotropic liquid crystals and aqueous phases

Abstract: The tools of surface science have enabled a thorough understanding of the ordering of liquid crystalline materials in contact with the surfaces of solids. This report moves beyond these past studies of solid–liquid crystal interfaces by describing the emergence of a set of experimental observations that revolve around the ordering of liquid crystals at interfaces to immiscible aqueous phases. The self-assembly of surfactants, lipids, proteins and synthetic polymers at these interfaces has recently been shown to lead to a rich but poorly understood spectrum of orientational ordering behaviors of liquid crystalline materials. It is also evident that the ordering of the liquid crystals influences the organization of the molecules that assemble at these interfaces. In particular, recent experiments have revealed the influence of the molecular structure and interfacial organization of adsorbed surfactants on the ordering of the liquid crystals. The time-dependent behaviors of liquid crystals, which can reorganize on time scales of tens of milliseconds, have been used to follow dynamic phenomena at these interfaces, including the adsorption, desorption and spatial reorganization of surfactants. At these same aqueous–liquid crystal interfaces, phospholipids have been shown to assemble into monolayers that mimic dynamic properties of biological membranes. Specific protein binding events and enzyme-catalyzed reactions at phospholipid-decorated interfaces of liquid crystals lead to changes in the organization of the phospholipids that are reported as ordering transitions in the liquid crystals. Larger amphiphilic molecules, including polymers and proteins, have also been shown to assemble at aqueous–liquid crystal interfaces and to couple to the ordering of the liquid crystal. For example, functional amphiphilic polymers impart a means to reversibly control the ordering of liquid crystals in response to changes in solution conditions (e.g., pH). These observations, when combined, suggest that interfaces formed between liquid crystals and aqueous phases represent a fundamentally interesting and technologically promising class of interfaces for chemical and biological sensing, active control of interfacial assemblies and realization of stimuli-responsive materials. These interfaces define a rich set of scientific challenges that warrant the attention of the surface science community.

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.

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

Twist-bend nematic phase of the liquid crystal dimer CB7CB: orientational order and conical angle determined by 129Xe and 2H NMR spectroscopy

Abstract: The liquid crystal dimer 1ʺ,7ʺ-bis(4-cyanobiphenyl-4ʹ-yl) heptane (CB7CB) has been shown to possess a nematic–nematic phase transition at 376 K. The phase below this temperature has been identified...