Christian Bahr

Chiral-racemic phase diagram of a ferroelectric liquid crystal with high spontaneous polarization

Abstract The ferroelectric properties in a chiral-racemic system of a liquid crystal with high spontaneous polarization are investigated by studying the spontaneous polarization in the SmC*-phase, the dielectric permittivity and the field induced tilt angle in the SmA-phase, and the SmC*-SmA transition temperature as a function of the enantiomeric excess. The results are compared with a simple Landau description of the SmC*-SmA transition. For the coupling between tilt angle and polarization a linear variation with the enantiomeric excess is found.

Electric field induced smectic A to smectic C transition in ferroelectric Liquid crystals

Abstract The influence of an electric field on the SmC*-SmA transition of two new ferroelectric liquid crystals is investigated. At temperatures just above the SmC*-SmA transition double hysteresis loops are observed similar to that known for solid Cerroelectrics indicating a field induced first order transition to the ferroelectric phase. The first order character of the SmC* -SmA transition of the two compounds under investigation is supported by DSC and tilt angle measurements. With field strength of about 50 kV/ cm the transition temperature is enhanced by 1 K. The shift of the SmC*-SmA transition temperature, which is found to vary linearly with applied field strength, can be calculated by simple thermodynamic arguments.

INFLUENCE OF DIMENSIONALITY AND SURFACE ORDERING ON PHASE TRANSITIONS: STUDIES OF FREELY-SUSPENDED LIQUID-CRYSTAL FILMS

Freely-suspended films of smectic liquid crystals can be regarded as membranes consisting of a stack of an integral number of molecular (smectic) layers with the layer planes being parallel to the two free surfaces. Because of their variable thickness (between thousands and only two layers) and the large variety of phase transitions between different smectic phases, freely-suspended films are excellent systems to study the influence of the dimensional cross-over from three to two dimensions on phase transitions. Further, because the free surface of a liquid crystal has a strong ordering effect (contrary to solids which exhibit generally surface-induced disorder), freely-suspended films are well-suited for the study of the effect of enhanced surface order on phase transitions. A review of the corresponding experimental work is given.

Solubilization of Thermotropic Liquid Crystal Compounds in Aqueous Surfactant Solutions

We study the micellar solubilization of three thermotropic liquid crystal compounds by immersing single drops in aqueous solutions of the ionic surfactant tetradecyltrimethylammonium bromide. For both nematic and isotropic drops, we observe a linear decrease of the drop size with time as well as convective flows and self-propelled motions. The solubilization is accompanied by the appearance of small aqueous droplets within the nematic or isotropic drop. At low temperatures, nematic drops expell small nematic droplets into the aqueous environment. Smectic drops show the spontaneous formation of filament-like structures which resemble the myelin figures observed in lyotropic lamellar systems. In all cases, the liquid crystal drops become completely solubilized, provided the weight fraction of the liquid crystal in the system is not larger than a few percent. The solubilization of the liquid crystal drops is compared with earlier studies of the solubilization of alkanes in ionic surfactant solutions.

Controlling Smectic Focal Conic Domains by Substrate Patterning

We describe a simple method to control the generation and the dimensions of focal conic domains in smectic liquid-crystal films. The surface of silicon substrates is structured in a way that areas with homeotropic anchoring conditions alternate with areas possessing random planar anchoring conditions. In smectic films on such substrates, the anchoring pattern translates into a presence-and-absence pattern of focal conic domains. The lateral dimensions of the planar anchoring areas determine an upper limit of the diameter of the focal conic domains. Thus, an almost arbitrary two-dimensional arrangement of focal conic domains can be achieved by controlling the size and position of individual domains.

Liquid crystal microfluidics: surface, elastic and viscous interactions at microscales

The hydrodynamic properties of nematic liquid crystals are characterized by a complex mutual coupling between flow, viscosity, and nematic order. While the flow behaviour of nematic bulk samples is well known, corresponding studies in microfluidic settings are still at an early stage. The presence of the four confining channel walls – and in particular the nature of the surface anchoring of the nematic order on the walls – adds new phenomena to the already rich and multifaceted flow behaviour. We present an overview of recent studies focusing on the microfluidics of nematic liquid crystals. Particular topics are the functionalization of the channel walls for defined surface anchoring conditions and the resulting structures of the nematic director field, the controlling and tuning of the flow velocity profile and director field configuration and resulting opto-fluidic applications, and the behaviour of topological defects in the flowing nematic and their application for a guided colloidal transport.

Lasing and waveguiding in smectic A liquid crystal optical fibers

We demonstrate a new class of soft matter optical fibers, which are self-assembled in a form of smectic-A liquid crystal microtubes grown in an aqueous surfactant dispersion of a smectic-A liquid crystal. The diameter of the fibers is highly uniform and the fibers are highly birefringent. They are characterized by a line topological defect in the core of the fiber with an optical axis pointing from the defect core towards the surface. We demonstrate guiding of light along the fiber and Whispering Gallery Mode (WGM) lasing in a plane perpendicular to the fiber. The light guiding as well as the lasing threshold are significantly dependent on the polarization of the excitation beam. The observed threshold for WGM lasing is very low (≈ 75μJ/cm(2)) when the pump beam polarization is perpendicular to the direction of the laser dye alignment and is similar to the lasing threshold in nematic droplets. The smectic-A fibers are soft and flexible and can be manipulated with laser tweezers demonstrating a promising approach for realization of soft photonic circuits.

Topological microfluidics for flexible micro-cargo concepts

State-of-the-art microfluidic techniques rely usually on an isotropic carrier fluid, the flow of which is modulated using morphological patterns on the microchannels, or application of external fields. In the present work, we demonstrate that replacing the isotropic fluid by an anisotropic liquid crystal introduces a flexible but versatile approach to guided transport of microscopic cargo in microfluidic devices. We show that topological line defects can be threaded at will through the microfluidic channels and used as a ‘soft rail’ whose position is controlled through easily accessible experimental parameters. Colloid particles and small water droplets, the ‘working horses’ of microfluidics, are trapped and consequently guided by the defect line through the microfluidic device. Furthermore, we demonstrate controlled threading of the defect line at a channel bifurcation. Topological microfluidics introduces a unique platform for targeted delivery of single particles, droplets, or clusters of such entities, paving the way to flexible micro-cargo concepts in microfluidic settings.

Landau second order to tricritical crossover behaviour for smectic A - smectic C* transitions

Abstract The smectic A (SA)-chiral smectic C (S*C) transition in binary mixtures of methylchloropentanoyloxy heptyloxybiphenyl (C7) and butyloxyphenyl decyloxybenzoate (10O.4) changes from first order to second order as a function of X, the mole per cent of 10O.4. High resolution heat capacity measurements show that the Cp behaviour is well-described by Landau mean field theory. Classical tricritical behaviour is observed near X tc ≃ 10·6 and the coefficient b of the quadratic term in the free energy varies like (X - X tc) in good agreement with results from polarization data.

Myelin Structures Formed by Thermotropic Smectic Liquid Crystals

We report on transient structures, formed by thermotropic smectic-A liquid crystals, resembling the myelin figures of lyotropic lamellar liquid crystals. The thermotropic myelin structures form during the solubilization of a smectic-A droplet in an aqueous phase containing a cationic surfactant at concentrations above the critical micelle concentration. Similar to the lyotropic myelin figures, the thermotropic myelins appear in an optical microscope as flexible tubelike structures growing at the smectic/aqueous interface. Polarizing microscopy and confocal fluorescence microscopy show that the smectic layers are parallel to the tube surface and form a cylindrically bent arrangement around a central line defect in the tube. We study the growth behavior of this new type of myelins and discuss similarities to and differences from the classical lyotropic myelin figures.