PbS nanoparticles stabilised blue phase liquid crystals

A kind of blue phase liquid crystal (BPLC), consisting of nematic liquid crystal, E7, and chiral dopants, CB15 and R1011, was investigated by doping PbS nanoparticles. The blue phase temperature range was extended from 3^oC to 4.6°C by doping PbS nanoparticles with diameters around 9.6 nm. A kind of porous texture was observed both in the forming process of PbS nanoparticles doped BPLCs as well as in the BPLCs (with/without PbS nanoparticles) under assisting electric field. The porous texture may indicate that the liquid crystals molecule should be reoriented during the formation process of PbS nanoparticles doped BPLCs.     

Linear response and stochastic resonance of superparamagnets

The standard mathematical expression for the response functions of the linear response theory for Markovian stochastic systems, constructed from the corresponding Fokker-Planck equation, is transformed into an expression which is very suitable for numerical simulations. The method is applied to a stochastic model for superparamagnetism presented previously by the authors. For convenient values of the parameters the model shows the phenomenon of stochastic resonance.     

A positive cell vertex Godunov scheme for a Beeler–Reuter based model of cardiac electrical activity

The monodomain model is a widely used model in electrocardiology to simulate the propagation of electrical potential in the myocardium. In this paper, we investigate a positive nonlinear control volume finite element scheme, based on Godunov's flux approximation of the diffusion term, for the monodomain model coupled to a physiological ionic model (the Beeler–Reuter model) and using an anisotropic diffusion tensor. In this scheme, degrees of freedom are assigned to vertices of a primal triangular mesh, as in conforming finite element methods. The diffusion term which involves an anisotropic tensor is discretized on a dual mesh using the diffusion fluxes provided by the conforming finite element reconstruction on the primal mesh and the other terms are discretized by means of an upwind finite volume method on the dual mesh. The scheme ensures the validity of the discrete maximum principle without any restriction on the transmissibility coefficients. By using a compactness argument, we obtain the convergence of the discrete solution and as a consequence, we get the existence of a weak solution of the original model. Finally, we illustrate by numerical simulations that the proposed scheme successfully removes nonphysical oscillations in the propagation of the wavefront and maintains conduction velocity close to physiological values.     

Cracking and oxygenation of YBaCuO bulk superconductors: application to c-axis elements for current limitation

A process to elaborate crack free, oxygenated c-axis samples in YBa₂Cu₃O_x bulk superconductors is reported. This includes texturing of crack free pellets and development of an adequate oxygen annealing treatment to avoid cracking during oxygen uptake. For current limitation in c-direction such a process is needed as c-axis elements are particularly susceptible to ab-plane cracking during oxygenation. Mechanical properties have been compared for different annealing treatments by flexural strength measurements. With the load applied in ab-direction the mean flexural strength is 58 MPa in as-textured c-axis bars and 48 MPa in c-axis bars which were oxygenated using a customized annealing treatment. The conventional oxygen annealing treatments lead to macroscopic oxygenation cracks which reduced flexural strength to 10–20 MPa. Bars cut in ab-direction do not show significant variation in flexural strength (load applied in c-direction) as a function of the annealing condition with an average value of 145 MPa.     

Director dynamics of uniformly aligned nematic liquid crystals in transient shear flow

We have developed a modular rheo-optical apparatus to study the flow properties of liquid crystals. Its main components are shearing device, strong magnetic field, and optical microscope. We performed experiments on well defined initial morphologies with uniform molecular alignment. The monodomains were achieved with strong magnetic fields (4.7T). Time-resolved conoscopy is the primary optical technique in our investigation. We propose a simple relation between the distribution of alignment angles over the sample thickness and the conoscopically measured angle, to quantitatively measure the alignment angle in shear flow.We followed the relaxation of a shear-induced splay deformation in small molecule model systems (N-(p-methoxybenzylidene) p-butylaniline (MBBA), pentyl-cyano-biphenyl (5 CB) and a commercially available mixture OM14244). We define a rotational director diffusivity (K _s splay elastic constant.i7s splay viscosity) from the relaxation process and devised a model, based on the diffusion equation to determine their values.The director alignment behavior of the small molecule liquid crystals (SMLC's) in shear flow is well described by the two-dimensional Leslie-Ericksen model. The effect of director elasticity can clearly be seen in our experiments, resulting in a decrease of the steady state alignment angle at smaller Ericksen numbers.We found that there is no strain rate dependence of the director vorticity from 0.002/s to 2/s for poly-(-benzyl-D/L-glutamate) (PBG). We determined ₂/₃ = –44 for a 2007o solution of 280000 molecular weight PBG in m-cresol at 20°C. The conoscopic interference pattern vanished after 8 strain units from an initially planar alignment and shearing could be reversed up to 10 strain units to completely recover the initial monodomain.Presented at 4th Meeting of European Rheologists, Sept. 4–9, 1994, Seville, SpainDedicated to Prof. H. Janeschitz-Kriegl at the occasion of his 70th birthday.     

Polydomain–Monodomain Orientational Process in Smectic‐C Main‐Chain Liquid‐Crystalline Elastomers

The polydomain–monodomain (PM) transformation takes place when a polydomain of a smectic‐C main‐chain liquid‐crystalline elastomer (SmC MCLCE) is uniaxially stretched. We present results based on a combination of mechanical and X‐ray experiments which show how the domains initially rearrange to finally form a perfect conical layer distribution (monodomain) when the sample is fully stretched. The rearrangement and orientational process of the domains is quantified and compared to the parallel and perpendicular uniaxial stress–strain deformations of a monodomain sample. The stress–strain behaviour of the polydomain lays between the uniaxial deformations, parallel and perpendicular to the director, of the monodomain sample.

     

Rotation of liquid crystalline macromolecules in shear flow and shear-induced periodic orientation patterns

We have measured the shear-induced rotation of the nematic director in a liquid crystalline polymer using poly benzyl glutamate (PBG) as model system. PBG is a well characterized synthetic poly ( amino acid) with rigid chain architecture and well defined conformations. For the experiments it is important to start out with a sample in which the molecules are highly aligned with a uniform director. This so-called monodomain morphology is obtained by use of strong magnetic fields and surface modifications of the sample holders. When shearing the monodomain at a constant rate, the macromolecules rotate initially homogeneously until a periodic director pattern develops. These spatially periodic structures emerge in a narrow range of shear strain and, as shearing continues, disintegrate into a chaotic texture. By varying the initial monodomain director with respect to the flow direction (but within the shear planes) we could show that the periodic patterns do not depend on the shear direction; they are governed by the director of the initial monodomain. We observe conoscopically that at high shear rates the texture becomes uniformly aligned. The molecules are aligned preferentially with an angle of about 4° to the shear direction (against vorticity direction). Interestingly, this agrees very well with predictions made by Larson (1990).Dedicated to Prof. Dr. J. Meissner on the occasion of his retirement from the chair of Polymer Physics at the Eidgenössische Technische Hochschule (ETH) Zürich, Switzerland