Competition between electroconvection and Fréedericksz distortions in nematic liquid crystals with slightly positive dielectric anisotropy.

Planar electroconvection in nematic liquid crystals with positive dielectric anisotropy is theoretically studied in the nonlinear regime. The system is characterized by a competition between the nonequilibrium electroconvection instability and the equilibrium Fréedericksz distortions. Near a resulting multicritical bifurcation point a splay-roll instability and a bistability between the convective and the homogeneous states occur.

Electroconvection in nematic liquid crystals with positive dielectric and negative conductivity anisotropy.

Electroconvection in an unusual nematic compound with strongly positive dielectric anisotropy and negative anisotropy of the conductivity is investigated. For homeotropic alignment, where one has a direct transition to rolls or squares depending on the frequency of the applied voltage, we present a quantitative theory. From the comparison we infer values for some viscosities, which are rather unusual, but not unreasonable in view of the vicinity of the nematic-smectic transition. For planar alignment, electroconvection sets in above a splay Freedericksz transition with "parallel rolls," which is also captured by the theory.

Direct transition to electroconvection in a homeotropic nematic liquid crystal.

We present an experimental and theoretical investigation of a variant of electroconvection using an unusual nematic liquid crystal in an isotropic configuration (homeotropic alignment). The significance of the system is a direct transition to the convecting state due to the negative conductivity anisotropy and positive dielectric anisotropy. We observe at onset rolls or squares depending on the frequency and amplitude of the applied ac voltage with a strong signature of the zigzag instability. Good agreement with calculations based on the underlying hydrodynamic theory is found. We also construct an extended Swift-Hohenberg model which allows us to capture complex patterns like squares with a quasiperiodic modulation.

Isotropic convection scenarios in an anisotropic fluid.

We study a new variant of electroconvection using a homeotropically aligned nematic liquid crystal. The novelty of this system is a direct transition to roll- or square-type patterns controlled by the frequency of the applied voltage with a rich crossover scenario and strong influence of the zigzag instability even at onset. From the weakly nonlinear theory and simulations of an adapted Swift-Hohenberg model one can understand essential features of the phase diagram. In particular we find a quasiperiodic pattern with square symmetry.

Competition of periodic and homogeneous modes in extended dynamical systems.

Despite their simple structure, spatially homogeneous modes can participate directly in pattern-formation processes. This is demonstrated by new experimental and theoretical results for thermo- and electroconvection in planar nematic liquid crystals, where two distinct homogeneous modes, twist and splay distortions of the director field, emerge. Their nonlinear excitation is due to certain spontaneous symmetry-breaking bifurcations.

Reduction of granulocyte activation during hemodialysis with regional citrate anticoagulation: dissociation of complement activation and neutropenia from neutrophil degranulation.

Neutropenia and degranulation of neutrophils during hemodialysis with cellulosic membranes have been linked to complement activation, whereas in the synthetic polymethyl methacrylate (PMMA) membrane, degranulation occurs without notable complement activation. The mechanisms of neutrophil degranulation under these conditions have not yet been elucidated. Ionized calcium is an important prerequisite of granulocyte activation during in vitro blood contact with both types of artificial surfaces. This study compared the effect of normal ionized calcium during heparin anticoagulation with the effect of extracorporeal calcium depletion during regional citrate anticoagulation on activation of blood components. Because ionized calcium is reduced only in the extra-corporeal circuit, citrate anticoagulation in addition helps to differentiate between extracorporeal and systemic activation phenomena. Twelve chronic hemodialysis patients were dialyzed with polymethyl methacrylate (PMMA, 16 treatments) or cuprophane (CUP, 16 treatments) membranes either during regional citrate anticoagulation or while anticoagulated with heparin. During hemodialysis with CUP, anticoagulation with citrate significantly reduced neutropenia, C3a levels, and lactoferrin release. Elastase concentrations, however, were not reduced by citrate, probably because elastase release occurred not locally in the cuprophane dialyzer, but mostly in the systemic circulation of the patient. PMMA did not elevate C3a levels, and neutropenia was only mild. Both parameters were not influenced by citrate anti-coagulation. However, PMMA profoundly induced elastase and lactoferrin release during heparin anti-coagulation. Depletion of ionized calcium markedly reduced PMMA-mediated neutrophil degranulation in the extracorporeal circuit. The results indicate that ionized calcium is a requirement for neutrophil degranulation during hemodialysis. In PMMA membranes, neutrophil degranulation occurs independent of high complement levels, occurs at least partially inside the dialyzer, and requires the presence of ionized calcium in the extracorporeal circuit. In cuprophane membranes, degranulation was uncoupled from neutropenia and did not correlate with the degree of complement activation. Even in cuprophane dialysis, degranulation of secondary granules was markedly dependent on ionized calcium levels in the extracorporeal circuit.