Nonlinear convection of binary liquids in a porous medium.

Thermal convection of binary mixtures in a porous medium is studied with stress-free boundary conditions. The linear stability analysis is studied by using the normal mode method. The effects of the material parameters have been studied at the onset of convection. Using a multiple scale analysis near the onset of the stationary convection, a cubic-quintic amplitude equation is derived. The influence of the Lewis number and the separation ratio on the supercritical-subcritical transition is discussed. Stationary front solutions and localized states are analyzed at the Maxwell point. Near the threshold of the oscillatory convection, a set of two coupled complex cubic-quintic Ginzburg-Landau type amplitude equations is derived, and implicit analytical expressions for the coefficients are given.

Reversible and dissipative macroscopic contributions to the stress tensor: active or passive?

The issue of dynamic contributions to the macroscopic stress tensor has been of high interest in the field of bio-inspired active systems over the last few years. Of particular interest is a direct coupling ("active term") of the stress tensor with the order parameter, the latter describing orientational order induced by active processes. Here we analyze more generally possible reversible and irreversible dynamic contributions to the stress tensor for various passive and active macroscopic systems. This includes systems with tetrahedral/octupolar order, polar and non-polar (chiral) nematic and smectic liquid crystals, as well as active fluids with a dynamic preferred (polar or non-polar) direction. We show that it cannot a priori be seen, neither from the symmetry properties of the macroscopic variables involved, nor from the structure of the cross-coupling contributions to the stress tensor, whether the system studied is active or passive. Rather, that depends on whether the variables that give rise to those cross-couplings in the stress tensor are driven or not. We demonstrate that several simplified descriptions of active systems in the literature that neglect the necessary counter term to the active term violate linear irreversible thermodynamics and lead to an unphysical contribution to the entropy production.

Active polar two-fluid macroscopic dynamics.

We study the dynamics of systems with a polar dynamic preferred direction. Examples include the pattern-forming growth of bacteria as well as shoals of fish, flocks of birds and migrating insects. Due to the fact that the preferred direction only exists dynamically, but not statically, the macroscopic variable of choice is the macroscopic velocity associated with the motion of the active units, which are typically biological in nature. We derive the macroscopic equations for such a system and discuss novel static, reversible and irreversible cross-couplings connected to a second velocity as a variable. We analyze in detail how the macroscopic behavior of an active system with a polar dynamic preferred direction compares to other systems with two velocities including immiscible liquids and electrically neutral quantum liquids such as superfluid (4)He and (3)He . We critically discuss changes in the normal mode spectrum when comparing uncharged superfluids, immiscible liquids and active system with a polar dynamic preferred direction. We investigate the influence of a macroscopic hand (collective effects of chirality) on the macroscopic behavior of such active media.

Chaotic dynamics of a magnetic nanoparticle.

We study the deterministic spin dynamics of an anisotropic magnetic particle in the presence of a magnetic field with a constant longitudinal and a time-dependent transverse component using the Landau-Lifshitz-Gilbert equation. We characterize the dynamical behavior of the system through calculation of the Lyapunov exponents, Poincaré sections, bifurcation diagrams, and Fourier power spectra. In particular we explore the positivity of the largest Lyapunov exponent as a function of the magnitude and frequency of the applied magnetic field and its direction with respect to the main anisotropy axis of the magnetic particle. We find that the system presents multiple transitions between regular and chaotic behaviors. We show that the dynamical phases display a very complicated structure of intricately intermingled chaotic and regular phases.

Macroscopic behavior of systems with an axial dynamic preferred direction.

We present the derivation of the macroscopic equations for systems with an axial dynamic preferred direction. In addition to the usual hydrodynamic variables, we introduce the time derivative of the local preferred direction as a new variable and discuss its macroscopic consequences including new cross-coupling terms. Such an approach is expected to be useful for a number of systems for which orientational degrees of freedom are important including, for example, the formation of dynamic macroscopic patterns shown by certain bacteria such a Proteus mirabilis. We point out similarities in symmetry between the additional macroscopic variable discussed here, and the magnetization density in magnetic systems as well as the so-called Î vector in superfluid (3)He-A. Furthermore we investigate the coupling to a gel-like system for which one has the strain tensor and relative rotations between the new variable and the network as additional macroscopic variables.

Macroscopic behavior of non-polar tetrahedratic nematic liquid crystals.

We discuss the symmetry properties and the macroscopic behavior of a nematic liquid crystal phase with D(2d) symmetry. Such a phase is a prime candidate for nematic phases made from banana-shaped molecules where the usual quadrupolar order coexists with octupolar (tetrahedratic) order. The resulting nematic phase is nonpolar. While this phase could resemble the classic D (infinityh) nematic in the polarizing microscope, it has many static as well as reversible and irreversible properties unknown to nonpolar nematics without octupolar order. In particular, there is a linear gradient term in the free energy that selects parity leading to ambidextrously helical ground states when the molecules are achiral. In addition, there are static and irreversible coupling terms of a type only met otherwise in macroscopically chiral liquid crystals, e.g. the ambidextrous analogues of Lehmann-type effects known from cholesteric liquid crystals. We also discuss the role of hydrodynamic rotations about the nematic director. For example, we show how strong external fields could alter the D(2d) symmetry, and describe the non-hydrodynamic aspects of the dynamics, if the two order structures, the nematic and the tetrahedratic one, rotate relative to each other. Finally, we discuss certain nonlinear aspects of the dynamics related to the non-commutativity of three-dimensional finite rotations as well as other structural nonlinear hydrodynamic effects.

Response of prestretched nematic elastomers to external fields.

We investigate the response of prestretched nematic side-chain liquid single-crystal elastomers to superimposed external shear, electric, and magnetic fields of small amplitude. The prestretching direction is oriented perpendicular to the initial nematic director orientation, which enforces director reorientation. Furthermore, the shear plane contains the direction of prestretch. In this case, we obtain a strongly decreased effective shear modulus in the vicinity of the onset and the completion of the enforced director rotation. For the same regions, we find that it becomes comparatively easy to reorient the director by external electric and magnetic fields. These results were derived using conventional elasticity theory and its coupling to relative director-network rotations.

Thermal convection in mixtures with an inversion of the separation ratio.

We have numerically investigated Rayleigh-Bénard convection in binary mixtures assuming a thermal diffusion ratio that depends on the local temperature and changes its sign within the cell. The stationary instability has been found to precede the oscillatory one for a wide range of negative mean psi values. The bifurcation diagram for stationary rolls turns out to be qualitatively different from that for constant Soret effect. Nevertheless, it can be mapped onto this special case by using a scaling argument, taking into account the fact that for small convection amplitudes the rolls are restricted to parts of the cell where the sign of the Soret coefficient favors instability.

Rosensweig instability of ferrogel thin films or membranes.

We derive the dispersion relation of surface waves for magnetic gel membranes or thin films at the interface between two fluids in the presence of an external magnetic field normal to the free surface. Above a critical field strength surface waves become linearly unstable with respect to a stationary pattern of surface protuberances. This linear stability criterion generalizes that of the Rosensweig instability for ferrofluid and ferrogel free surfaces to take into account bending elasticity and intrinsic elastic and magnetic surface properties of the film or membrane, additionally. The latter is of interest for uniaxial ferrogel film or membranes, which show a locked-in permanent magnetization.

Convection patterns in colloidal solutions.

We investigate the bifurcation properties and pattern selection in Rayleigh-Bénard convecting binary mixtures for parameter combinations that are typical for colloidal fluids using the Galerkin method. In such fluids the typical Lewis numbers and separation ratios differ by orders of magnitude from the values found in molecular mixtures like alcohol/water. We study stationary rolls, square and crossroll patterns for positive separation ratios, and traveling waves for negative separation ratios. Results are compared to those for molecular mixtures.

Splay-bend textures involving tetrahedratic order.

We show that defect-free splay-bend textures are less energetic compared to uniform states in liquid-crystalline phases that possess both quadrupolar (nematic) and octupolar (tetrahedratic) order. This is because, in such systems, there is a symmetry-allowed linear gradient term in the energy. Another unusual feature of these splay-bend textures is the fact that they have a non-homogeneous, space-dependent free-energy density. These results may help clarify some mysterious features noted for the B7 liquid-crystal phase formed by achiral banana-shaped molecules.

A phenomenological theory of the isotropic to chiral smectic-C phase transition.

In this paper we discuss the direct isotropic to chiral smectic-C phase transition on the basis of a phenomenological theory. The model free energy is written in terms of the coupled order parameters including the spontaneous polarization. We present a detailed analysis of the different phases that can occur and analyze the question under which conditions a direct isotropic to chiral smectic-C phase transition is possible when compared to other phase transitions. On the basis of this model the isotropic-smectic-C* transition is always of first order. The theoretical predictions are compared with the available experimental results.

Mechanical properties of mono-domain side chain nematic elastomers.

We investigate the behavior of the shear rigidity modulus G = G' + iG" of three mono-domain side chain liquid-crystalline elastomers composed of side chain polysiloxanes cross-linked by either flexible or rigid cross-linkers. The measurements were taken in a frequency domain ranging from approximately 0.02 Hz to approximately 10(4) Hz applying the shear in a plane perpendicular to or containing the director. The measurements as a function of temperature show an anisotropy of G' which appears around T(NI), when decreasing the temperature, and which is due to the expected lowering of G'(//) coming from the coupling between the shear and the director. The measurements as a function of frequency show that G has two components for both geometries, in both the isotropic phase and in the nematic phase around the phase transition. One reflects the network behavior in its hydrodynamic regime ( G' is constant and G'' is approximately f, where f is the frequency), the other which appears at higher frequencies is characterized by a scaling law behavior (G' is approximately G" is approximately f(0.5)) of the Rouse type. We discuss the results in the framework of available theories and show that the three elastomers present a non-soft behavior, even for the elastomer for which the contrary was claimed, and that there is no separation of time scales between the director and the network. We also present data on a poly-domain sample and a non-mesomorphic one which complement these results.

Free fatty acids exert a greater effect on ocular and skin blood flow than triglycerides in healthy subjects.

BACKGROUND: Free fatty acids (FFAs) and triglycerides (TGs) can cause vascular dysfunction and arteriosclerosis. Acute elevation of plasma FFA and TG concentration strongly increase ocular and skin blood flow. This study was designed to discriminate whether FFA or TG independently induce hyperperfusion by measuring regional and systemic haemodynamics. METHODS: In a balanced, randomized, placebo-controlled, double-blind, three-way, crossover study nine healthy subjects received either Intralipid (Pharmacia and Upjohn, Vienna, Austria) with heparin, Intralipid alone or placebo control. Pulsatile choroidal blood flow was measured with laser interferometry, retinal blood flow and retinal red blood cell velocity with laser Doppler velocimetry, and skin blood flow with laser Doppler flowmetry during an euglycaemic insulin clamp. RESULTS: A sevenfold increase of FFA during Intralipid/heparin infusion was paralleled by enhanced choriodal, retinal, and skin blood flow by 17 +/- 4%, 26 +/- 5% (P < 0.001), and 47 +/- 19% (P = 0.03) from baseline, respectively. In contrast, a mere threefold increase of FFA by infusion of Intralipid alone did not affect outcome parameters, despite the presence of plasma TG levels of 250-700 mg dL(-1); similar to those obtained during combined Intralipid/heparin infusion. Systemic haemodynamics were not affected by drug infusion. CONCLUSIONS: Present findings demonstrate a concentration-dependent increase in ocular and skin blood flow by FFA independently of elevated TG plasma concentrations. As vasodilation of resistance vessels occur rapidly, FFA may play a role in the development of continued regional hyperperfusion and deteriorate microvascular function.

Macroscopic dynamics of uniaxial magnetic gels.

We present the derivation of the macroscopic equations for uniaxial ferrogels. In addition to the usual hydrodynamic variables for gels we introduce the magnetization and the relative rotations between the magnetization and the network as macroscopic variables. The relative rotations introduced here for a system with magnetic degrees of freedom are the analog of the relative rotations introduced by de Gennes in nematic elastomers for rotations of the director with respect to the elastomeric network. These variables give rise to a large number of static as well as dynamic effects due to their coupling to the magnetization, the strain field, and the density of linear momentum. A few of them are discussed for specific geometries, for example, the case of a shear-induced magnetization perpendicular to the preferred direction.

Hydrodynamics of isotropic ferrogels.

We derive the complete set of macroscopic dynamic equations for ferrogels under an external magnetic field, including the magnetization as an independent dynamic degree of freedom. The magnetoelasticity comes in the form of magnetostriction and through the magnetic part of the Maxwell stress. Various dynamic couplings of the elastic degree of freedom with the magnetization and the magnetic field are found. We discuss static elongation, shear deformations, and the modified sound spectrum in the presence of an external magnetic field.

ANP but not BNP reflects early left diastolic dysfunction in type 1 diabetics with myocardial dysinnervation.

OBJECTIVE: We investigated whether plasma concentrations of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) reflect impaired diastolic relaxation or its improvement after ACE inhibition. METHODS: 7 long-term Type 1 diabetic patients with normal systolic but impaired diastolic function and with sympathetic myocardial dysinnervation and 10 controls were included. Exercise tolerance and maximal O 2 uptake were evaluated by bicycle exercise prior to the study. ANP, BNP and norepinephrine/epinephrine (NE/E) were determined at baseline and at 80 % .VO2 max workload and after recovery, before and following 12 weeks of treatment with fosinopril (10 mg/d). RESULTS: Isovolumetric relaxation time (IVRT) and A/E wave ratio were increased by 26.7 +/- 11.5 % and 54.4 +/- 26.1 % in diabetic patients as compared to controls, respectively (p < 0.02). After 12 weeks of fosinopril treatment, no differences in IVRT or A/E wave ratio were detectable between groups. ANP was enhanced in Type 1 diabetes as compared to controls (baseline: 9.2 +/- 3.0 vs. 4.5 +/- 1.1; exercise: 22.4 +/- 7.7 vs. 7.9 +/- 1.2; recovery: 20.3. +/- 4.6 vs. 9.5 +/- 2.0 fmol/ml, p < 0.02). Fosinopril treatment abolished any differences between groups. BNP plasma levels did not differ between groups and no exercise dependent changes were observed. NE- and E-increase was greater at 80 % .VO2 max work load in Type 1 diabetes than in controls (p < 0.05). Again, fosinopril abolished differences between groups. CONCLUSION: In Type 1 diabetes, impaired diastolic function is associated with elevated ANP and catecholamine plasma levels that are normalized after ACE inhibition. Thus, ANP but not BNP appears to be a sensitive biochemical marker for early diastolic dysfunction in Type 1 diabetes.

Hydrodynamic instabilities in ferronematics.

In the hydrodynamic description of ferronematics there are various dynamic magnetic-field effects, linear in the field strength, that are negligible in usual nematics, but can play a role in ferronematics. Here, we investigate theoretically the influence of these new terms on the thermal convection (Rayleigh-Bénard) and the viscous fingering (Saffman-Taylor) instability in ferronematics in the presence of a strong magnetic field. We find that the instabilities are qualitatively changed due to the occurrence of a finite vorticity component--a feature that is known from simple liquids in the case of a superimposed mechanical rotation. We suggest to use the additional effects (cross-flow within convection rolls, oblique rolls, rotating fingers) for measuring the phenomenological coefficients involved.

Deformable tetrahedratic phases: the effects of external fields and flows.

We discuss changes in the symmetry and physical properties of an isotropic phase which has initially tetrahedral symmetry characterized by four unit vectors. In its undeformed state, these four vectors are at the tetrahedral angle (109.47 degrees) to each other. We find that this optically isotropic phase becomes uniaxial under the influence of an external electric field, E, resulting in a phase with C3v symmetry. For an applied simple shear flow, the system becomes biaxial and a time-dependent state with C1 symmetry arises. We discuss to what extent deformations induced by external forces and flows on this optically isotropic phase, which we call a "deformable tetrahedratic phase", are consistent with observations at the isotropic-B7 transition found recently in compounds composed of banana-shaped molecules and suggest a number of experiments to test the conclusions of this model.

The renal and systemic hemodynamic effects of a nitric oxide-synthase inhibitor are reversed by a selective endothelin(a) receptor antagonist in men.

There is evidence for an interaction between nitric oxide (NO) and endothelin (ET) at the level of the renal vasculature. We hypothesized that acute renal effects of systemic NO synthase inhibition (NG-monomethyl-l-arginine, L-NMMA) may be blunted by coadministration of a specific ET(A) receptor antagonist (BQ-123) in healthy humans. Fifteen healthy young male subjects participated in this randomized, double-blind, placebo-controlled 3-way crossover study. These sodium-repleted volunteers received L-NMMA alone, or BQ-123 alone, or L-NMMA with a subsequent coinfusion of BQ-123. Renal plasma flow (RPF) and glomerular filtration rate (GFR) were determined with the PAH and inulin clearance method, respectively. Mean arterial pressure (MAP) and pulse rate were measured noninvasively at baseline and every 15 min after the start of the study period. L-NMMA alone reduced RPF (-22%, P < 0.001) and GFR (-8%, P < 0.009) and increased MAP (+10%, P < 0.001). BQ-123 alone did not affect these parameters. However, coinfusion of BQ-123 blunted the effects of L-NMMA on RPF (P < 0.001), GFR (P < 0.001), and MAP (P = 0.006). Peripheral and renal hemodynamic effects of acute systemic NO synthase inhibition are at least partially reversed by ET(A) receptor blockade with BQ-123. This indicates a functional antagonism between specific ET(A) receptor antagonist and NO synthase inhibitors at the level of the renal vasculature.