Biomedical subjects

Helmut R Brand

Publications and source records attributed to Helmut R Brand.

13 recordsLinked to original sources

PubMed2006

Rotatoelectricity in cholesteric side-chain liquid single crystal elastomers.

We analyze the phenomenon of rotatoelectricity which is characteristic of cholesteric side-chain liquid single crystal elastomers. Using a linearized macroscopic continuum description and our previous work we show that if such a material is exposed to a static external electric field oriented parallel to the cholesteric helical axis, the director of the liquid crystalline phase will rotate around the helical axis. The material considered is assumed to be a perfect electric insulator. We propose an experiment in which the effect of rotatoelectricity should be directly observable and from which the ratio of the material parameters involved will be accessible.

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PubMed2006

Macroscopic dynamics of polar nematic liquid crystals.

We present the macroscopic equations for polar nematic liquid crystals. We consider the case where one has both, the usual nematic director, n[over ] , characterizing quadrupolar order as well as the macroscopic polarization, P , representing polar order, but where their directions coincide and are rigidly coupled. In this case one has to choose P as the independent macroscopic variable. Such equations are expected to be relevant in connection with nematic phases with unusual properties found recently in compounds composed of banana-shaped molecules. Among the effects predicted, which are absent in conventional nematic liquid crystals showing only quadrupolar order, are pyro-electricity and its analogs for density and for concentration in mixtures as well as a flow alignment behavior, which is more complex than in usual low molecular weight nematics. We also discuss the formation of defect structures expected in such systems.

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PubMed2006

Noise-induced spatiotemporal patterns in a bistable reaction-diffusion system: photoelectron emission microscopy experiments and modeling of the oxidation reaction on Ir(111).

We use photoelectron emission microscopy (PEEM) measurements to study the spatiotemporal patterns obtained for the CO oxidation reaction on Ir(111) as a function of the noise strength we superpose on the CO and the oxygen fractions of the constant total reactant gas flux. The investigations are focused on the bistable regime this reaction displays including its monostable vicinity. Simultaneously we analyze numerically the underlying reaction-diffusion (RD) equations in two spatial dimensions. For intrinsic and/or small strength of the external noise we find transitions from the locally stable to the globally stable branch via slow nucleation and growth of islands of the globally stable state: oxygen or CO, respectively. With increasing noise strength the number of islands as well as their growth rate increases. These phenomena are very well reproduced by numerical calculations of the RD model. For sufficiently large noise strength we observe bursts from CO rich to oxygen rich and back as well as switching between the two states. While such phenomena are also obtained from the model calculations, their experimentally observed spatial scales were not satisfactorily reproduced using the same approach as for the lower noise strengths.

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PubMed2006

Phase winding in chiral liquid crystalline monolayers due to Lehmann effects.

We model recent experimental results by Tabe and Yokoyama on chiral liquid crystalline monolayers driven by the transmembrane transfer of water molecules. We point out that the target patterns found for the orientation of the in-plane preferred direction can be described as phase winding due to Lehmann effects, which are specific to macroscopically chiral systems. The model presented has only one variable and is driven and dissipative. Including the effects of noise leads to an accurate representation of the patterns observed experimentally. Future experimental tests of the model are suggested.

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PubMed2005

Stable stationary and breathing holes at the onset of a weakly inverted instability.

We show numerically different stable localized structures including stationary holes, moving holes, breathing holes, stationary and moving pulses in the one-dimensional subcritical complex Ginzburg-Landau equation with periodic boundary conditions, and using two classes of initial conditions. The coexistence between different types of stable solutions is summarized in a phase diagram. Stable breathing moving holes as well as breathing nonmoving holes have not been described before for dissipative pattern-forming systems including reaction-diffusion systems.

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PubMed2005

The Gray-Scott model under the influence of noise: reentrant spatiotemporal intermittency in a reaction-diffusion system.

We investigate the influence of noise on the spatiotemporal behavior of the Gray-Scott model, a prototype for a simple reaction-diffusion system. In the parameter regime studied it is characterized deterministically by a stable fixed point. As the noise increases a regular periodic pattern is replaced first by an irregularly oscillating periodic pattern and then by spatiotemporal intermittency. With further increasing noise strength the spatiotemporal intermittency is first replaced by a low amplitude noisy regime followed by spatiotemporal intermittency (STI) embedded into a noisy background. At sufficiently high noise intensity high amplitude noise prevails. We point out that the transition from spatiotemporal intermittency to low amplitude noise can be traced back to the fact that the spatially homogeneous state is a global attractor. As the noise strength grows further the "noisy" fixed point starts to communicate with STI leading to noise-induced spatiotemporal intermittency as an excitable state. At high enough noise strength high amplitude noise is left over wiping out all details of the underlying deterministic dynamical system.

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PubMed2005

Spatiotemporal patterns of external noise-induced transitions in a bistable reaction-diffusion system: photoelectron emission microscopy experiments and modeling.

The rate of CO oxidation on Ir(111) surfaces exhibits bistability at T=500 K in a range of the CO fraction Y in the CO+O reactant gas flux. Measured CO2 rates as a function of the noise strength imposed on Y are well reproduced by parameter-free modeling. We present photoelectron emission microscopy measurements and 2D calculations of the spatiotemporal patterns of CO- and O-rich domains. The role of combined multiplicative and additive noise on Y for CO and O domain wall motion and island nucleation-growth-coalescence processes is analyzed.

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PubMed2005

Low-voltage-driven electromechanical effects of swollen liquid-crystal elastomers.

We experimentally investigate, in detail, electromechanical effects in liquid-crystal elastomers (LCEs) previously swollen with low-molecular-weight liquid crystals (LMWLCs). Both polydomain (POLY) and monodomain (MONO) LCEs were studied. We used a well known LMWLC, 4-n-pentyl-4-cyanobiphenyl (5CB) as a solvent. After swelling POLY and MONO LCEs (LSCE) with 5CB, shape changes were measured by recording the displacement of the edge of the swollen LCE at different voltages, V, and temperature. With 100 microm distance between electrodes, measurable shape changes (approximately 1-20 microm) are observed with small voltages (V approximately 0.5-10 V). In particular, we note that, compared to unswollen L(S)CEs, a dramatic approximately 200 times decrease of the threshold field was found for electromechanical effects in swollen L(S)CEs. While swollen MONO LCEs showed electromechanical effects in the planar geometry, homeotropic MONO swollen with homeotropically oriented 5CB did not. This is easy to understand because, in the homeotropic case, the liquid-crystal preferred axis is already aligned with the field so the field has no reorienting effect. The inverse of the response time when the field was switched on in both POLY and MONO was proportional to E2, which is the same field dependence as the response time of LMWLCs. When the field was switched off, the relaxation time showed a field dependence different from that of LMWLCs that we attribute to relaxation of the LCE network.

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PubMed2004

Coexistence of stable particle and hole solutions for fixed parameter values in a simple reaction diffusion system.

We present a simple autocatalytic reaction-diffusion model for two variables, which shows for fixed parameter values the simultaneous stable coexistence of particle solutions as well of two types of hole solutions. The associated spatially homogeneous system is characterized by the coexistence of one stable fixed point and a stable limit cycle solution. We compare our results to other dissipative systems which have for fixed parameters either stable particle or stable hole solutions including the quintic complex Ginzburg-Landau equation and the envelope equation for optical bistability as well as other reaction-diffusion models.

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PubMed2004

External noise imposed on the reaction-diffusion system CO+O2-->CO2 on Ir(111) surfaces: experiment and theory.

We study experimentally and theoretically the influence of noise on the fractions of CO and oxygen in the constant gas flow directed at an Ir(111) surface during CO oxidation. Depending on the noise strength and the fraction Y of CO we observe in the deterministically bistable region a large variety of different types of behavior. These include bistable behavior for small noise intensities, transitions from the upper to the lower branch of the bistable loop and vice versa, island nucleation and growth and noise-induced switching. Near the boundary of the bistable region and in the presence of noise the transition between the two branches takes place via very slow domain wall motion with time scales of the order of 10(4)-10(5) s. The experiments were carried out in an UHV system for which the mass flow could be controlled very precisely. The modeling was using the reaction-diffusion system underlying the reaction studied for which all the kinetic coefficients are known rather precisely. Our numerical analysis was performed for one and two spatial dimensions showing qualitatively similar behavior. The comparison between the experimental results and the modeling shows semiquantitative to quantitative agreement.

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PubMed2004

Swelling dynamics of liquid crystal elastomers swollen with low molecular weight liquid crystals.

We experimentally investigated the swelling behavior of thin films (approximately 150 microm) of liquid crystalline elastomers (LCEs) by low molecular weight liquid crystals (LMWLCs). The two LMWLCs used are the well-known nematic liquid crystals, 4-n-pentyl-4-cyanobiphenyl, and 4-methoxy-benzilidene-4-butyl-aniline. Both polydomain (POLY) and monodomain (MONO) LCE swelling are studied. In MONO LCEs (LSCEs), the director n empty set is uniformly oriented throughout the film. POLY films are made of many domains with different orientations. Its swelling behavior was similar to isotropic gels. In contrast, LSCEs revealed interesting results not anticipated by any theory. First, the LMWLC enters the LSCE by front propagation about three-times faster axially n empty set than radially n empty set. Second, only the LSCE dimensions radially n empty set expanded, while that axially n empty set did not change at all. Third, when the LMWLC director and the LSCE director are aligned (MONO2 samples), swelling takes place about twice as fast as when they are not aligned. Volume change dynamics of swollen L(S)CEs investigated as a function of temperature revealed several phase transitions by optical and calorimetry techniques.

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PubMed2004

Analytical approach to localized structures in a simple reaction-diffusion system.

We study from an analytical point of view a simple reaction-diffusion model, which admits stable oscillating localized structures as a consequence of the coexistence between a stable limit cycle and a stable fixed point. Using a generalized matching approach we are able to find approximate analytical expressions for localized oscillating structures in this reaction-diffusion model capturing all the essential ingredients of these breathing particlelike solutions.

Diffusion↗

PubMed2002

Shear-induced instabilities in layered liquids.

Motivated by the experimentally observed shear-induced destabilization and reorientation of smectic-A-like systems, we consider an extended formulation of smectic-A hydrodynamics. We include both, the smectic layering (via the layer displacement u and the layer normal p(circ)) and the director n(circ) of the underlying nematic order in our macroscopic hydrodynamic description and allow both directions to differ in nonequilibrium situations. In an homeotropically aligned sample the nematic director does couple to an applied simple shear, whereas the smectic layering stays unchanged. This difference leads to a finite (but usually small) angle between n(circ) and p(circ), which we find to be equivalent to an effective dilatation of the layers. This effective dilatation leads, above a certain threshold, to an undulation instability of the layers. We generalize our earlier approach [G. K. Auernhammer, H. R. Brand, and H. Pleiner, Rheol. Acta 39, 215 (2000)] and include the cross couplings with the velocity field and the order parameters for orientational and positional order and show how the order parameters interact with the undulation instability. We explore the influence of various material parameters on the instability. Comparing our results to recent experiments and molecular dynamic simulations, we find a good qualitative agreement.

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