Control and coupling of spiral waves in excitable media

We report the complex dynamics of spiral waves observed in the ferroin-catalyzed BZ reaction. The reaction is run in an open unstirred reactor (CFUR) with the catalyst immobilized on a polysulfone membrane. The catalyst-loaded membrane is placed between two well stirred compartments which are fed with solutions of sulfuric acid/malonic acid/bromide and sulfuric acid/bromate, respectively. An electrical field perpendicular to the membrane can be applied via Pt-ring electrodes or, alternatively, via transparent electrodes made of ITO-coated glass. In the field-free case relatively simple target and spiral patterns are observed in the membrane. If an alternating electrical field is applied the spiral core drifts through the membrane. The actual trajectory of the spiral tip depends on the amplitude and frequency of the applied electrical field. If the perturbation parameters are chosen properly the wave fronts break up and new spiral cores emerge under the influence of the alternating field. Complex spatio-temporal patterns may be induced which are reminiscent of "spiral-chaos". After switching off the perturbation the system returns to its previous, "simple" behaviour. Our experimental observations are confirmed by model calculations based on the Barkley model of spiral waves. The technique of using modulated excitability to control the dynamics of spiral waves is further extended to the coupling of two spirals in two CFURs. We present numerical simulations based on two identical excitable reaction-diffusion (RD) systems which are mutually coupled. The coupling is based on the observation of an arbitrarily chosen point inside each of the RD systems: If a chemical wave passes the point of optical observation in system 1 an electric field is applied to system 2 and vice versa. Thus, a local observation made in one system is transformed in a global perturbation of the second CFUR. We report the observation of CFUR states where the two spiral waves are spatially and temporally coupled to each other.     

Control of spiral breakup by an alternating advective field

The control of spiral breakup due to Doppler instability is investigated. It is found that applying an alternating advective field with suitable amplitude and period can prevent the breakup of spiral waves. Further numerical simulations show that the growing meandering behavior of a spiral tip caused by decreasing the excitability of the medium can be efficiently suppressed by the alternating advective field, which inhibits the breakup of spiral waves eventually.     

Transition from Turing stripe patterns to hexagonal patterns induced by polarized electric fields

The effect of a circularly polarized electric field on the Turing stripe patterns is studied. The numerical results show that stripe patterns may change to hexagonal wave patterns by choosing the intensity and the frequency of the circularly polarized electric field suitably. Our findings indicate that a pattern tends to organize itself to the pattern with the same symmetry of the applied field with the fact that compared to the stripe patterns, hexagonal wave patterns possess hexagonal symmetry which is closer to the rotation symmetry of the circularly polarized electric field.     

Modelling the internal field distribution in human erythrocytes exposed to MW radiation

This paper studies the internal electric field distribution in human erythrocytes exposed to MW radiation. For this purpose, an erythrocyte cell model is exposed to linearly polarized electromagnetic (EM) plane waves of frequency 900 MHz and the electric field within the cell is calculated by using a finite element (FE) technique with adaptive meshing. The results obtained show the dependence of the induced electric field distribution on the main modelling parameters, i.e., the electrical properties (permittivity and conductivity) of the membrane and cytoplasm and the orientation of the cell with respect to the applied field. It is found that for certain orientations, the field amplification within the membrane of the erythrocyte shape cell can be higher than the one observed in an equivalent simple spheroidal geometry cell, commonly used in bioelectromagnetism. The present work shows that a better insight of the interaction of electromagnetic fields with basic biological structures is obtained when the most possible realistic cell shape is used.     

Programmed Locomotion of an Active Gel Driven by Spiral Waves

Active media that host spiral waves can display complex modes of locomotion driven by the dynamics of those waves. We use a model of a photosensitive stimulus‐responsive gel that supports the propagation of spiral chemical waves to study locomotive transition and programmed locomotion. The mode transition between circular and toroidal locomotion results from the onset of spiral tip meandering that arises via a secondary Hopf bifurcation as the level of illumination is increased. This dynamic instability of the system introduces a second circular locomotion with a small diameter caused by tip meandering. The original circular locomotion with large diameter is driven by the push‐pull asymmetry of the wavefront and waveback of the simple spiral waves initiated at one corner of gel. By harnessing this mode transition of the gel locomotion via coded illumination, we design programmable pathways of nature‐inspired angular locomotion of the gel.     

Spectroscopic analysis of electrically polarized polyacrylonitrile

Electrically polarized and unpolarized films of polyacrylonitrile (PAN) have been investigated by both infrared attenuated total reflection (ATR) and thermally stimulated discharge (TSD). ATR analysis of PAN films has been aimed at explaining the molecular origin of thermally stimulated currents, and consequently that of phenomena contributing to electrical polarization in this material. Preferred orientation of nitrile dipoles along the thickness direction (applied electric field direction) has been detected by ATR in both polarized and unpolarized films. It is suggested that dipolar alignment in unpolarized solvent-cast films could result from internal electric fields associated with space charges. The observed orientational anisotropy is found to disappear gradually as films (both polarized and unpolarized) are heated from ambient temperature to 100°C and rotational motion in the backbone is thermally activated. TSD currents observed in this thermal range are thus associated with randomization of molecular dipoles. TSD currents observed above 100°C are suggested to originate from drift of space charges, since both an isotropic orientation of dipoles and onset of considerable diffusion are detected by ATR in this temperature range. PAN films polarized by high-intensity electric fields (5 × 10⁵Vcm^?1, as opposed to 5 × 10⁴Vcm^?1) are found to retain orientational anisotropy above 100°C, and this is believed to be associated with a structural rearrangement induced by electrical polarization.     

Electronic properties of capped, finite-length armchair carbon nanotubes in an electric field

This study investigates the electronic properties of finite-length armchair carbon nanotubes in an electric field (E) using a single-pi-band tight-binding model. Three different tip configurations are considered, namely, open ends with hydrogen terminations (H-terminations), one end capped with half of C60 fullerene and the other end open with H-terminations, and both ends capped with half of C60 fullerene. In general, the electronic states and energy gaps of low-energy electronic structures are highly sensitive to changes in the direction and magnitude of the applied electric field and to the tip configuration. The present results show that the electric field induces a strong modulation of the state energies and energy gaps of the current nanotubes, changes their energy spacings, and prompts the occurrence of semiconductor-metal transitions (SMTs). It is found that the SMTs occur more frequently as the direction of the electric field approaches the symmetry axis or when its magnitude becomes sufficiently large. The present results also indicate that the Fermi levels and energy gaps of the three nanotubes considered in this study are strongly influenced by the cap configuration. Finally, the convergent decay behavior of the energy gap which is observed as the length of the nanotube is increased is also strongly dependent on the tip configuration.     

非水介质中明胶水凝胶电场驱动行为的研究

研究了明胶水凝胶在绝缘硅油中的电场响应行为。结果表明,在硅油中,明胶水凝胶在外加高压直流电场作用下可发生运动,其运动由转动和平动两部分组成。存在一个运动所需的最小阈值电场,只有外加电场在此阈值以上时,才可观察到水凝胶明显的运动。水凝胶的运动速度随外加电场的增大而增大,其运动可通过外加电场的大小来调控。由硅油很稳定且在电场中会电解,因此避免了传统电场驱动水凝胶在水介质中响应时不可避免的电解缺点,为建立一种新的电响应凝毅然驱动方式提供了可能。     

Dynamics of dissipative structures and the transition to turbulence in a nematic liquid crystal

It has been observed for the first time that the domain oscillations of a nematic liquid crystal subject to a transverse DC electric field are self-organized in space as concentric and spiral phase waves. The formation of the phase wave sources has been described. The transition of the hexagonal convective structure to turbulence has been investigated by the temporal Fourier analysis method. It has been found that a route to turbulence passes through a continuous widening of peaks in the power spectrum of light intensity transmitted through the cell.     

LOCAL ELECTRIC FIELD EFFECT ON THE CHLOROPHYLL FLUORESCENCE

Abstract. Polarized absorption and fluorescence of chlorophyll a and bacteriochlorophyll solutions in nematic liquid crystal mixtures have been studied in the presence of an external electric field.
The electric field caused a reorientation of the pigment molecules as concluded from changes in the polarized absorption. However, no correlation was found between the change in the polarized absorption and the change in the polarized fluorescence as a function of the field strength. The field influence was much stronger than expected only from the molecular orientation. The fluorescence polarized parallel to the direction of the liquid crystal was found to increase, whereas the similarly polarized absorption decreases. As a whole, the fluorescence yield significantly increased. It is suggested that the additional electric field is reinforced by a local field, most probably due to a protonation of the liquid crystal molecules.
Charged solvent molecules are reoriented in an external electric field which changes the local electric field acting on chlorophylls. Similar changes in CHI fluorescence yield due to local electric field can be created in vivo by the shift of charged molecules present in surrounding pigments. Such effects can be at least partially responsible for slow induction of fluorescence phenomena.     

迁移管的电场强度对真空紫外电离-离子迁移谱仪性能的影响

离子迁移管是离子迁移谱仪的核心部分,它用来产生均匀的电场,以使不同迁移率的离子进行分离。本研究以丙酮为例,详细研究了本课题组所研制的真空紫外电离源-离子迁移谱仪中迁移管的电场参数对离子的灵敏度和分辨率的影响,发现电压的增大灵敏度增大,但是分辨率存在一个最佳的电压,这些结果可用于迁移谱的优化设计。     

Isothermal crystallization of poly(vinylidene fluoride) in the presence of high static electric fields. I. Primary nucleation phenomenon

Primary nucleation of poly(vinylidene fluoride) (PVF₂) γ-phase crystals from the melt is affected by the presence of an electric field. Crystallization under the electric field is studied by polarized optical microscopy, thermal analysis, and wide-angle x-ray diffraction. Preliminary results indicate that the γ-phase crystal nucleation rate and content are enhanced by the electric field. A modification of the classical theory of homogeneous nucleation of a crystalline phase is proposed to account for the experimental observations. Electrostatic interaction between the nucleus total polarization and the electric field contributes to the free energy of nucleation to a very large extent at low undercooling. Theoretical predictions indicate that a static electric field will increase the nucleation rate of a polar phase and will decrease the nucleation rate of a nonpolar phase. Confirmation of the former fact is observed experimentally.     

Proposal of a new method to determine tilt angles of absorption transition moments in host-guest liquid crystal cells

An idea for a method of experimental determination of tilt angles and order parameters of cylindrosymmetric transition moments of liquid crystals and of dyes in host-guest systems is described. Furthermore, first experimental hints are given. With freely rotating molecules, all transition moments result in an isotropic absorbance for UV/VIS-spectroscopy using polarized light. The resulting absorbance becomes anisotropic if the transition moment of the molecules is polarized, the molecules are aligned in a preferential direction, and the free rotation is suppressed. In this case, the measurement of Aiso, Atilt, and Atilt using polarized light is sufficient to calculate the related tilt angle Psi, the order parameter S, and the absorbance A as a function of the polarization angle. The local electric field and its influence on the dichroic ratio of an absorption band has not been considered. The precision of this method is still limited by the photometric accuracy of the spectrometer and the mechanical precision of the goniometric adjustment of the experimental apparatus.     

Stimulated light scattering in smectic A liquid crystals

The non-linear interaction of arbitrary polarized light with smectic layer deformations in smectic A liquids (S_A) is considered. It is shown that the combined effect of anisotropy, fluidity and a characteristic kind of deformation cause a number of specific non-linear optical phenomena. Two-wave mixing in S_A transforms into a partly degenerate four-wave mixing (FWM) when the polarization and the direction of propagation of the coupled electromagnetic (EM) waves are arbitrary. The interference of the EM waves gives rise to a dynamic grating of layer deformations without a change of mass density of S_A. In the resonant case a propagating mode of a second sound (SS) is excited. The non-linear phenomena are analysed by solving the self-consistent system of the Maxwell equations for the non-linear anisotropic inhomogeneous medium and the hydrodynamic equations of S_A in the external EM field. The explicit expressions of the EM and SS waves amplitudes are obtained. It is shown that the coupled fundamental EM waves undergo the parametric amplification and the phase cross-modulation, and their amplitudes as well as the SS wave amplitude are spatially localized. The energy transfer between the coupled EM waves is non-reciprocal. The scattering of the fundamental EM waves by the dynamic grating results in the appearance of additional harmonics with combination frequencies and wavevectors. The light induced dynamic grating also generates a longitudinal electric field due to the flexoelectric effect.     

Delay-induced inward and outward spiral waves in oscillatory medium

Inward and outward spiral waves as well as inward target waves are induced by local delay feedback in a reaction-diffusion system exhibiting a Turing hexagon pattern spontaneously. The system gives rise to large-amplitude sinusoidal oscillations when the inward spiral waves are observed. Compared with the inward spirals, the outward spiral waves usually possess longer wavelength and exhibit larger amplitude relaxation oscillations. Varying the feedback parameters continuously, the inward spiral waves are transformed into outward spirals through a turbulent state. The dispersion analysis about the delayed system reveals that there is an unstable band on the dispersion curve, where the turbulent state arises. The dispersion curve is divided into two parts by the unstable band. The inward spiral waves exist in the left part with negative group velocity, i.e., domega/dk<0, while the outward spiral waves are stable in the right part with domega/dk>0 (omega is frequency and k wavenumber).     

Electric field effect on the vibration of single CO molecules in a scanning tunneling microscope junction

A low-temperature scanning tunneling microscope (STM) and ab initio calculations were used to study the electric field effect on the vibration of single CO molecules in an STM junction at 13 K. The vibrational energy of CO molecules adsorbed on silver atoms, measured by STM-based inelastic electron tunneling spectroscopy, depends on the direction of the electric field applied between the STM tip and the silver species. This characteristic can be explained by the charge separation model. The electric field modifies the binding characteristics of CO on silver as a result of a change in the charged states of the species, which leads to an increase (or a decrease) of the energies of the hindered rotation and the CO stretch on silver.     

Nanoscale electrowetting effects observed by using friction force microscopy

We report the study of electrowetting (EW) effects under strong electric field on poly(methyl methacrylate) (PMMA) surface by using friction force microscopy (FFM). The friction force dependence on the electric field at nanometer scale can be closely related to electrowetting process based on the fact that at this scale frictional behavior is highly affected by capillary phenomena. By measuring the frictional signal between a conductive atomic force microscopy (AFM) tip and the PMMA surface, the ideal EW region (Young-Lippmann equation) and the EW saturation were identified. The change in the interfacial contact between the tip and the PMMA surface with the electric field strength is closely associated with the transition from the ideal EW region to the EW saturation. In addition, a reduction of the friction coefficient was observed when increasing the applied electric field in the ideal EW region.     

Expanded theory for the resolving power of a linear ion mobility spectrometer

An expanded theory for the resolving power of a linear ion mobility spectrometer (IMS) is derived. By definition, the resolving power is directly proportional to the total drift time for the ion through the drift tube divided by the full-width-at-half-height (FWHH) of the observed ion mobility peak. Two approaches to theoretically estimating these two parameters are possible, depending on the operating parameters of the IMS cell. The drift time is given by the first moment of the IMS response. If the electric fields (assumed uniform) are equal in both the shutter/aperture and aperture/collector region, the FWHH is given by a difference in error functions. If the electric fields (again assumed uniform) are not equal, the FWHH is given by the second central moment of the IMS response and can only be known to within a multiplicative factor. The effectiveness of these two approaches is demonstrated using IMS data from the published literature.The additional peak broadening often observed in a linear IMS has several possible sources. One depends on the construction of the cell and the parallelism (or lack thereof) that might exist between the aperture grid and ion collector. Another depends on electric fields used to bias the cell. If the electric field in the aperture/collector region is less than in the shutter/aperture region, peak broadening occurs. Induction effects in the aperture/collector region not only shorten drift times, but also create diffusion-like broadening of the peak. Shortening the distance between the aperture grid and ion collector, or using a higher electric field in that region, minimizes induction effects. Drift time calibration requires adjustments for induction effects.     

酞菁聚合物薄膜的电场取向和光导性能研究

通过电场取向法成功地制备了酞菁类聚合物(PPc)有序薄膜,并采用偏振荧光和TEM技术对其取向进行了研究.分别以该PPC有序薄膜和腙类作为载流子发生层和载流子传输层制备光电导体,发现其光导性能优于不加电场制备的PPc薄膜,呈现了光导性能提高的材料结构低维化效应.     

The influence of layer curvature on switching in ferroelectric B2 phases of liquid crystals having bent-shape molecules

In the B₂ phase of liquid crystalline compounds with bent-shape molecules ferroelectric switching can occur either by molecular rotation on the cone or by rotation of the molecule about its long axis (so-called chirality flipping), or by both mechanisms simultaneously. When the smectic layers of the B₂ phase are non-deformed and parallel the rotation of molecules under an external electric field occurs readily on the surface of the cone, while rotation around the long molecular axis is hindered by an energy barrier. Imposed deformation of smectic layers leads to interaction between local layer curvatures and molecular orientation, which results in the energy barrier hindering the molecular rotation by a cone. For appropriate constants describing this interaction the energy barrier can be so high that chirality flipping becomes the principal switching mode. An increase in the electric field can eliminate layer curvature, and therefore the energy barrier, so that switching with molecular rotation on the cone becomes possible. In the present contribution these mechanisms of switching are discussed and the influence of layer curvature on the switching mode is demonstrated.