Electrohydrodynamic instability in some nematic cyanobiphenyls in an a.c. electric field

Abstract

Electrohydrodynamic instability in homeotropically oriented nematic samples of 4′-n-octyl-4-cyano-biphenyl and 4′n-alkyloxy-4-cyanobiphenyl, (n = 8.9) have been studied in an a.c. electric field. The domain patterns during the instability in these compounds in a very low frequency a.c. field are very similar to those in a d.c. field. The domain patterns observed at higher frequencies have been identified as ‘maltese crosses’ or ‘crossed isogyres’. The electro-convective ‘isotropic’ flows near the electrode play an important role in the observed instability.     

Superlattice patterns and spatial instability induced by delay feedback

Various oscillatory superlattice patterns in a reaction-diffusion system are observed by means of delay feedback (DF) in the parametric domain where the system without DF displays uniform bulk oscillation. By varying DF parameters within an appropriate range, the system undergoes transitions to oscillatory hexagons, stripes and squares, and square superlattices with different wavenumbers are also obtained. Linear stability analysis reveals that the patterns do not result from the Turing instability and a possible mechanism of pattern formation is suggested and proved analytically: DF induces instability of a homogeneous limit cycle with respect to spatial perturbations even if the Turing instability does not occur, so that oscillatory patterns possessing the corresponding spatial modes are produced. The different behavior of the dominant characteristic multiplier seems to be connected to the pattern selection. Here it is clearly demonstrated that DF can play a destabilizing role in spatially extended system instead of stabilizing the periodic orbits or turbulent states, which most earlier works have usually focused on.     

Electrohydrodynamic instability in a binary mixture of cyanobiphenyls in a d.c. field

Electrohydrodynamic instability in a homeotropically oriented liquid crystal mixture, with positive dielectric anisotropy, has been studied under a d.c. electric field. Various studies on textures at different voltage were also made. The existence of charge injection, charge diffusion and convective flow of liquid in the liquid crystal cells was found. The effect of viscosity, dielectric anisotropy and other physical parameters on electric current through the sample cells, is discussed. A small change in phase transition temperature on application of an electric field was found in the thin cell of the sample material.     

Joule heating effects on electrokinetic flows with conductivity gradients

Instability occurs in the electrokinetic flow of fluids with conductivity and/or permittivity gradients if the applied electric field is beyond a critical value. Understanding such an electrokinetic instability is significant for both improved transport (via the suppressed instability) and enhanced mixing (via the promoted instability) of liquid samples in microfluidic applications. This work presents the first study of Joule heating effects on electrokinetic microchannel flows with conductivity gradients using a combined experimental and numerical method. The experimentally observed flow patterns and measured critical electric fields under Joule heating effects to different extents are reasonably predicted by a depth-averaged numerical model. It is found that Joule heating increases the critical electric field for the onset of electrokinetic instability because the induced fluid temperature rise and in turn the fluid property change (primarily the decreased permittivity) lead to a smaller electric Rayleigh number.     

Magnetic Fluid Structural Transformations in Electric and Magnetic Fields

Periodic structural lattices formed in thin layers of an originally uniform magnetic fluid exposed to a constant electric field were studied experimentally. Peculiarities of their transformations under the effect of an additional magnetic field were described. The structural transformations and the development of an electrohydrodynamic instability in an aggregated magnetic fluid subjected to both alternating electric and constant magnetic fields were investigated. The observed patterns of the light diffraction on the studied structures were reported.     

Pattern formation and morphology evolution in Langmuir monolayers

We present a study of how patterns formed by Langmuir monolayer domains of a stable phase, usually solid or liquid condensed, propagate into a metastable one, usually liquid expanded. During this propagation, the interface between the two phases moves as the metastable phase is transformed into the more stable one. The interface becomes unstable and forms patterns as a result of the competition between a chemical potential gradient that destabilizes the interface on one hand and line tension that stabilizes the interface on the other. During domain growth, we found a morphology transition from tip splitting to side branching; doublons were also found. These morphological features were observed with Brewster angle microscopy in three different monolayers at the water/air interface: dioctadecylamine, ethyl palmitate, and ethyl stearate. In addition, we observed the onset of the instability in round domains when an abrupt lateral pressure jump is made on the monolayer. Frequency histograms of unstable wavelengths are consistent with the linear-instability dispersion relation of classical free-boundary models. For the case of dendritic morphologies, we measured the radius of the dendrite tip as a function of the dendrite length as well as the spacing of the side branches along a dendrite. Finally, a possible explanation of why Langmuir monolayers present this kind of nonequilibrium growth patterns is presented. In the steady state, the growth behavior is determined by Laplace's equation in the particle density with specific boundary conditions. These equations are equivalent to those used in the theory of morphology diagrams for two-dimensional diffusional growth, where morphological transitions of the kind observed here have been predicted.     

AC electric field induced helix unwinding in planar texture of a ferroelectric liquid crystal

The helix unwinding induced by an AC electric field in a thick ferroelectric S*_C sample with planar orientation has been investigated experimentally. Four patterns of instabilities have been observed below the unwinding critical field E_c. The corresponding thresholds and are found to be dependent on frequency. The formation of two instabilities (λ pattern and rhombic lattice pattern) is discussed on the basis of the present theory. The behaviour above the critical field is briefly described.     

Comparison of bulk and thin film structures of the liquid crystal 28OBC: measurements and simulations

Freely-suspended liquid crystalline films of ethyl 4'-n-octyloxybiphenyl-4-carboxylate (28OBC) were prepared and transferred onto different substrates which enable detailed structural characterization. The structures of these thin film assemblies, which are only accessible in this way, were determined and compared with the crystal and molecular structure of 28OBC as formed by crystallization from toluene solution. The compound crystallizes in the monoclinic space group P21/c, a = 11.168(1) A, b = 7.595(2) A, c = 49.106(1) A, beta = 94.01(1) degree, Z = 8. The two symmetrically independent molecules of the asymmetric unit have been used as starting geometries for semi-empirical MO calculations. The difference between the experimentally observed and the optimized molecular structures is interpreted as the influence of the crystal field. The structures of the crystalline and E film phases have been investigated by SAXR and TED and the former has been found to be different from the bulk structure. The structural relationships between the different phases are discussed and a suggestion for the crystalline film structure is given as deduced from simulations of electron diffraction patterns.     

Admittance measurements on protein layers adsorbed at the Pt/solution interface: effect of d.c. potential and a.c. field

The effect of changes in the electric field on the structure of protein layers adsorbed at the Pt electrode/solution interface has been investigated by means of admittance measurements. The measurements have been performed over a wide range of d.c. potentials, at different frequencies and amplitudes of the a.c. electric field, and at various pH values. The proteins used, cytochrome C and serum albumin, differ considerably with respect to their molecular masses, points of zero charge and structure stabilities. In contrast to serum albumin, cytochrome C has a relatively strong electric dipole moment. Nevertheless, the results for the two proteins are very similar. Both proteins stay adsorbed at the interface over the d.c. potential range studied and at every pH, irrespective of any electrostatic repulsion. Apparently, for both proteins factors other than electrostatic interactions are dominant in their final binding to the Pt/solution interface. In line with this, no indications were obtained that the orientation of adsorbed cytochrome C molecules is modulated by low-frequency (200–1000 Hz) reversal of the electric field of the interface. A strong a.c. field leads to irreversible structural changes in the adsorption layer for both proteins.     

Pattern formation in phase separating binary mixtures

We experimentally investigate the interplay of thermodynamics with hydrodynamics during phase separation of (quasi-) binary mixtures. Well defined patterns emerge while slowly crossing the cloud point curve. Depending on the material parameters of the experimental system, two distinct scenarios are observed. In quasi-binary mixtures of methanol-hexane patterns appear before macroscopic phase separation sets in. In course of time the patterns turn faint while the overall turbidity of the sample increases until the mixtures become completely turbid. We attribute this pattern formation to a latent heat induced instability resembling a Rayleigh-Bénard instability. This is confirmed by calorimetric data and an estimate of its Rayleigh number. Mixtures of C(4)E(1)-water doped with decane phase separate under heating. After passing the cloud point curve these mixtures first become homogenously turbid. While clearing up, pattern formation is observed. We attribute this type of pattern formation to an interfacial tension induced Bénard-Marangoni instability. The occurrence of the two scenarios is supported by the relevant dimensionless numbers.     

Orientation of DNA and the agarose gel matrix in pulsed electric fields

Transient electric birefringence has been used as an analytical tool to study the orientation of DNA in agarose gels, and to study the orientation of the matrix alone. The sign of the birefringence of DNA oriented in an agarose gel is negative, as observed in free solution, indicating that the DNA molecules orient parallel to the direction of the electric field. If the median pore diameter of the gel is larger than the contour length of the DNA molecule, the DNA effectively does not see the matrix and the birefringence relaxation time is the same as observed in free solution. However, if the median pore diameter of the gel is smaller than the contour length of the DNA, the DNA molecule becomes stretched as well as oriented. For DNA molecules of moderate size (less than or equal to 4 kb), stretching in the gel causes the birefringence relaxation times to increase to the values expected for fully stretched molecules. Complete stretching is not observed for larger DNA molecules. The orientation and stretching of DNA molecules in the gel matrix indicates that end-on migration, or reptation, is a likely mechanism for DNA electrophoresis in agarose gels. When the electric field is rapidly reversed in polarity, very little change in the orientation of the DNA is observed if the DNA molecules were completely stretched and had reached their equilibrium orientation before the field was reversed in direction. Hence completely stretched, oriented DNA molecules are able to reverse their direction of migration in the electric field with little or no loss of orientation. However, if the DNA molecules were not completely stretched or if the equilibrium orientation had not been reached, substantial disorientation of the DNA molecules is observed at field reversal. The forced rate of disorientation in the reversing field is faster than the field-free rate of disorientation. Complicated patterns of reorientation can be observed after field reversal, depending on the degree of orientation in the original field direction. The effect of pulsed electric fields on the orientation of the agarose gel matrix itself was also investigated.(ABSTRACT TRUNCATED AT 400 WORDS)     

Flexoelectric response at defect sites in nematic inversion walls

Electric field experiments have been carried out on +1 and -1 defects formed in alignment inversion walls, in a planarly aligned nematic phenyl benzoate. The results show that the defects are non-singular in the core and exhibit a flexoelectric response to an applied d.c. or low frequency a.c. field. When the c-director flux lines are circular, as in a +1 defect in a wall parallel to the easy axis, flexoelectro-optic switching characterized by an azimuthal angle variation is observed. When the c-director flux is radial, the response is seemingly through polar angle variations involving no rotation of the extinction brushes due to crossed polarizers. This conclusion follows from the field-induced structural distortions observed at a -1 defect having a combination of radial and tangential c-director fields.     

Ion excitation in a linear quadrupole ion trap with an added octopole field

Modeling of ion motion and experimental investigations of ion excitation in a linear quadrupole trap with a 4% added octopole field are described. The results are compared with those obtained with a conventional round rod set. Motion in the effective potential of the rod set can explain many of the observed phenomena. The frequencies of ion oscillation in the x and y directions shift with amplitude in opposite directions as the amplitudes of oscillation increase. Excitation profiles for ion fragmentation become asymmetric and in some cases show bistable behavior where the amplitude of oscillation suddenly jumps between high and low values with very small changes in excitation frequency. Experiments show these effects. Ions are injected into a linear trap, stored, isolated, excited for MS/MS, and then mass analyzed in a time-of-flight mass analyzer. Frequency shifts between the x and y motions are observed, and in some cases asymmetric excitation profiles and bistable behavior are observed. Higher MS/MS efficiencies are expected when an octopole field is added. MS/MS efficiencies (N(2) collision gas) have been measured for a conventional quadrupole rod set and a linear ion trap with a 4% added octopole field. Efficiencies are chemical compound dependent, but when an octopole field is added, efficiencies can be substantially higher than with a conventional rod set, particularly at pressures of 1.4 x 10(-4) torr or less.     

Dielectric spectroscopy of a smectic liquid crystal

Complex dielectric spectroscopy (frequency range 5 Hz–13 MHz) has been used to analyse the frequency, temperature and bias‐field dependences of the molecular dynamics of a very high‐spontaneous‐polarization ferroelectric liquid crystalline material exhibiting SmA, SmC* and unknown SmX smectic phases. Different smectic phase transition temperatures have been observed from the study of the temperature dependence of the dielectric strength and the relaxation frequency. The phase transition temperatures (crystalline to isotropic phases) have also been described very accurately from the temperature‐dependent symmetric and asymmetric shape parameters of the relaxation function and also the dc conductivity. In a planar aligned cell, two symmetric modes (Goldstone mode and domain mode) have been observed in both the SmX and SmC* phases. One asymmetric mode (X‐mode) observed in the SmC* and SmA phases could be related to the interaction of dipoles of the ferroelectric liquid crystals being affected by the surface of the cell. The soft mode, which usually appears very close to the SmC*–SmA phase transition was not observed until the bias field was applied. The second order nature of the SmC*–SmA phase transition was revealed.     

Helix unwinding process in the chiral smectic C phase of MHPOBC as observed by conoscopy

Conoscopic studies have been made on the helix unwinding process in the chiral smectic C phase of MHPOBC. The unwinding process is rather unique in the sense that abrupt changes of the biaxiality and apparent tilt angle are followed by further gradual changes before the complete helix unwinding. The process is qualitatively explained by simulating the conoscopic images using the 4 x 4 matrix method with consideration of model molecular distributions, by taking account of both the ferroelectric and dielectric coupling between molecules and a field. The transmittance loss due to selective reflection was also measured under the application of a stepwise d.c. field. The shift of the dip position due to the loss, toward a longer wavelength region, was observed at low field. In contrast, the emergence of a transmittance loss at the same wavelength as that of an unperturbed state was observed. This phenomenon is explained by the ferroelectric deformation of a helix with the pitch unchanged.     

射频溅射制备的多晶ZnO膜表面氧的吸附和脱附

多晶的六角密排的ｃ轴平行于衬底的ＺｎＯ膜已用射频溅射的方法制备出来，一电场感应的氧吸附在这些膜上被观察到由紫外光照射或与Ａｒ离子反应所产生的氧的脱附可使膜的电导率增加６－７个数量级，并且一个积累层在膜的表面显现出来。     

Mobility-induced instability and pattern formation in a reaction-diffusion system

Ions undergoing a reaction-diffusion process are susceptible to electric field. We show that a constant external field may induce a kind of instability on the state stabilized by diffusion in a reaction-diffusion system giving rise to formation of pattern even when the diffusion coefficients of the reactants are equal. The origin of the pattern is due to the difference in mobilities of the two species and is thus markedly different from that of deformed Turing pattern in presence of the field. While this differential flow instability had been shown earlier to result in traveling waves, we realize in the context of stationary pattern formation in a typical reaction-diffusion-advective system. Our analysis is based on a numerical simulation of a generic model on a two-dimensional domain.     

Colloidal electroconvection in a thin horizontal cell. II. Bulk electroconvection of water during parallel-plate electrolysis

We recently have reported [J. Chem. Phys. 122, 164701 (2005)] a family of electroconvective patterns that arise when charge-stabilized colloidal dispersions are driven by constant (dc) vertical electric fields. Competition between gravity and electrokinetic forces acting on the individual spheres in this system leads to the formation of highly organized convective instabilities involving hundreds of spheres. Here, we report a distinct class of electroconvective patterns that emerge in confined aqueous dispersions at higher biases. These qualitatively resemble the honeycomb and labyrinthine patterns formed during thermally driven Rayleigh-Benard convection, but arise from a distinct mechanism. Unlike the localized colloidal electroconvective patterns observed at lower biases, moreover, these system-spanning patterns form even without dispersed colloidal particles. Rather, they appear to result from an underlying electroconvective instability during electrolysis in the parallel plate geometry. This contrasts with recent theoretical results suggesting that simple electrolytes are linearly stable against electroconvection.