Dynamics of surface anchoring breaking in a nematic liquid crystal

We present a simple dynamical model which describes the time dependence of the threshold electric field for breaking the surface anchoring of nematics. This model includes a surface friction, resulting from volume hydrodynamic dissipation. Experimental data with AC and DC pulsed fields are explained by this model. For DC, the threshold difference versus the field polarity is well-described by the flexo-electric effects in a uniform field for short times and in a non-uniform field for long times.     

Simple Model for Calculating the Rate Constants of Plasma-Chemical Reactions in Low-Pressure DC Magnetron Discharges

A simple model which allows one to calculate the rate coefficients of plasma-chemical reactions in low-pressure DC magnetron discharges is presented. In this model, the electron cyclotron frequency is assumed to be much greater than any electron collision frequency. We also assume that plasma-chemical reactions take place in the near-cathode bright region where the magnetic field, the electric field, the electron density, and the electron energy are maximum. The collision probabilities have been calculated for an electron moving in crossed E × B fields by averaging the cross-sections of plasma-chemical reactions along its trajectory and over all its possible initial pitch angles. Based on this model we calculated the rate constants of the plasma-chemical reactions taking place in DC magnetron reactive sputtering in argon–oxygen gas mixtures.     

Decorating nickel phthalocyanine periphery by aryl-1,3,4-oxadiazole pendants: synthesis,characterization, and conductivity studies

We have decorated nickel phthalocyanine (NiPc) periphery by four different aryl-[1,3,4]-oxadiazole pendants. Introduction of aryl-[1,3,4]-oxadiazole pendants into the NiPc core results in improved thermal stability, fine-tuning of the position of the Q-band with decrease in band gap, indicating effective electronic communication between the two different ring systems with NiPc core. The magnitude of DC electrical conductivity for aryl-[1,3,4]-oxadiazole-substituted NiPcs 3a–3d is found to be ～10⁵ times higher than the parent NiPc (1). The temperature-dependent DC conductivity studies reveal semiconducting nature of the newly synthesized NiPc compounds with significant decrease in thermal activation energy (ΔE) compared to parent NiPc.     

Dopant effect on threshold electric field of antiferroelectric liquid crystal switching

Abstract

The influence of dopants on the threshold electric field for switching anti-ferroelectric liquid crystals was determined from the optical response curve obtained by using a triangular wave. The effect of dopant concentration on the transition temperatures of the mixtures with a host material was also investigated. The threshold electric field was diminished depending on (i) the chemical structure of the additive and (ii) increasing amounts of the additive. The upper limits of the temperature region of the S*_{C A} phases also decreased with increasing amount of additive, and these phases disappeared at 40 mol% of additive in all cases. Compound (C) is the most effective with respect to the threshold electric field. It is a two ring compound and has a chiral part similar to that of the host antiferroelectric liquid crystal compound.     

The impact of the modified Poisson–Boltzmann model on protein bound to a lipid coated silicon nanowire field effect transistor biosensor in an electrolyte environment

The aim of this work was to analyse the electrostatic potential profile, various effects of electrolyte concentrations, and the influences of surface charge on a protein bound to a lipid coated Silicon nanowire field effect transistor (Si-NW FET) biosensor by implementing the modified Poisson–Boltzmann (MPB) model. In this work, we modelled a lipid monolayer-coated Si-NW FET for the sensing of proteins, which consisted of variable amounts of aspartic acid. The electrostatic potential profile, protein charge distributions, the response to various electrolyte concentration, and the impacts of various surface charge were studied by implementing the MPB model with the Si-NW FET biosensor. Additionally, a comparison between the use of the MPB and the Poisson–Boltzmann model in studying the effects of various surface charges was carried out. Taken together, it was found that the MPB model showed a higher resolution in studying the Si-NW FET biosensor model when higher concentrations and surface charges were administered.     

A New Mussel‐Inspired Polydopamine Sensitizer for Dye‐Sensitized Solar Cells: Controlled Synthesis and Charge Transfer

The efficient electron injection by direct dye‐to‐TiO₂ charge transfer and strong adhesion of mussel‐inspired synthetic polydopamine (PDA) dyes with TiO₂ electrode is demonstrated. Spontaneous self‐polymerization of dopamine using dip‐coating (DC) and cyclic voltammetry (CV) in basic buffer solution were applied to TiO₂ layers under a nitrogen atmosphere, which offers a facile and reliable synthetic pathway to make the PDA dyes, PDA‐DC and PDA‐CV, with conformal surface and perform an efficient dye‐to‐TiO₂ charge transfer. Both synthetic methods led to excellent photovoltaic results and the PDA‐DC dye exhibited larger current density and efficiency values than those in the PDA‐CV dye. Under simulated AM 1.5 G solar light (100 mW cm^?2), a PDA‐DC dye exhibited a short circuit current density of 5.50 mW cm^?2, corresponding to an overall power conversion efficiency of 1.2 %, which is almost 10 times that of the dopamine dye‐sensitized solar cell. The PDA dyes showed strong adhesion with the nanocrystalline TiO₂ electrodes and the interface engineering of a dye‐adsorbed TiO₂ surface through the control of the coating methods, reaction times and solution concentration maximized the overall conversion efficiency, resulting in a remarkably high efficiency.     

Synthesis and Characterization of One-dimensional Dicyclohexyl-18-crown-6 Complexes with M2[Cd(mnt)2] (M = Na, K)

Two supramolecular crown ether complexes [Na(DC18C6-A)(H₂O)]{[Na(DC18C6-A)][Cd(mnt)₂]} (1) and [K(DC18C6-A)]₂[Cd(mnt)₂] (2) (DC18C6-A = cis-syn-cis-dicyclohexyl-18-crown-6, isomer A; mnt = maleonitriledithiolate) have been synthesized and characterized by elemental analysis, FT-IR spectroscopy and X-ray single crystal diffraction. Complex 1 is composed of one [Na(DC18C6-A)(H₂O)]⁺ complex cation and one {[Na(DC18C6-A)][Cd(mnt)₂]}⁻ complex anion and displays an infinite chain-like structure through N–Na–N interactions. In complex 2, [K(DC18C6-A)]⁺ complex cation and [Cd(mnt)₂]²⁻ complex anion afford a novel 1D ladder-like structure by N–K–N, N–K–S interactions.     

A new type of electrohydrodynamic instability in nematic liquid crystals with positive dielectric anisotropy

Abstract

A new type of electrohydrodynamic instability originally reported in nematic liquid crystal mixtures with positive dielectric anisotropy and as moderately thick samples is further studied. The ability of homogeneously aligned nematics with positive dielectric anisotropy, in the presence of a magnetic field, to exhibit Williams domains as a threshold effect is numerically investigated. The variation of the threshold voltage for domain formation and dielectric alignment with dielectric anisotropy is calculated theoretically and compared with the experimental results as moderately thick and thin samples.     

The Goldstone mode in mixtures containing ferroelectric liquid crystals

Abstract

Measurements of complex electric permittivity of room temperature ferroelectric liquid crystal mixtures have been made on aligned samples with the electric measuring field being parallel to the layer planes. The spontaneous polarization, the tilt angles and pitch have been measured in these mixtures. By theoretical fitting of the experimental points of electric permittivity for the Cole–Cole modification of the Debye equation dielectric parameters, the dielectric strength, relaxation frequency, and distribution parameter for the Goldstone mode have been computed. The dielectrically observed Goldstone mode in our mixtures is shown to have both DC bias field and AC field dependences.     

Anomalous charging and discharging processes in liquid crystals

Abstract

Transient charging and discharging currents produced by a DC voltage applied to a parallel plate capacitor filled with a liquid crystal sample have been studied experimentally. A discontinuous jump of the current magnitude in the nematic phase has been observed for the first time. The shape of the charging and discharging current curves has been shown to depend on the run numbers. Experimental results have been explained with a simple model based on molecular alignment.     

Plane hydrodynamic flow in thin free suspended nematic liquid crystal films

Abstract

Radial hydrodynamic flows in free suspended films (with thickness h ≥ 20 μm) of some liquid crystal materials was observed in a narrow temperature range before the line nematic-smectic C transition. The observed flows are explained by a non-linear temperature dependence of the surface tension (Marangoni effect).

Upon thinning the liquid crystal films to a thickness 6 μ < h < 20 μm the hydrodynamic flow changes its character: instead of radial flow with sinking seeding particles we observe two circular plane hydrodynamic flows symmetrical to the film's diameter. The temperature distribution in the free film and the surface tension field are discussed. A model for the established circular flows in thin liquid crystal films is presented.     

Electro-deposition under a modulated electrical field as an enhanced method for the preparation of an efficient photoanode of dye-sensitized solar cells

This paper describes an application of a new electro-deposition method in a modulated electrical field in order to have an efficient semiconductor coating on a conductive substrate. The prepared film was used as a photoanode of dye-sensitized solar cells (DSSCs). Electro-deposition of nanoparticles usually was performed by applying a DC electrical field in a suspension. In the DC field, a homogeneous layer could not be performed because of unwanted electrochemical reactions that might occur on the substrate surface. Modulated electrical fields based on pulsed AC and a sweeping voltage profile were used. The photovoltaic performance of the assembled solar cells showed a significant difference between the films produced in different electrical field conditions. Under the illumination of simulated AM 1.5 sunlight (100 mW cm^?2) with an aperture black mask, the energy conversion efficiency of 2.45% (V _OC = 768 mV, J _SC = 4.74 mA cm^?2, FF = 67%) was obtained with a thin layer of TiO₂ nanoparticles deposited in a pulsed waveform voltage. A crack-free and uniform porous layer produced in this condition showed an enhancement of about seven times over the photoanode prepared using conventional DC electrical field with the same voltage amplitude. The solar cell efficiency was increased to 4.22% (V _OC = 735 mV, J _SC = 7.92 mA cm^?2, FF = 72%) by just increasing the TiO₂ film thickness and using a blocking layer beneath the semiconductor layer. Moreover, a higher electron recombination lifetime presented better electron transport and collection efficiency of the film deposited in the pulsed electrical field conditions.     

The influences of surface polarization on NLC cells

The influence of the surface polarization, P_s , on a nematic liquid crystal (NLC) cell is investigated analytically. Flexoelectric polarization is considered, but selective ion absorption is ignored. The differential equations are derived for tilt angle, θ, of director n and the corresponding boundary conditions based on Gibbs free energy, and their solutions discussed. Equations for the reduced threshold voltage, u_th , and the reduced saturation voltage, u_sat , are deduced and the relationships between u_th , u_sat and reduced strength of surface polarization, p, derived.     

Threshold flexoelectric deformations in homeotropic nematic layers

Elastic deformations of nematic liquid crystal layers subjected to a d.c. electric field were studied numerically. The flexoelectric properties of the nematic material and the presence of ionic space charge were taken into account. Homeotropic alignment with finite surface anchoring strength was assumed. The director orientation and the electric potential distribution were calculated; the space charge density was also determined. It was found that the threshold voltage strongly depended on the parameters of the system. In particular, a threshold as low as a few tenths of a volt occurred under suitable circumstances. In the case of a negative dielectric anisotropy, Δ ε, such low values of the threshold voltage existed when the ion concentration was sufficiently high, and given sufficiently large magnitudes of the flexoelectric coefficients and a sufficiently small anchoring energy. If the ion concentration was low or if the flexoelectric coefficients were small or if the surface anchoring was strong, the threshold was equal to several volts. In the case of positive dielectric anisotropy, the threshold amounted to several tenths of a volt for a weakly anisotropic and highly conductive material. If the dielectric anisotropy was sufficiently high or if the ion concentration was sufficiently low, the threshold voltage increased with Δ ε and reached tens of volts. These results can be explained as the effect of the inhomogeneous electric field arising in the vicinity of the surfaces, due to the ionic space charge redistributed by the external voltage. They are qualitatively consistent with earlier experiments which show the effect of the ion concentration on the elastic deformations in flexoelectric nematics. They correspond also with theoretical results concerning the effect of the electric field produced by the surface polarization or by the adsorption of ions.     

Electrically induced linear locomotion of polymer gel in air

A electromechanical gel, which could be driven in air by a DC electric field, was developed using poly(vinyl alcohol)/dimethylsulfoxide gel. When the applied electric field exceeds a certain threshold, the gel exhibited a continuous and linear crawling motion. The result indicated that, under the applied electric field of 275 V/mm, the maximum crawling velocity of the gel could reach v = 1.63 mm/s, which is about 80 times larger than that reported in earlier works. At a proper range of the driving time, the average crawling speed and crawling direction could be well controlled by the external electric field. Furthermore, some factors, which have influence on the critical driving electric field of the gel, such as the swelling degree of the gel and the apparent contact area between the gel and substrate, were studied. For the first time, a mechanism based on the strong electrostatic interaction between the external electric field and the charges accumulated in the gel–air and substrate–gel interfacial regions was put forward to explain the novel motion. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1187–1197, 2007     

The elastic distortion and stability of biaxial nematic liquid crystal on the surface grooves

Fukuda et al. reexamined the Berreman's model which attributes the surface anchoring to the elastic distortion of the uniaxial nematic liquid crystal induced by the grooves of a surface. They showed that at the variance with the assumption made in the original approach of Berreman, the azimuthal distortion of the director cannot be considered as negligibly small. Now this method is generalized to the biaxial nematic liquid crystals, with some approximations for the elastic constants. We obtain an additional term in the elastic distortion energy per unit area which depends on the second power of the cosine of the angle made between the main director n at infinity and the direction of the surface grooves. This additional term describes the distortion energy of the minor director m induced by the surface grooves when the n director is anchored exactly along the grooves. We have studied the stability of the n director around the grooves, and in one-constant model for each director the stability condition is that the elastic constant of the n director is the maximum.     

A 2D electrohydrodynamic model for electrorotation of fluid drops

A theoretical analysis of spontaneous electrorotation of deformable fluid drops in a DC electric field is presented with a 2D electrohydrodynamic model. The fluids in the system are assumed to be leaky dielectric and Newtonian. If the rotating flow is dominant over the cellular convection type of electrohydrodynamic flow, closed-form solutions for drops of small deformations can be obtained. Because the governing equations are in general nonlinear even when drop deformations are ignored, the general solution for even undeformed drop takes a form of infinite series and can only be evaluated by numerical means. Both closed-form solutions for special cases and numerical solutions for more general cases are obtained here to describe steady-state field variables and first-order drop deformations. In a DC electric field of strength beyond the threshold value, spontaneous electrorotation of a drop is shown to occur when charge relaxation in the surrounding fluid is faster than the fluid inside the drop. With increasing the strength of the applied electric field from the threshold for onset of electrorotation, the axis of drop contraction deviates from from that of the applied electric field in the direction of the rotating flow with an angle increasing with the field strength.     

Influence of Rubber Formulation on Surface Modifications Produced by RF Plasma

The effectiveness of nitrogen, oxygen and air Radio Frequency (RF) plasma treatments on two styrene-butadiene vulcanized rubbers with a different formulation has been studied. The presence of an antiadherent surface layer containing low-molecular weight ingredients (sulfur-rich vulcanization agents and wax) from SW (Sulfur-Wax) rubber formulation requires an extended plasma treatment capable of removing this surface layer. When the percentage of antiadherent moieties is reduced in ZS (Zinc Stearate) rubber formulation, shorter plasma treatment times are enough to modify rubber surface and increase its polarity by the creation of C–O and C=O polar groups that enhance adhesion towards a polyurethane adhesive. Air and oxygen plasma treatments are more aggressive than nitrogen plasma and therefore they are more effective in removing the antiadherent layer of the outermost rubber surface layer prior to oxidation of the rubber surface.     

Dielectric investigation of the liquid crystal compound with the directSmA*–SmCA* phase transition

New compound showing a direct SmA*–SmC_A* phase transition was synthesised. As far as authors know there are a few pure compounds showing para- and antiferroelectric phases without SmC* between them. Direct current (DC) field applied into a planar-oriented cell induces ferroelectric SmC* phase in an investigated compound. Typical for SmC*, Goldstone mode starts to be detectable. DC field also shifts down the temperature of a SmC_A* phase creation. Moreover, modes in the appearing antiferroelectic phase are enhanced by DC field. This paper shows and discusses relations between modes detected in SmA*, SmC_A* and SmC* (SmC* phase – nucleated by DC field) phases. Parameters of observed modes are calculated using the Cole–Cole relaxation model and a calculation procedure useful especially for high frequency relaxations (higher than 200 kHz).