Structure parameter of electrorheological fluids in shear flow

A structure parameter, Sn = η(c)γ/τ(E), is proposed to represent the increase of effective viscosity due to the introduction of particles into a viscous liquid and to analyze the shear behavior of electrorheological (ER) fluids. Sn can divide the shear curves of ER fluids, τ/E(2) versus Sn, into three regimes, with two critical values Sn(c) of about 10(-4) and 10(-2), respectively. The two critical Sn(c) are applicable to ER fluids with different particle volume fractions φ in a wide range of shear rate γ and electric field E. When Sn < 10(-4), the shear behavior of ER fluids is mainly dominated by E and by shear rate when Sn > 10(-2). The electric current of ER fluids under E varied with shear stress in the same or the opposite trend in different shear rate ranges. Sn(c) also separates the conductivity variation of ER fluids into three regimes, corresponding to different structure evolutions. The change of Sn with particle volume fraction and E has also been discussed. The shear thickening in ER fluids can be characterized by Sn(c)(L) and Sn(c)(H) with a critical value about 10(-6). As an analogy to friction, the correspondence between τ/E(2) and friction coefficient, Sn and bearing numbers, as well as the similarity between the shear curve of ER fluids and the Stribeck curve of friction, indicate a possible friction origin in ER effect.     

The electrorheological efficiency of polyaniline particles with various conductivities suspended in silicone oil

Electrorheological (ER) behavior of silicone oil suspensions of particles of polyaniline protonated to various doping levels with ortho-phosphoric and tetrafluoroboric acids has been studied. The dynamic yield stress obtained by extrapolation of shear stress to zero shear rate using Herschel–Bulkley equation was used as a criterion of the ER efficiency. At a same molar concentration of doping acids, various protonation effects appeared and the dependences of the yield stress on the acid concentration differed. The comparison of the yield stresses with dielectric characteristics calculated from the Havriliak–Negami equation revealed that the particle conductivity, in contrast to particle permittivity, dominates the polarization process especially at higher protonation degrees. Consequently, particle conductivity or dielectric relaxation time proved to be the parameters providing the common dependences of the yield stress regardless of the way of polarization.     

The dielectric properties and flow of electrorheological fluids based on polymer-coated nanodispersed barium tetraacetate titanyl particles upon a dynamic shear in electric fields

The shear stress in flowing electrorheological fluids consisting of PMS-20 poly(dimethylsiloxane) filled with nanodispersed barium tetraacetate titanyl particles coated with polymers (polyethyleneimine, poly(ethylene glycol), and polyethyloxazoline) has been studied as depending on the strengths of direct- and alternating-current (f = 50 Hz) electric fields. Results of analyzing the dielectric spectra of electrorheological fluids in a frequency range of 25–10⁶ Hz have been presented. The values of the shear stress in the flowing fluids as depending on the nature of a polymer adsorbed on the particle surface decrease in a series corresponding to a reduction in the Maxwell–Wagner relaxation times of the suspensions. The current-voltage characteristics of the electrorheological fluids at high voltages (up to 5 kV) indicate the realization of the mechanism of currents limited by the space charge. The influence of an adsorbed polymer on the magnitude of the electrorheological effect is reduced to blocking polar groups on the particle surface and variations in the conductivity, effective dielectric permittivity, and loss tangents of filler materials. An increase in the contribution from these factors leads to a gradual decrease in the magnitude of the electrorheological effect.     

Structure of electrorheological fluids under an electric field and a shear flow: experiment and computer simulation

It is known that macroscopic properties of colloidal suspensions are often determined by the microstructure of the particles in the suspensions, depending on the interparticle, Brownian, and hydrodynamic (if any) forces. We take electrorheological (ER) fluids as an example. By using a computer simulation and an experimental approach, we investigate the structure of ER fluids subjected to both an electric field and a shear flow. The microstructure evolution from random structure, to chains, and then to stable lamellar patterns, observed in the experiments, agrees very well with that obtained in the simulations. It is shown that the formation of such lamellar patterns originates from the difference between the dipole moment induced in the particles suspended in the ER fluids without shear and the one with shear. The results on the relaxation process of structural formation and the internal structure of layers are also presented. Thus, it seems possible to achieve various structures and hence desired macroscopic properties of colloidal suspensions by adjusting external fields and, simultaneously, a shear flow.     

Hydrogen sensors based on conductivity changes in polyaniline nanofibers

Hydrogen causes a reversible decrease in the resistance of a thin film of camphorsulfonic acid doped polyaniline nanofibers. For a 1% mixture of hydrogen in nitrogen, a 3% decrease in resistance is observed (DeltaR/R = -3%). The hydrogen response is completely suppressed in the presence of humidity. In contrast, oxygen does not inhibit the hydrogen response. A deuterium isotope effect on the sensor response is observed in which hydrogen gives a larger response than deuterium: (DeltaR/R)H/(DeltaR/R)D = 4.1 +/- 0.4. Mass sensors using nanofiber films on a quartz crystal microbalance also showed a comparable deuterium isotope effect: DeltamH/DeltamD = 2.3 +/- 0.2 or DeltanH/DeltanD = 4.6 +/- 0.4 on a molar basis. The resistance change of polyaniline nanofibers is about an order of magnitude greater than conventional polyaniline, consistent with a porous, high-surface-area nanofibrillar film structure that allows for better gas diffusion into the film. A plausible mechanism involves hydrogen bonding to the amine nitrogens along the polyaniline backbone and subsequent dissociation. The inhibitory effect of humidity is consistent with a stronger interaction of water with the polyaniline active sites that bind to hydrogen. These data clearly demonstrate a significant interaction of hydrogen with doped polyaniline and may be relevant to recent claims of hydrogen storage by polyaniline.     

Structural and conductivity changes during the pyrolysis of polyaniline base

Polyaniline base has been exposed to various temperatures between 100 °C and 1000 °C for 2 h in air. The mass loss has increased with increasing temperature. FTIR and Raman spectroscopies show the gradual destruction of the PANI structure, the possible formation of intermediate oxime and nitrile groups, and the final conversion to graphitic material. The elemental analysis confirmed the dehydrogenation while the content of nitrogen was nearly constant even after treatment at 800 °C. The conductivity of PANI base, 10⁻⁸ S cm⁻¹, increased to ∼10⁻⁴ S cm⁻¹ after treatment at 1000 °C; most of the products, however, were non-conducting. Another series of experiments involved the polyaniline base heated at 500 °C for 1-8 h. The studies were performed in connection with the potential flame-retardant application of polyaniline.     

A new approach of enhancing the shear stress of electrorheological fluids of montmorillonite nanocomposite by emulsion intercalation of poly-N-methaniline

Poly-N-methaniline/montmorillonite (PNMA-MMT) nanocomposite particles with high dielectric constant as well as suitable conductivity were synthesized by an emulsion intercalation method and characterized by FT-IR, XRD, and TEM spectrometry, respectively. The electrorheological (ER) properties of the suspensions of PNMA-MMT particles in silicone oil (20 wt%) were investigated under DC electric fields. It was found that the shear stress of poly-N-methaniline/montmorillonite electrorheological fluid (ERF) is 6.0 kPa in 3 kV/mm (74.5 s(-1)), which is 3.6 times that of electrorheological fluid at zero field, and also much higher than that of pure poly-N-methaniline (PNMA) and montmorillonite (MMT). In the range of 10-90 degrees C, the shear stress changes slightly with the temperature. The sedimentation ratio of PNMA-MMT ERF was about 97% after 60 days. Furthermore, the dielectric constant of PNMA-MMT nanocomposite was increased 3.74 times that of PNMA and 1.99 times that of MMT at 1000 Hz, the dielectric loss tangent also increased about 1.58 times that of PNMA. It is apparent that the notable ER effect of PNMA-MMT ER fluid was attributed to the prominent dielectric property of the poly-N-methaniline/montmorillonite nanocomposite particles.     

Yield stress analysis of 1D calcium and titanium precipitate-based giant electrorheological fluids

1D calcium and titanium composite nanorods synthesized were applied as electrorheological (ER) active materials with extremely high static yield stresses, i.e., giant ER effect. The yield stress of this giant ER fluid was analyzed using a new universal yield stress scaling equation in the form of the modified Bessel functions with two different limiting behaviors in a low and high electric field region. The universal yield stress equation collapsed the yield stress data onto a single curve.     

Analysis of shear flow alignment of nematic liquid crystals at low shear stress based on catastrophe theory

The low stress shear flow alignment of a nematic liquid crystal in the presence of strong anchoring at the boundaries is analysed. Layers with pretilted director orientation are taken into account. Two kinds of symmetric deformations are assumed. They differ in the director distribution in the vicinity of the boundaries. The analysis is carried out using an expansion of the free energy of the layer in powers of the maximum deformation angle. The results have qualitative character. The condition for the threshold behaviour and the stability of the solutions are discussed. The deformation may develop continuously or discontinuously. The transition between two kinds of deformation is predicted. The facts already known are confirmed and supplemented.     

A microfluidic generator of dynamic shear stress and biochemical signals based on autonomously oscillatory flow

Biological cells in vivo typically reside in a dynamic flowing microenvironment with extensive biomechanical and biochemical cues varying in time and space. These dynamic biomechanical and biochemical signals together act to regulate cellular behaviors and functions. Microfluidic technology is an important experimental platform for mimicking extracellular flowing microenvironment in vitro. However, most existing microfluidic chips for generating dynamic shear stress and biochemical signals require expensive, large peripheral pumps and external control systems, unsuitable for being placed inside cell incubators to conduct cell biology experiments. This study has developed a microfluidic generator of dynamic shear stress and biochemical signals based on autonomously oscillatory flow. Further, based on the lumped-parameter and distributed-parameter models of multiscale fluid dynamics, the oscillatory flow field and the concentration field of biochemical factors has been simulated at the cell culture region within the designed microfluidic chip. Using the constructed experimental system, the feasibility of the designed microfluidic chip has been validated by simulating biochemical factors with red dye. The simulation results demonstrate that dynamic shear stress and biochemical signals with adjustable period and amplitude can be generated at the cell culture chamber within the microfluidic chip. The amplitudes of dynamic shear stress and biochemical signals is proportional to the pressure difference and inversely proportional to the flow resistance, while their periods are correlated positively with the flow capacity and the flow resistance. The experimental results reveal the feasibility of the designed microfluidic chip. Conclusively, the proposed microfluidic generator based on autonomously oscillatory flow can generate dynamic shear stress and biochemical signals without peripheral pumps and external control systems. In addition to reducing the experimental cost, due to the tiny volume, it is beneficial to be integrated into cell incubators for cell biology experiments. Thus, the proposed microfluidic chip provides a novel experimental platform for cell biology investigations.     

The dependence of electro-osmotic flow on current density and time

The dependence of the water transport number on current density is examined for three membranes whose characteristics cover a wide spectrum: poly(vinylbenzenesulfonate), porous Vycor glass and cellulose. Experiments and theory show that non-linear volume—time plots in electro-osmotic experiments arise from displacements of the membrane in the electric field, and that reliable water transport numbers can be obtained at a given current density. When the current density is varied, experiments show that the observed water transport number can: (a) increase at low current densities because of osmotic flow superimposed on water transport by the electric field; (b) decrease at higher current densities because of accumulation of salt in the membrane; (c) decrease more at current densities near and above the limiting value because of an increased contribution of hydrogen and hydroxide ions to transport. These phenomena arise from a combination of diffusion films at both membrane—solution interfaces and from the dependence of counteflon and water transport numbers on external salt concentration.     

Criteria of negative thixotropy for polymer solutions determined from the time dependence of the shear and normal stress

Summary The time dependences of shear and normal stress measured with a Weissenberg rheogoniometer were used in an investigation of the negative thixotropy of poly(methyl methacrylate) solutions in a low-viscosity solvent (mixture of cyclohexanone and isopropanol). A set of criteria was suggested for a complex quantitative evaluation of this phenomenon. The reproducibility of these criteria and their sensitivity towards a change in the velocity gradient were determined.

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Zusammenfassung Es wurde die negative Thixotropie der Lösungen von Polymethylmethacrylat in niedrigviskosem Lösungsmittel (Gemischen von Cyclohexanon und Isopropanol) studiert. Zeitänderungen der Schub- und Normalspannung wurden mit dem Weissenbergschen Rheogoniometer verfolgt und Kriterien zur komplexen quantitativen Beschreibung dieses Effektes wurden entworfen. Die Reproduzierbarkeit und Empfindlichkeit auf die Änderungen des Geschwindigkeitsgefälles wurden ermittelt.

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Dedicated to Prof. Dr.G. Rehage on the occasion of his 60th birthday.

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With 6 figures and 1 table     

Electrical conductivity of poly(vinylidene fluoride)/polyaniline blends under oscillatory and steady shear conditions

Blends of poly(vinylidene fluoride) (PVDF) and polyaniline (PAni) were prepared through melt blending in a batch mixer. The morphology, rheological behavior and electrical conductivity were investigated through transmission electron microscopy (TEM) and combined electro-rheological measurements. Through TEM analysis, it was possible to observe that all blends showed typical phase separation with the presence of conductive polymer aggregates. Deformations imposed during a strain sweep caused, not only disturbance of the linear viscoelastic behavior, but also changes in electrical conductivity. The oscillatory shear altered the morphology, breaking the PAni domains into smaller ones. This effect increases the distance between them and, consequently, resulted in a decrease of the electrical conductivity. The measurements under quiescent conditions and steady shear proved that the disturbance in morphology for PVDF/PAni system is non-recoverable. Through combined electrical and rheological measurements, it was possible to achieve good correlation between the electrical and flow behavior of PVDF/PAni blends.     

A fast and sensitive continuous flow nanobiodetector based on polyaniline nanofibrils

Polyaniline nanofibrils, fabricated as a freestanding nanonetwork in a micro gap between two gold electrodes, have been applied in a fast and sensitive analytical device for detecting microorganism cells in a flowing liquid, including the “on-line” regime. The electrical response of the device (in this case an increase in the electrical conductivity of the nanonetwork) depends on the number of cells in the sample analyzed: it is linear within the range of 0–5000 and 0–15 × 10⁶ cells injected for yeast and bacteria examined, respectively. The detection limit depends on the type of microorganism: it is about 300 and 1.9 × 10⁵ (per mL) for yeast (Saccharomyces cerevisiae) and bacteria cells (Lactobacillus rhamnosus), respectively. The results are registered within few seconds (typically 5–15 s). The device is designed to detect a low level of bio-contamination and it is expected to be useful in biomedical applications, environmental protection and anti bioterrorism systems, including “on-line” monitoring.     

Structure and characteristics of composite materials based on polyaniline and Nylon-6

A comparative systematic study addresses the specific features of polymerization of aniline adsorbed on the Nylon matrices in the solutions containing monomer and in the monomer-free solutions. The characteristics of the formed composite materials are investigated. In the presence of aniline in the solution, a uniform and conducting composite material is obtained and its electrical conductivity reversibly depends on pH of the medium and is several orders of magnitude higher than the electrical conductivity of the composite material prepared by the polymerization of aniline in the Nylon matrix in the monomer-free solution.     

Electrorheological fluids based on glycerol-activated titania gel particles and silicone oil with high yield strength

This article describes an electrorheological (ER) fluid based on glycerol-activated titania organic-inorganic hybrid gel particles and silicone oil with high yield strength. Based on a physical picture of a water-activated ER system, glycerol that has a high dielectric constant and boiling point is in situ prepared in the amorphous titania gel during the sol-gel processing. A small amount of ionic surfactant hexadecyltrimethylammonium bromide (CTAB) is employed to enhance charge carriers in particles. FTIR and XRD techniques are used to determine the nature and structure of the hybrid gel. Rheology test results show that a large static yield stress greater than 12.6 kPa is obtained when 3 kV/mm dc electric field is applied. This value is close to the value predicted by H. Conrad (MRS Bull. 8 (1998) 35) in theory. Furthermore, dynamic shear stress as a function of shear rate and temperature is also investigated. This ER fluid exhibits strong temperature dependence and a wide working temperature range from 0 to 120 degrees C, while its leaking current density is still low. More interesting is that the glycerol content is demonstrated to have an influence on ER effect and temperature dependence. Measurement of the dielectric properties of ER fluids shows enhancement of the dielectric constant and dielectric loss due to addition of glycerol and a regular dependence of them on temperature, which well explains the strong ER effect.     

Dispersive micro-solid-phase extraction of benzodiazepines from biological fluids based on polyaniline/magnetic nanoparticles composite

In this study, diverse types of Fe₃O₄ nanocomposites modified by polyaniline, polypyrrole, and aniline–pyrrole copolymer were synthesized through chemical oxidative polymerization process for dispersive-μ-solid phase extraction (D-μ-SPE) in the presence of various dopants. The results showed that the nanocomposite modified by polyaniline with p-toluene sulfonic acid as a dopant demonstrated higher extraction efficiency for lorazepam (LRZ) and nitrazepam (NRZ). Also the synthesized magnetic sorbents were characterized. The nanocomposite sorbent in combination with high performance liquid chromatography–UV detection was applied for the extraction, preconcentration and determination of lorazepam and nitrazepam in urine and plasma samples. Different parameters influencing the extraction efficiency including: sample pH, amount of sorbent, sorption time, elution solvent and its volume, salt content, and elution time were optimized. The obtained optimal conditions were: sample pH, 6; amount of sorbent, 5 mg; sorption time, 5.0 min; elution solvent and its volume, 0.5 mM cethyltrimethyl ammonium bromide in acetonitrile, 150 μL; elution time, 2.0 min and without addition of NaCl. The calibration curves were linear in the concentration range of 1–2000 μg L⁻¹. The limits of detection (LODs) were achieved in the range of 0.5–1.8 μg L⁻¹ for NRZ and 0.2–2.0 μg L⁻¹ for LRZ, respectively. The percent of extraction recoveries and relative standard deviations (n = 5) were in the range of 84.0–99.0, 6.1–7.8 for NRZ and 90.0–99.0, 4.1–7.0 for LRZ, respectively. Ultimately, the applicability of the method was successfully confirmed by the extraction and determination of NRZ and LRZ in human urine and plasma samples.     

Parallel superposition of small- and large-amplitude oscillations upon steady shear flow of polymer fluids

The parallel superposition of small- and large-amplitude oscillations upon steady shear flow of elastic fluids has been considered. Theoretical results, obtained by numerical methods, are based upon the Leonov viscoelastic constitutive equation. Steady-state components, amplitude, and phase angle of oscillatory components of the shear stress, the first and second normal-stress differences as a function of shear rate, deformation amplitude, and frequency have been calculated. These oscillatory components include the first harmonic of the shear stresses and the first and second harmonic of the normal stresses. In the case of small-amplitude superposition, the effect of the steady shear flow upon frequency-dependent storage and loss moduli has been determined and compared with experimental data available in the literature for polymeric solutions and melts. In the case of large-amplitude superposition, the effect of oscillations upon the steady shear flow characteristics has been determined and compared with our experimental data for a polymeric melt. The experimental results for shear stress components have been found to be in good agreement with theoretical predictions, although there are some deviations for storage modulus at high shear rates. The deviations seem to be dependent on material. Moreover, the theory is unable to describe experimental data available for the first harmonic of normal stresses.