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.     

Phase behavior of a simple dipolar fluid under shear flow in an electric field

Nonequilibrium molecular dynamics simulations are performed on a dense simple dipolar fluid under a planar Couette shear flow. Shear generates heat, which is removed by thermostatting terms added to the equations of motion of the fluid particles. The spatial structure of simple fluids at high shear rates is known to depend strongly on the thermostatting mechanism chosen. Kinetic thermostats are either biased or unbiased: biased thermostats neglect the existence of secondary flows that appear at high shear rates superimposed upon the linear velocity profile of the fluid. Simulations that employ a biased thermostat produce a string phase where particles align in strings with hexagonal symmetry along the direction of the flow. This phase is known to be a simulation artifact of biased thermostatting, and has not been observed by experiments on colloidal suspensions under shear flow. In this paper, we investigate the possibility of using a suitably directed electric field, which is coupled to the dipole moments of the fluid particles, to stabilize the string phase. We explore several thermostatting mechanisms where either the kinetic or configurational fluid degrees of freedom are thermostated. Some of these mechanisms do not yield a string phase, but rather a shear-thickening phase; in this case, we find the influence of the dipolar interactions and external field on the packing structure, and in turn their influence on the shear viscosity at the onset of this shear-thickening regime.     

Preparation and electrorheological characteristic of Y-doped BaTiO3 suspension under dc electric field

The electrorheological (ER) effects of BaTiO₃ or other perovskite materials with high dielectric constant are presumed to be large. However, their weak ER activity is very puzzling. In this study, we choose cubic BaTiO₃ and first achieve its ER enhancement under dc electric field by modifying its intrinsic structure with doping rare earth Y ions, which are synthesized by means of sol-gel technique. DSC-TG, FT-IR, XRD, ICP and XPS techniques are used to characterize thermal, structure and component change of materials. It is demonstrated that Y³⁺ substitutes for Ba²⁺, which causes lattice-distorting defects. Rheological experiments show that Y-doped BaTiO₃ suspension has notable ER effect and clear fibrillation structure under dc electric field, while the pure cubic BaTiO₃ suspension suffers from electrophoretic effects and its ER effect is very weak. The ER effect of typical Y-doped BaTiO₃ ER suspension is ten times that of pure BaTiO₃ ER suspension. Based on the electrical measurements, the enhancement of ER activity of BaTiO₃ may be attributed to the increase of conductivity due to Y-doping. The enhancement in ER activity of cubic BaTiO₃ under dc electric field by doping rare earth Y ions is helpful to further understand the perovskite-based ER materials with high dielectric constant but low ER activity.     

Force acting on the microparticles in electrorheological solids under the application of a nonuniform ac electric field

Under the application of electric fields, the structure of electrorheological (ER) solids can be changed from the body-centered tetragonal lattice to other lattices. We have derived the dipole factor for the lattice by taking into account the local-field effect through the Ewald–Kornfeld formulation, and expressed it in the spectral representation exactly. It is found that when the ER solid is subject to a nonuniform ac electric field, the force acting on the microparticle can be affected by the structure transformation, and local-field effect as well as field frequency. Our results are very well understood in the spectral representation theory.     

Transient effects in electrolytic cells submitted to an external electric field

We analyse transient effects in an electrolytic cell submitted to an external voltage and determine the relaxation time of the redistribution of the ions and of the potential. We consider the case in which adsorption effects at the interface with the electrodes are present and show that the typical relaxation time, for applied voltage V ₀25 mV, is of the order of tens of seconds for commercial nematic liquid crystals. When V ₀>25 mV the linearized analysis is no longer valid. In this case, the relaxation time depends on the applied voltage. Increasing V ₀, but still remaining in the low amplitude limit, the relaxation time starts increasing. This indicates that the reduction of the actual field in the sample, for moderate values of V ₀, plays an important role. For large values of V ₀, the relaxation time is a decreasing function of V ₀. This result is interpreted in terms of a simple model, according to which the ionic charge is localized in a surface layer whose thickness depends on the amplitude of the applied voltage.     

Differences in the electrorheological response of a particle suspension under direct current and alternating current electric fields

We have observed an unusual reduction of shear stress with increasing shear rate under direct current electric fields, for an electrorheological fluid composed of sulfonated poly(styrene-co-divinylbenzene) particles dispersed in silicone oil. At all shear rates, the shear stress under the electric field is larger than that in the absence of the field, indicating that there is still some field-induced agglomeration of the particles. In contrast, the behavior under alternating current electric fields is the Bingham-fluid-type response commonly observed with electrorheological fluids. It is suggested that the conventional dipole–dipole interaction approach based on simplified microstructural models would be unable to explain these phenomena. Received: 27 November 2000 Accepted: 22 May 2001     

Ca ion permeation through liposome membranes with heat generation by square-wave electric field

Phospholipid liposomes that contain Ca ions in inside compartment were subjected to external alternating electric fields of square wave form at several frequencies: 10, 100 Hz, 1, 10, 100 kHz. The leakage of Ca ions from inside to bulk solution caused by the electric fields was detected by the fluorescence change due to Ca-Quin 2 (fluorescent dye) complex formation in the bulk aqueous solution. The temperature increments of the sample solution in the chamber were also measured. The amplitude of the electric field and time interval between application of the electric field were varied. The frequency dependency of both the leakage of Ca ion and the temperature increments was observed. It was observed that the efficiency of the leakage had a minimum at 1 kHz. The origin of the frequency dependency of the leakage is discussed, and it is concluded that the energy consumption on a microscopic scale in the liposome membrane zone can be the reason for membrane permeation in this experimental system.     

Influence of an electric field on the non-Newtonian response of a hybrid-aligned nematic cell under shear flow

The authors study shear flow in hybrid-aligned nematic cells under the action of an applied electric field by solving numerically a hydrodynamic model. The authors apply this model to a flow-aligning nematic liquid crystal (4'-n-pentyl-4-cyanobiphenyl) and obtain the director's configuration and the velocity profile at the stationary state. The authors calculate the local and apparent viscosities of the system and found that the competition between the shear flow and the electric field gives rise to an interesting non-Newtonian response with regions of shear thickening and thinning. The results also show an important electrorheological effect ranging from a value a bit larger than the Miesowicz viscosity etab [Nature (London) 17, 261 (1935)] for small electric fields and large shear flows to etac for large electric fields and small shear flows. The analysis of the first normal stress difference shows that for small negative shear rates, the force between the plates of the cell is attractive, while it is repulsive for all other values of shear rates. However, under the application of the electric field, one can modify the extent of the region of attraction. Finally, the authors have calculated the dragging forces on the plates of the cell and found that it is easier to shear in one direction than in the other.     

The response of a colloidal suspension to an alternating electric field

This paper is concerned with the calculation of the complex conductivity K* of a suspension, a quantity which may be determined experimentally from the measurement of the alternating current which flows between a pair of electrodes in the suspension due to an alternating voltage difference. A semi-analytic formula is derived for the complex conductivity of a dilute suspension of spherical particles with small dielectric constant which is reasonably accurate for ?-potentials of less than 50 mV. For such suspensions this formula represents a very economical alternative to the exact computer calculation of K* described by DeLacey and White (ref. 2). Although the formula for K* is derived for particles with fixed surface charge, it is shown that the formula can also be applied to a more general class of suspensions, in which the surface charge arises from the dissociation of a single type of surface group.     

Quasi-static electrorheological properties of hematite/silicone oil suspensions under DC electric fields

In this work, a modified rheometer has been used to gain information on the "start-up" of the shear flow of an electrorheological (ER) fluid consisting of hematite particles dispersed in silicone oil. The results show that unelectrified suspensions behave essentially as fluids, continuously deforming upon application of shear. However, this behavior changes in the presence of an electric field. For low fields and low volume fractions of solids, a solidlike (drastic increase in shear stress after the strain is applied) behavior is observed for small deformations. If the strain is increased, the yield starts and a transition to a viscoelastic-plastic nature is observed. Finally, a plastic behavior is characteristic of the post-yield regime. If the field strength and solids content are high, a discontinuous flow profile develops. These results, together with direct structural observations, suggest that the observed behavior is compatible with the formation of layers of particles electrophoretically deposited on the electrodes; the layers turn into rings when the shear field is applied. It is the slip of the fluid between these rings that can be considered responsible for the ER effect in these suspensions.     

Orientation of liquid crystalline 5CB under an external electric field

The orientation characteristics of pre-aligned liquid crystalline 5CB (4-n-pentyl-4′-cyanobiphenyl) in a germanium cell with unidirectional rubbed polyimide-coated surfaces have been investigated. Orientation of 5CB molecules near the polyimide surface and those representing average properties of the system (i.e., the bulk) are compared. The orientation of the bulk is monitored by transmission Fourier transform infrared (FT-IR) spectroscopy while that of the molecules next to the surface is observed via attenuated total reflection (ATR) FT-IR spectroscopy. There are significant differences in orientation characteristics between the two groups of molecules. For molecules near the polyimide surface, there is an observable difference in orientation of the soft and hard segments of the liquid crystalline. Moreover, they show depth dependent orientation.     

Conformational analysis of diphenylacetylene under the influence of an external electric field

Theoretical investigation of the torsional potentials of a molecular wire, diphenylacetylene, was carried out at the B3LYP/6-311+G** level by considering the influence of the external electric field (EF). It demonstrates that many molecular features are sensitive to the EF applied. In particular, the torsional barrier increases and the LUMO-HOMO gap decreases with the increase of EF. Quantitative correlations between these features and the external EF were revealed. The current-voltage behavior corresponding to different conformers was studied as well by non-equilibrium Green's function method combined with the density functional theory. Further, the evolution of the LUMO-HOMO gap and the spatial distribution of molecular orbital were used to analyze these structure-property relationships.     

Dynamic light scattering of an ionic SDS micellar solution under an AC electric field

The phase separation behavior of near-critical ionic sodium-dodecyl-sulfate (SDS) micellar solution under a sinusoidal electric field was investigated by phase-contrast optical microscopy and by the small-angle dynamic light scattering method. The sinusoidal electric field significantly deformed the concentration domains and shifted the phase separation temperature. The autocorrelation function under a sinusoidal electric field was measured in the vicinity of a phase separation temperature range of 0.10 K < T_p – T < 0.97 K, where T_p – T is the temperature distance from the phase separation temperature in quiescent state T_p. The correlation function with an oscillatory part in the longer correlation time region was observed. The occurrence of the oscillatory mode, which depended on both the applied field frequency and the ambient temperature, indicates deformation of concentration fluctuation domains by dynamical coupling between the phase separation and the applied sinusoidal electric field. Received: 9 June 2000 Accepted: 31 August 2000     

Nonlinear response of aluminum surface to electric field

The static limit of the perpendicular second-harmonic response of the exposed single-crystal planes of aluminum to applied electric field is studied using the variant of stabilized-jellium model which allows to account for the anisotropy of surface potential. The self-consistently calculated linear and second-order induced charge density distributions are used to determine the normal component of the polarization vector. The surface structure is found to have a pronounced effect on the anisotropy of second-harmonic response. © 1997 John Wiley & Sons, Inc.     

Simulation of dynamic response to an electric field of surface stabilized antiferroelectric liquid crystals

Responses of surface stabilized antiferroelectric liquid crystals (SSAFLCs) to an electric field are simulated based on a model of the AFLC phase. The apparent tilt angle, the polarization reversal current and the optical transmission under applied triangular wave voltages are calculated. The results obtained systematically explain the experimental observations fairly well. The effects of parameters such as amplitude and frequency of applied voltages, an antiferroelectric coupling parameter and surface anchoring strength on these phenomena are discussed.     

Crystal nucleation in an electric field

Nucleation of polar and apolar crystals in an electrostatic field has been analyzed. The analysis is based on the extended nucleation theory which takes into account orientation of amorphous kinetic elements and the resulting crystals. In an electric field free energy of transformation is orientation-dependent which leads to orientation and field effects in thermodynamic (critical crystallization temperature) and kinetic crystallization characteristics (thermal and athermal nucleation rates).     

Simulations of a weakly conducting droplet under the influence of an alternating electric field

We investigate the electrohydrodynamics of an initially spherical droplet under the influence of an external alternating electric field by conducting axisymmetric numerical simulations using a charge-conservative volume-of-fluid based finite volume flow solver. The mean amplitude of shape oscillations of a droplet subjected to an alternating electric field for leaky dielectric fluids is similar to the steady-state deformation under an equivalent root mean squared direct electric field for all possible electrical conductivity ratio and permittivity ratio of the droplet to the surrounding fluid. In contrast, our simulations for weakly conducting media show that this equivalence between alternating and direct electric fields does not hold for . Moreover, for a range of parameters, the deformation obtained using the alternating and direct electric fields is qualitatively different, that is, for low and high , the droplet becomes prolate under alternating electric field but deforms to an oblate shape in the case of the equivalent direct electric field. A parametric study is conducted by varying the time period of the applied alternating electric field, the permittivity and the electrical conductivity ratios. It is observed that while increasing has a negligible effect on the deformation dynamics of the droplet for , it enhances the deformation of the droplet when for both alternating and direct electric fields. We believe that our results may be of immense consequence in explaining the morphological evolution of droplets in a plethora of scenarios ranging from nature to biology.     

On the dipolar electric field response of large systems

Whereas the definition of the dipole moment operator for any finite system, independent of its size, is trivial, it is only within the last 1?C2 decades that expressions have been proposed (based on the independent particle approximation) for the equivalent operator of an infinite and periodic system. Using a quasi-one-dimensional system as an example, we show how different versions of these expressions can be derived and thereby formulate, for the first time, a multi-determinant treatment of the effect of electron correlation. Based on another version, an MP2 correlation treatment is suggested that relies entirely on already available procedures. In contrast with what is often assumed, we demonstrate that the so-called branch dependence of the dipole moment per unit of an infinite periodic system is directly related to bulk physical observables that depend on the terminations even for samples of a size that is well above the thermodynamic limit. In particular, the structural response to an electrostatic field is studied for a model system. It is shown how the effect of the terminations on the converse piezoelectric coefficient can be calculated and measured. In addition, we demonstrate the viability of the finite field nuclear relaxation treatment for determining the vibrational contribution to static (hyper)polarizabilities and dynamic non-linear optical processes in periodic systems.     

In situ speciation studies of copper in the electroplating sludge under an electric field

Speciation of copper in the electrokinetic treatments of an industrial sludge was studied by in situ extended X-ray absorption fine structural (EXAFS) and X-ray absorption near-edge structural (XANES) spectroscopies in the present work. The least-square fits (LSF) of the XANES spectra indicated that the main copper species in the sludge were Cu(NO)(3) (82%) and adsorbed copper (Cu/SiO(2)) (17%). Copper in the sludge possessed a Cu-O bond distance of 1.97A with a coordination number (CN) of 3.8. In the second shells, the bond distance of Cu-(O)-Si was 3.03A with a CN of 1.5. Most of Cu/SiO(2) was dissolved in the aqueous phase since little Cu-(O)-Si species in the sludge was found after 180 min of the electrokinetic treatments. About 69% of total Cu(II) was dissolved into the aqueous phase and 51% of which was migrated to the cathode under the electric field (5V cm(-1)) for 180 min.