Particles In Electrorheological Fluids Research Articles

Study on microscale flow characteristics of electrorheological Fluids

Electrorheological fluid is a kind of intelligent material whose rheological properties change rapidly and reversibly under the action of external electric field, It has a broad prospect in practical engineering application. It is found in the research progress of electrorheological fluids. The time scale and space scale of the electrorheological effect are very small, and the flow process has some micro-scale flow characteristics. In this paper, the dynamics of dielectric particles in electrorheological fluids under static and dynamic coupling fields is numerically simulated by dissipative particle dynamics method. The influence of surface force under static field on the submicrostructure of Electrorheological fluid and the influence of electroviscosity under dynamic coupled field on the flow rate of Electrorheological fluid are also studied.

Electrostatic Force between Two Dielectric Particles in Electrorheological Fluids: beyond Spherical Particles

In an effort to increase the shear yield stress of dielectric electrorheological fluids, we focus on the electrostatic force of different forms of particles in a dielectric polarization model. By solving Laplace‘s equation and applying the multiple image method and the finite element method, the analytical and numerical solutions of the electrostatic force of a two-sphere structure have been studied. The results suggest that when the dielectric mismatch factor is large and when the positions of the two spheres are nearly in contact with each other, most of the analytical solutions either over-or underestimate the force. Additionally, the structure of particles beyond the spherical form is considered. Three example cases are studied to shed light on how different geometries of particles may affect the electrostatic force, thereby influencing the shear yield stress of the fluid.

Enhanced Electrorheological Performance of Nb-Doped TiO2 Microspheres Based Suspensions and Their Behavior Characteristics in Low-Frequency Dielectric Spectroscopy.

Titanium dioxide and Nb-doped titanium dioxide microspheres with the same size were fabricated by a simple sol-gel method, and the formation mechanism of Nb-doped titanium dioxide microspheres was proposed. Titanium dioxide and Nb-doped titanium dioxide microspheres were adopted as dispersed materials for electrorheological (ER) fluids to investigate the influence of the charge increase introduced by Nb doping on the ER activity. The results showed that Nb doping could effectively enhance the ER performance. Combining with the analysis of dielectric spectroscopy, it was found that the interface polarization of Nb-doped TiO2 ER fluid was larger than that of TiO2 ER fluid, which might be caused by more surface charges in Nb-TiO2 microspheres due to Nb(5+) doping and resulting in enhancement of electric field force and strengthening of fibrous structure. In addition, by comparing and analyzing the permittivity curves of Nb-TiO2/LDPE solid composite and Nb-TiO2/silicone-oil fluid composite, it could be concluded that the enhancement of permittivity at low frequency resulted from the increase of the order degree of dispersed particles in ER fluid rather than from the quasi-dc (QDC) behavior. Moreover, the absolute value of slope of permittivity curves (K) at 0.01 Hz could be utilized as the standard for judging the ability to maintain the chainlike structure. The relationships between polarizability of dispersed particles, dielectric spectrum, parameter K, and ER properties were discussed in detail.

Wear Characteristics Under Boundary Lubrication Contacts in Phosphorated Starch-Based Electrorheological Fluid

This experimental study examined wear characteristics under boundary lubrication contacts associated with electrorheological (ER) fluid composed of phosphorated starch particles and silicone oil. The ER fluids with starch phosphate are manufactured and adopted as lubricants. A pin-on-disc wear tester is then designed and constructed to evaluate wear rates of pin specimens. Specimens used for the experiments include steel, copper, and aluminum pins. The wear tests with the lubrication of ER fluid are experimentally performed under consideration of several operational factors such as normal load, sliding distance, and sliding speed as well as the specimen materials. Subsequently, microscopic surface and roughness changes of the worn surface of pin specimens are analyzed in order to validate the measured wear characteristics by using the scanning electron microscope (SEM) as well as surface profilometer. Friction coefficients of the ER fluid are also experimentally evaluated with respect to the sliding ranges. In addition, the chemical wear characteristics are investigated by using energy dispersive X-ray spectroscopy (EDS). In order to verify the effect of starch phosphate particles in ER fluid, the wear test results with ER fluid are compared with test results with only silicone oil, which is base carrier of ER fluid. The results clearly indicate that the phosphorated starch-based ER fluid tested shows the stabilized wear as well as friction characteristics after a certain period of operations but increased the wear rate as compared to the case with only silicone oil.

Dynamic Polarizability of Rotating Particles in Electrorheological Fluids

A rotating particle in electrorheological (ER) fluid leads to a displacement of its polarization charges on the surface which relax toward the external applied field E0, resulting in a steady-state polarization at an angle with respect to E0. This dynamic effect has shown to affect the ER fluid properties dramatically. In this paper, we develop a dynamic effective medium theory (EMT) for a system containing rotating particles of finite volume fraction. This is a generalization of established EMT to account for the interactions between many rotating particles. While the theory is valid for three dimensions, the results in a special two-dimensional configuration show that the system exhibits an off-diagonal polarization response in addition to a diagonal polarization response, which resembles the classic Hall effect. The diagonal response monotonically decreases with increasing rotational speed, whereas the off-diagonal response exhibits a maximum at a reduced rotational angular velocity omega0, compared to the case of isolated rotating particles. This implies a way of measurement on the interacting relaxation time. The dependencies of the diagonal and off-diagonal responses on various factors, such as omega0, the volume fraction, and the dielectric contrast, are discussed.

MEASUREMENT OF INTERACTIVE SURFACE FORCE OF SUSPENDED PARTICLES IN ER AND MR SUSPENSIONS UNDER ELECTRIC AND MAGNETIC FIELD

Interactive surface force between each particle in suspension is very important to explain the dispersion and flocculation phenomena in fluid. The new equipment has been developed to measure the interactive surface force of particles, which was calculated by using Derjaguin's formula, in Electro-Rheological (ER) Fluid and Magneto-Rheological (MR) Suspensions/fluids under electric and magnetic fields, respectively. Firstly, the interactive force of particles in ER fluid dispersed by melamine particles under electric fields was measured. Secondly, the interactive force of particles in MR suspensions dispersed by carbonyl iron particles under magnetic field was investigated. Considering the measurement of interactive surface force of ER and MR fluids, they have a original characteristic frequency due to the attractive and repulsive force of particles interactions initiated by the electric, magnetic and compressive force. Moreover, clusters formed according to the size of particles, particles movement model and formation are estimated by the characteristic frequency of ER fluid and MR suspensions under various electrical and magnetic fields. It indicates that these estimation models might be lead to the interaction kinetics of suspended particles in ER/MR under electrical and magnetic fields.

Asymptotic estimation for the system of electro-rheological fluids

By the movement model of ball particles in electrorheological(ER) fluids, the asymptotic behavior of the movement of the particles in ER fluids is considered when the mass of the particles is small. Using the theory of differential inequalities, the expression of asymptotic estimation is given and proved.

Conducting poly(methyl methacrylate)/polyaniline (core/shell) particles for electrorheological fluids

A core/shell particle with a poly(methyl methacrylate) (PMMA) core and a conducting polyaniline (PANI) shell was prepared and used as a dispersed phase for an electrorheological (ER) fluid, which is a suspension of polarizable particles in a non-conducting medium. PMMA particles, which were obtained using the dispersion polymerization method and spherical in shape with a monodisperse size distribution, were used as the core material for three different particle sizes. A PANI shell was introduced to the PMMA particle surface through in situ polymerization of aniline by an oxidant within an aqueous acidic solution, and its thickness was controlled via the amount of aniline monomer. These synthesized particles were dispersed in a silicone oil to prepare an ER fluid. The ER fluid, possessing Newtonian characteristics in the absence of an external electric field, exhibited a viscoelastic solid behavior with a non-vanishing yield stress, which were systematically controlled by the applied electric field strengths.

COMPUTER SIMULATIONS OF ER FLUIDS IN DID MODEL

Theoretical investigations on electrorheological (ER) fluids are usually concentrated on monodisperse systems. Real ER fluids must be polydisperse in nature, i.e., the suspended particles can have various sizes and/or different dielectric constants. An initial approach for these studies would be the point-dipole (PD) approximation, which is known to err considerably when the particles approach and finally touch due to multipolar interactions. A dipole-induced-dipole (DID) model is shown to be both more accurate than the PD model and easy to use. The DID model is applied to simulate the athermal aggregation of particles in ER fluids and the aggregation time is found to deviate significantly as compared to the PD model. Moreover, the inclusion of DID force further complicates the results because the symmetry between positive and negative contrasts will be broken by the presence of dipole-induced interactions.

Phase transition of solid-liquid electrorheological system in flow percess

Considering the short-range interaction of multi-particles, we have employed the equivalent plate conductivity model to simulate the phase transition of solid-liquid electrorheological(ER) fluids in flow process. We also devised experiment to observe the phase transition of ER fluids. The conclusions show that under the certain pressure the ER fluids begin to flow, here the flow is the two-phase flow. But when the strength of external electric field reaches a certain threshold value, the particles in ER fluids stop flowing, so the flow of ER fluids changes into one-phase flow, the phase transition happens. The phase transition observed in our experiment complies with the simulation fundamentally. The threshold strength of external electric field increases with the increasing pressure, and decreases with the concentration of particles in ER fluids.

A theory of induced interaction between rotating particles in electrorheological fluids

Recent experiments on electrorheological effects provide evidence that the rotation of particles weakens the induced electric forces between dielectric particles. To study the induced electric force exerted on rotating particles phenomenologically, we establish a differential equation to describe the relaxation process of the surface polarized charge distribution on rotating particles in an electrorheological fluid. In the case of a uniformly rotating particle, a general solution to the relaxation equation for the polarized charge is obtained. The surface charge distribution of a two-particle system consisting of a particle at rest and a uniformly rotating particle is calculated. Based on the Green’s function approach, we evaluate the electric potential in both the host and the rotating particle. A formula relating the angular velocity of rotating particle to the induced electric force on the particle is then derived.

Computer simulations of electrorheological fluids in the dipole-induced dipole model.

We have employed the multiple image method to compute the interparticle force for a polydisperse electrorheological (ER) fluid in which the suspended particles can have various sizes and different permittivities. The point-dipole (PD) approximation, being routinely adopted in the computer simulation of ER fluids, is known to err considerably when the particles approach and finally touch due to multipolar interactions. The PD approximation becomes even worse when the dielectric contrast between the particles and the host medium is large. From the results, we show that the dipole-induced-dipole (DID) model yields very good agreements with the multiple image results for a wide range of dielectric contrasts and polydispersity. As an illustration, we have employed the DID model to simulate the athermal aggregation of particles in ER fluids, both in uniaxial and rotating fields. We find that the aggregation time is significantly reduced. The DID model partially accounts for the multipolar interaction and is simple to use in the computer simulation of ER fluids.

The Electrostatic Interaction between Coated Particles in Electrorheological Fluids

A theoretical method is developed to calculate the interaction force between coated particles in a host liquid under an electric field. With the nonlinear conduction effect of the oil taken into consideration, the Poisson's equation of the potential φ is transformed into a Laplace's equation. At the same time, the multipolar expansion method is applied to solve the problem. By self-consistent calculation, the conductivity of the host liquid and the local field strength can be obtained simultaneously. On the basis of the model, a single chain structure spanning two parallel electrodes is dealt with. We find that the interaction between neighboring particles is strengthened when a dielectric layer is coated on the surface of the metallic spheres in comparison with that of the dielectric particles, and the interaction force is nearly independent on the dielectric constant of the coating layer except that the layer is a sort of insulator and its dielectric constant is rather small. Further, the interaction force in a dc field varies with the field strength more slowly than a parabola, and approaches a straight line in some cases, even has a maximum.

Novel Material Processing for Powder Surface. Surface Effects of Particles in Electrorheological Fluids.

On the application of electric fields, the suspensions consisting of polarizable particles dispersed in insulating oils undergo a rapid transition from Newtonian liquids to rigid solids, and this effect is referred to as electrorheology (ER). In electrified suspensions, the dipole-dipole interactions cause chains of particles in the direction parallel to the field vector. The fibrous structures are responsible for the solidlike responses of suspensions. When the dielectric constants of the particles and the oils are substantially different, each particle acquires an induced dipole in electric fields. Basically, the intrinsic mechanism of ER is a dielectric mismatch. However, in actual suspensions, the ER effect is strongly affected by the substances at the particle surface, rather than the particle material itself. The effects of surface conditions on the ER are discussed in relation to the method of formulating excellent ER fluids.

334 分散系機能性流体の粒子クラスタ

The viscosity variation of a suspension due to applied electric field strength is called Electrorheological(ER) effect, and several kinds of ER fluid have been developed so that it may be applied to mainly semi-active devices in mechanical engineering field. ER fluid generally consists of solid particles suspended in dielectric liquid. The particles in the liquid build up chain structures between electrodes in the direction of the applied electric field, whose tensile strength causes yield shear stress variation of ER fluid. In the present paper, the cluster formation of particles in ER fluid under an electric field was simulated by Cellular Automata(CA) and ER effect was quantified.

N-substituted copolyaniline for electrorheological material

An electrorheological (ER) fluid is a colloidal dispersion consisting of semiconducting particles and insulating oil. When an electric field is imposed, its rheological properties vary by showing a characteristic fibrillation, with the strings of the particles oriented along the electric field direction. This particle structuring is caused by the dielectric constant mismatch of the particles and the suspending oil [1]. Due to this mechanism, many typical properties of ER fluids, such as yield phenomenon, viscoelasticity, and drastic increase of viscosity, have been observed. In an electric field, ER fluids solidify in the order of milliseconds, and fluidize under applied deformation, which can destroy the chain structure. Therefore, ER fluids are often regarded as Bingham fluids. There are two ER fluid systems: wet-base (hydrous) and dry-base (anhydrous). The difference between these systems is the type of carrier species of the particle polarization. The absorbed moisture and the ions move through the electric field in wet-base ER fluids, whereas the electrons move inside the molecules of the particles in dry-base ER fluids. Dry-base ER fluid systems, including polyaniline, have the advantages of a relatively low density, a controllable conductivity, and thermal stability [2]. Polyaniline is very stable in the atmosphere, and also has good thermal properties. Furthermore, it has a relatively higher solubility in general organic solvents than other conducting polymers. In order to develop the processibility of polyaniline further, many workers have investigated polyaniline dispersion [3], the synthesis of copolyaniline, the introduction of a side group into the main chain [4], and types of dopants [5]. Among various copolymers, SO3y and alkyl sulfonate substituents are often put into the polymer main chain. In this type of polymer, the sulfonate ion acts as a self-dopant. Thus, the conductivity of the polymers are little affected by the pH. The value of the conductivity, however, is lower than that of homopolymers, due to the disturbance of the conjugation of the electrons through the main chain. In this study, N-substituted copolyaniline was synthesized and used as the particles in the ER fluid. Its molecular chain structure is supposed as following: To synthesize N-substituted copolyaniline [6], monomer mixtures of aniline and diphenyl-4-sulfonic acid sodium salt (molar feed ratio ˆ 3:1) were put into a l-neck flask of lM HCl. FeSO4 was also added as a catalyst. The flask was kept in a controlled thermostatic bath (0 0.1 8C). A prechilled (NH4)2S2O8 solution in lM HCl was then added dropwise, to the mixture of aniline and lM HCl for one hour in a nitrogen environment. The system was stirred for approximately 2 h to complete the reaction. The solution was then allowed to precipitate for 30 h, and the product was separated using a glass filter. The pH of the product was adjusted to 8.26 by adding either NH4OH or HCl solution. The synthesized polymer was washed three times using distilled water to remove initiator (oxidant), unreacted monomer, and oligomer. It was then further washed with ethanol and cyclohexane, sequentially, in order to make the surface of the synthesized particles hydrophobic [2]. This process is advantageous for wetting between the particles and oil during ER fluid preparation. The product was finally put into a vaccum oven for approximately two days to dry. The conductivity of the particles was measured to be 2.33 3 10y6 S=cm by using the 2-probe method with a pressed disk of polymer. The particle size was controlled by a 38 im sieve, and the size of the particles was measured by a particle analyser (Malvern), using ethanol as the dispersant (average diameter : 15.4 im, median : 7.3 im). The laminate structure of the polymer particles was further identified by a scanning electron microscope. The structural analysis of copolyaniline was performed using FT-IR (BIO-RAD, FRS-40). Fig. 1 shows the FT-IR spectrum determined using KBr pellets. The peaks at 1586 and 1490 cmy1 are from aromatic C–C stretching vibrations, those at 1309 and 1144 cmy1 are from aromatic amine stretching, and the peak at 824 cmy1 is from the out of plane H deformation for aromatic rings, which is from the

Relationship Between the Formation of Clusters in Electrorheological Fluids and Their Fractal Dimension

In this paper, the scaling properties of aggregation of particles in electrorheological fluids (ERF) are analyzed. It is found that the cluster size is related to both the space dimension and its fractal dimension, i.e. the structure of cluster. By analyzing the experimental data and the results of computer simulations we have shown that the fractal dimension of the particle clusters in ERF is one for 2-dimensional space and 5/4 for 3-dimensional space.

Cluster formation in electrorheological fluids: Theory and computer simulation

It is well known that dispersed particles in electrorheological fluid (ERF) form chain‐like clusters under an electric field. Many authors consider this cluster formation to be the origin of the electrically induced shear stress τe. If the shear rate γ is increased, however, the clusters should be broken into shorter segments. Is this consistent with the experimental fact that τe is roughly independent of γ? To answer this question, we have studied the cluster formation under shear flow by simple theory and computer simulation, which are based on the force balance between the hydrodynamic resistance and electrostatic attraction among particles. The results show that, when γ is increased, the length of clusters becomes shorter in proportion to γ−1/2, while the stress due to the clusters remains roughly constant. This indicates that the clusters are indeed the origin of the induced stress.

Finite-element analysis of particle-particle forces in electrorheological fluids

The first finite-element analysis of the forces between particles in electrorheological fluids is presented. The shear modulus for chains of particles arrayed on a square lattice is calculated. Two limiting cases are considered: (1) dielectric particles in a dielectric fluid and (2) infinitely conducting particles in an insulating fluid. In (1), the modulus increases linearly with the ratio of dielectric constants Kp/Kf as the ratio becomes large, contrary to expectations from a simple dipole approximation where it would saturate. For case (2), the modulus depends sensitively on the interparticle spacing and diverges when the particles touch.