International Journal of Modern Physics B

Electrorheological (ER) fluids are a type of colloids whose rheological characteristics can be varied reversibly with the application of an electric field. The traditional ER mechanism is based on induced polarization arising from the dielectric constant contrast between the suspended solid particles and the fluid. However, for induced polarization there is a maximum value for the dimensionless electric susceptibility χ ∼ 0.24. That implies an upper bound for the traditional ER effect. For permanent molecular dipoles, χ can be as high as 4–50. Thus, one to two orders of magnitude higher electric energy can be gained by harnessing the molecular dipoles. Indeed, the recent discovery of the giant electrorheological (GER) effect, in nanoparticles of urea-coated barium titanate oxalate suspended in silicone oil, has shown that the theoretical upper bound of the traditional ER effect is no longer applicable to this new type of electrorheological fluid. A phenomenological model of the GER mechanism, based on aligned molecular dipoles of urea molecules in the contact regions of the nanoparticles, yields an adequate account of the observed effect but without a microscopic picture on how this can occur. More recently, by using molecular dynamics (MD) to simulate the urea-silicone oil mixture confined between two bounding surfaces of a nanocontact, we find that the urea molecules, which has a molecular dipole moment of 4.6 Debye, can form aligned dipolar filaments that penetrate the oil film to bridge the two substrates, with the attendant lowering of the aligning field for the urea dipoles. This phenomenon is explainable on the basis of a 3D to 1D crossover in urea molecules' microgeometry, realized through the confinement effect provided by the oil chains. The resulting electrical energy density is shown to give an excellent account of the observed yield stress variation as a function of the electric field.

The aggregation of magnetic particles in the presence of a magnetic field is the basic phenomenon which underlies all the physics of magnetorheological (MR) fluids. Although these interactions are well understood when the suspending fluid is a simple liquid, new MR fluids based on dispersion of magnetic microparticles in a ferrofluid or MR elastomers based on dispersion of magnetic particles in a rubber matrix, present some unusual properties which are not well described by conventional theories. We analyze in this work the motion of magnetic particles dispersed in a ferrofluid and submitted to a magnetic field and discuss the possible applications.

Experimental and simulation results are presented that illustrate how the addition of nonmagnetizable particles to a magnetorheological (MR) fluid can enhance the field-induced yield stress. Efforts to understand the underlying mechanisms are described.

When neutrally buoyant poly alpha olefin particles in corn oil were exposed to a gradient ac electric field generated by a spatially periodic electrode array, these particles experienced the negative dielectrophoresis and instability in all the suspensions of concentration range from 0.01% to 5% (v/v). One critical particle concentration was experimentally determined as 1% (v/v) below which the particles in corn oil were segregated to form island-like structures in the lower electric field regions; and above which, particles only formed straight stripes. The island-like structure was suspended in the lowest electric field area. Specially designed experiments with a suspension of 1.126% (v/v) confirmed that there exists particle instability. Anisotropic properties of electric interactions are responsible for particle instability in all the suspensions of different concentrations and island-like structures were formed only in the dilute suspensions in which the particle instability has enough space to be developed.

We used numerical simulations of a continuous model and the molecular dynamics model to understand the particle instability, formation of island-like structures and existence of one critical particle concentration of 1% (v/v) for formation of island-like structures in the suspension in a gradient AC electric field reported in Paper I. The simulations of the continuous model show that the critical concentration of 1% (v/v) is the concentration of which the particles of a suspension are just fully filling the lower field region finally. According to the MD simulations, the particles instability does exist in the corn oil in a gradient AC electric field, anisotropic polarization interactions among the particles are responsible for the particle instability and have memory, and the memory is still kept even when the particles are transported by a dielectrophoresis force. The island-like structures can be regarded as signature of the memory. We explored possibilities to apply our findings in biomedical fields.

The model of an artificial cilia as a flexible ferromagnetic filament in a rotating magnetic field is proposed. Numerical algorithm for the simulation of its behavior is developed and the characteristic shapes of the filament with one fixed end under the action of a rotating field are found. It is concluded that ferromagnetic filaments may be used as mixers in microfluidics.

Our aim in this work was to propose the use of a ternary blend of two carbonyl iron powder CIP, mixed with water atomized iron powder (WAIP), to reduce the off-state viscosity, without prejudice of MRF performance in terms of yield stress and torque output. The idea of mix water atomized iron powder with carbonyl iron powder is not new. The US Pat. # 5,900,184 by Weiss et al. (1999) describes that a binary blend, half-to-half, can reduces the viscosity of MRF in the absence of magnetic field, and increase the torque output under field.

A MR suspension was prepared by dispersing silica-coated iron alloy particles into a liquid gallium. In other words, the iron alloy particles of 30 to 50 nm in diameter were first prepared and then coated with silica. Next, the particles were then suspended in a liquid Ga (assay: 99.9999%). In addition, the magnetic properties of the synthesized particles and suspension under the influence of the magnetic field were investigated. One of the main findings of this study is that the prepared powder showed a temperature sensitive of magnetization within the testing temperature range of 293–353 K. The saturation magnetization of silica-coated FeNbVB particles was about 0.55 T, whereas the saturation magnetization (297 K) of the synthesized MR suspension was 0.019 T.

In recent years, a new type ER fluids named as polar-molecule-dominated electrorheological (PM-ER) fluids have been developed, of which the yield stress can reach more than 100 kPa and behaves a linear dependence on the electric field. A brief description on the composition and synthesizing method for the materials is given. The main merits of PM-ER fluid are as follows: high yield stress, the shear stress increasing with shear rate up to more than 10³s^-1, low current density, rapid electric response and anti-sedimentation. Some perspectives on PM-ER fluid and its applications are presented.

The progress in the synthesis of new magnetic nanoparticles and agglomerates stimulates the development of novel ferrofluids with enhanced rheological properties. In the current work ferrofluids based on Co-nanoplatelets and clustered iron oxide nanoparticles have been considered. Steady-shear experiments and yield stress measurements of these ferrofluids have been performed using rotational rheometry.

To understand the dynamic rheological behavior of polar molecular electrorheological (PMER) fluids, the shear stress and viscosity of the colloids are compared with the parameters of their lamellar structures which are obtained simultaneously with the rheological characteristics using an electrorheoscope. The results of the experiments and molecular dynamics simulation indicate that the shear stress is mainly contributed by the moving particle rings, and there is an inverse correlation between the width of the moving particle rings and the shear stress.

The high shear rate behavior of MR fluids is investigated using a concentric rotational cylinder viscometer fabricated in-house. The rotational cylinder viscometer is designed such that a high shear rate of up to 30,000 s^-1 can be applied to the MR fluid in a pure shear flow mode. As a comparison, the maximum shear rate of a commercially available parallel disk type rheometer is only up to 1,000 s^-1. To determine the shear rate of the MR fluid in the viscometer, an exact expression between torque and angular velocity is established. The yield stress and viscosity of the MR fluid is determined by fitting the expression into the measured torque and angular velocities, and the shear stress as a function of the shear rate is further derived. The magnetic filed strength across the fluid gap is determined based on an electromagnetic field analysis, and the yield stress and viscosity of the fluid as a function of the magnetic filed is established. Specifically, the stability of the MR fluid at high shear rate is also evaluated. Two commercially available MR fluids, i.e., Lord's MRF-132DG and MRF-140CG, are investigated using the rotational cylinder viscometer, and the testing results are compared to the manufacturer's data.

Nanocrystalline strontium hexaferrite powder has been prepared by a new mechanochemical method in which the single phase hexaferrite was obtained via a sol–gel autocombustion process followed by an intermediate high energy milling step and subsequent annealing. The effects of the intermediate milling on the phase evolution, crystallite size and annealing behavior of the final products were investigated using the X-ray diffraction (XRD) technique. The single phase strontium hexaferrite was obtained at an annealing temperature of 800°C, while this temperature was 1,000°C for the powder synthesized without milling. It could be seen that an intermediate milling accelerates the formation of strontium hexaferrite during the calcination process. The results showed that in the milled powder, the average crystallite size of the ferrite was about 40 nm and much smaller than that of the nonmilled powder. Magnetic properties were also measured by a vibrating sample magnetometer (VSM). The particle morphology was then studied by scanning and transmission electron microscopes (SEM and TEM).

In-doped zinc oxide (ZnO:In) thin films with thickness from 157 nm to 592 nm have been deposited on glass substrates by radio frequency (RF) magnetron sputtering. The effect of the film thickness on the structural, electrical and optical properties of ZnO:In thin films has been investigated. It is found that the films are hexagonal wurtzite structure with c-axis perpendicular to the substrate, and with increasing thickness, the crystallinity, the grains size and the conductivity of the films increases, but the strains along c-axis and the transmittance decrease. The decrease of the resistivity in a thicker film is attributed to the slight increase of the carrier concentration and the significant increase of Hall mobility. The transmittance of all the films is over 80% in the visible region (400–800 nm) and the band gap decrease with the increase of film thickness. The film with the thickness of around 303 nm has the resistivity of 6.07 × 10^-3 Ω⋅cm and the transmittance of 90% in the visible range. Based on the good conductivity and high transmittance, the ZnO:In films prepared by magnetron sputtering can be regarded as a potential transparent electrode.

We derive an expression for the linewidth of phonons involved in the order–disorder phase transitions in solids. The linewidth is obtained as a function of the dwell time from which the activation energy can be computed. A new viewpoint is presented on the assumption that the fluctuations in the random variable of the noise, which is carried by the random particles involved in the order–disorder mechanism, affect the linewidth of the phonons.

Nanocrystalline FeNi and Ni₃Fe alloys were prepared by mechanical alloying of Fe and Ni elemental powders using a planetary ball mill under protection atmosphere. X-ray diffraction measurements were performed to follow alloy formation process in these alloys. A heat treatment of 1 h at 800°C was carried out to relax the internal stresses of the milled samples. Morphological evolution of powder particles was revealed by scanning electron microscopy. The value of lattice parameter was reached to 0.35762 nm and the hardness was found to be 686 HV at 30 h milled FeNi powder. In the case of Ni₃Fe the values of 0.3554 nm and 720 HV were obtained for lattice parameter and hardness, respectively.

We demonstrate theoretically that terahertz (THz) fundamental band-gap between the electron mini-band in the InAs layer and the heavy-hole mini-band in the GaSb layer can be realized in InAs/GaSb-based type II superlattices (SLs). The THz band-gap can be tuned by varying the widths of the InAs/GaSb layers. The presence of such band-gap can result in a strong cut-off on optical absorption at THz frequencies. For typical sample structures, the THz cut-off of the optical absorption depends sensitively on temperature and a sharper cut-off can be observed at relatively high temperatures. This study is pertinent to the application of InAs/GaSb type II SLs as THz photodetectors.