Dependence of the viscosity coefficients of magnetic fluids on parameters of state

Dependences of the viscosity coefficients of magnetic fluids on parameters of state were investigated numerically using previously derived dynamic equations. It was shown that the volume viscosity and shear viscosity coefficients of a magnetic fluid based on kerosene increase with increasing density and concentration and decrease with increasing temperature; the coefficients increase with an increase in the magnetic field gradient. The results obtained are in satisfactory agreement with the experimental data.     

水基自形成CoFe2O4 磁性液体的零磁场粘滞特性

The plot of viscosity versus particle volume fraction for the water carrier of self-formed CoFe2O4 magnetic fluid is abnormal in zero magnetic field. However,the viscosity theory of the suspension with the global rigid particle filling cannot explain the experiment well. That is because the nanoparticles have aggregated before preparation of magnetic fluid. The fact is found that the sedimentation without magnetic field and the becoming chains in magnetic field of this type of magnetic fluid need the big particles which core are pre-aggregates by researching the interaction of particles of magnetic fluid. Around the big particles,nanoparticles are absorbed with the type of dynamic state. It is on that idea that the model of fluctuant aggregation is made. So,the average diameter,Einstein ratio and particles size distributive deviation of free suspended bodies in zero magnetic fluid are the functions of the particles volume fraction. And then,Popplewell’s formula of the viscosity is modified with this model. As a result,a well-fitted curve is obtained.     

Nano-sized Fe soft-magnetic particle and its magnetorheology

As a new magnetoresponsive magnetorheological (MR) material under an applied magnetic field, magnetic Fe nanoparticles were synthesized from a simple process of thermal decomposition of pentacarbonyl iron using oleyl amine and kerosene at 150 °C. Morphology of the fabricated Fe nanoparticle was examined using both scanning electron microscopy and transmission electron microscopy. MR characteristics of the nano-sized magnetic particle-based MR fluid dispersed in non-magnetic carrier fluid was investigated using a rotational rheometer under different external magnetic field strengths, focusing on their flow behaviors at a steady shear mode and yield stress. Flow curve was also found to be fitted well with the Casson equation.     

Rotational viscosity of viscoelastic magnetic fluid

The influence of a magnetic field on the viscosity of dilute dispersion of spherical magnetohard particles in viscoelastic matrix, the Maxwell liquid with a single relaxation time, is studied. Calculations are performed on the basis of the equation of rotational diffusion derived previously by the authors. It is shown that the effective viscosity of magnetic fluid decreases in the presence of a field. This unexpected result is clearly explained in physical terms.     

Soft magnetic carbonyl iron microsphere dispersed in grease and its rheological characteristics under magnetic field

Magnetorheological (MR) grease, comprised of a suspension of soft magnetic carbonyl iron (CI) microspherical particles dispersed in a grease medium, was fabricated by a mechanical stirring method. As potential medium oil for MR system, shear viscosity of the pure grease was measured as a function of temperature. Its MR characteristics were investigated using a rotational rheometer under an external magnetic field. Flow curve responses (shear stress and shear viscosity), yield stress, and elasticity were investigated using various magnetic field strengths ranging from 0 to 342 kA/m. It was confirmed that MR grease has a yield stress under no external magnetic field due to the inherent property of grease. In addition, CI based MR grease exhibited a characteristic of a Bingham fluid.     

Viscosity measurements of a ferrofluid: comparison with various hydrodynamic equations

Effective viscosity of a magnetic fluid as a function of applied magnetic field oriented in the perpendicular direction of the capillary flow is determined. Close agreement with the Shliomis expression derived on the basis of effective field method is observed.     

Destabilizing effect of time-dependent oblique magnetic field on magnetic fluids streaming in porous media

The present work studies Kelvin-Helmholtz waves propagating between two magnetic fluids. The system is composed of two semi-infinite magnetic fluids streaming throughout porous media. The system is influenced by an oblique magnetic field. The solution of the linearized equations of motion under the boundary conditions leads to deriving the Mathieu equation governing the interfacial displacement and having complex coefficients. The stability criteria are discussed theoretically and numerically, from which stability diagrams are obtained. Regions of stability and instability are identified for the magnetic fields versus the wavenumber. It is found that the increase of the fluid density ratio, the fluid velocity ratio, the upper viscosity, and the lower porous permeability play a stabilizing role in the stability behavior in the presence of an oscillating vertical magnetic field or in the presence of an oscillating tangential magnetic field. The increase of the fluid viscosity plays a stabilizing role and can be used to retard the destabilizing influence for the vertical magnetic field. Dual roles are observed for the fluid velocity in the stability criteria. It is found that the field frequency plays against the constant part for the magnetic field.     

Dispersion of magnetic susceptibility and the microstructure of magnetic fluid

The microstructure of magnetic fluid produced on the basis of kerosene with oleic acid as a stabilizer is studied experimentally. An analytical procedure based on the known dependence of the time of Brownian relaxation of the magnetic moment of the colloidal particle on its size and the expansion of a low-frequency spectrum of dynamic susceptibility into the series of Debye functions is used. Magnetic susceptibility is measured at frequencies from 10 Hz to 100 kHz and temperatures from 225 to 360 K for colloidal solutions with the volume fraction of magnetite from 0.08 to 0.17. The clusters with uncompensated magnetic moments and sizes varying from 50 to 70 nm that are three-or fourfold larger than the mean diameter of a single colloidal particle are found. It is revealed that characteristic sizes of clusters are virtually independent of temperature and concentration of colloidal particles. The contribution of clusters to the equilibrium susceptibility of magnetic fluid grows exponentially with decreasing temperature, being manyfold larger at low temperatures than that of single particles. The obtained temperature dependence of equilibrium susceptibility is compared with that predicted from current theoretical models.     

Rotational and spin viscosities of water: Application to nanofluidics

In this paper we evaluate the rotational viscosity and the two spin viscosities for liquid water using equilibrium molecular dynamics. Water is modeled via the flexible SPC/Fw model where the Coulomb interactions are calculated via the Wolf method which enables the long simulation times required. We find that the rotational viscosity is independent of the temperature in the range from 284 to 319 K. The two spin viscosities, on the other hand, decrease with increasing temperature and are found to be two orders of magnitude larger than that estimated by Bonthuis et al. [Phys. Rev. Lett. 103, 144503 (2009)] We apply the results from molecular dynamics simulations to the extended Navier-Stokes equations that include the coupling between intrinsic angular momentum and linear momentum. For a flow driven by an external field the coupling will reduce the flow rate significantly for nanoscale geometries. The coupling also enables conversion of rotational electrical energy into fluid linear momentum and we find that in order to obtain measurable flow rates the electrical field strength must be in the order of 0.1?MV?m(-1) and rotate with a frequency of more than 100 MHz.     

Glasslike behavior in aqueous electrolyte solutions

When salts are added to water, generally the viscosity increases, suggesting that the ions increase the strength of the water's hydrogen-bond network. However, infrared pump-probe measurements on electrolyte solutions have found that ions have no influence on the rotational dynamics of water molecules, implying no enhancement or breakdown of the hydrogen-bond network. Here, we report optical Kerr effect and dielectric relaxation spectroscopic measurements, which have enabled us to separate the effects of rotational and transitional motions of the water molecules. These data show that electrolyte solutions behave like a supercooled liquid approaching a glass transition in which rotational and translational molecular motions are decoupled. It is now possible to understand previously conflicting viscosity data, nuclear magnetic resonance relaxation, and ultrafast infrared spectroscopy in a single unified picture.     

Rotational viscosity of fluids composed of linear molecules: an equilibrium molecular dynamics study

In this paper, we investigate the rotational viscosity for a chlorine fluid and for a fluid composed of small linear molecules by using equilibrium molecular dynamics simulations. The rotational viscosity is calculated over a large range of state points. It is found that the rotational viscosity is almost independent of temperature in the range studied here but exhibits a power-law dependency on density. The rotational viscosity also shows a power-law relationship with the molecular length, and the ratio between the shear and rotational viscosities approaches 0.5 for the longest molecule studied here. By changing the number of atoms or united atomic units per molecule and by keeping the molecule length fixed, we show that fluids composed of molecules which have a rodlike shape have a lower rotational viscosity. We argue that this phenomenon is due to the reduction in intermolecular connectivity, which leads to larger fluctuations around the values possessed by the fluid on average. The conclusions here can be extended to fluids composed of uniaxial molecules of arbitrary length.     

Magnetic rotational spectroscopy with nanorods to probe time-dependent rheology of microdroplets

In situ characterization of minute amounts of fluids that rapidly change their rheological properties is a challenge. In this paper, the rheological properties of fluids were evaluated by examining the behavior of magnetic nanorods in a rotating magnetic field. We proposed a theory describing the rotation of a magnetic nanorod in a fluid when its viscosity increases with time exponentially fast. To confirm the theory, we studied the time-dependent rheology of microdroplets of 2-hydroxyethyl-methacrylate (HEMA)/diethylene glycol dimethacylate (DEGDMA)-based hydrogel during photopolymerization synthesis. We demonstrated that magnetic rotational spectroscopy provides rich physicochemical information about the gelation process. The method allows one to completely specify the time-dependent viscosity by directly measuring characteristic viscosity and characteristic time. Remarkably, one can analyze not only the polymer solution, but also the suspension enriched with the gel domains being formed. Since the probing nanorods are measured in nanometers, this method can be used for the in vivo mapping of the rheological properties of biofluids and polymers on a microscopic level at short time intervals when other methods fall short.     

Droplet migration in emulsion systems measured using MR methods

The migration of emulsion droplets under shear flow remains a largely unexplored area of study, despite the existence of an extensive literature on the analogous problem of solid particle migration. A novel methodology is presented to track the shear-induced migration of emulsion droplets based on magnetic resonance imaging (MRI). The work is in three parts: first, single droplets of one Newtonian fluid are suspended in a second Newtonian fluid (water in silicone oil (PDMS)) and are tracked as they migrate within a Couette cell; second, the migration of emulsion droplets in Poiseuille flow is considered; third, water-in-silicone oil emulsions are sheared in a Couette cell. The effect of (a) rotational speed of the Couette, (b) the continuous phase viscosity, and (c) the droplet phase concentration are considered. The equilibrium extent of migration and rate of migration increase with rotational speed for two different emulsion systems and increased continuous phase viscosity, leads to a greater equilibrium extent of migration. The relationship between the droplet phase concentration and migration is however complex. These results for semi-concentrated emulsion systems and wide-gap Couette cells are not well described by existing models of emulsion droplet migration.     

The role of activation energy and reduced viscosity on the enhancement of water flow through carbon nanotubes

Molecular dynamics simulations are carried out to study the pressure driven fluid flow of water through single walled carbon nanotubes. A method for the calculation of viscosity of the confined fluid based on the Eyring theory of reaction rates is proposed. The method involves the calculation of the activation energy directly from the molecular dynamics trajectory information. Computations are performed using this method to study the effect of surface curvature on the confined fluid viscosity. The results indicate that the viscosity varies nonlinearly with the carbon nanotube diameter. It is concluded that the reason behind the observed enhancement in the rate of fluid flow through carbon nanotubes could be the nonlinear variation of viscosity.     

Measurement of the rotational viscosity of ferroelectric liquid crystals based on a simple dynamical model

Rotational viscosity and spontaneous polarization are the most important properties of a ferroelectric liquid crystal with regard to its switching time in surface stabilized or a.c. field stabilized displays. Whereas there is an abundant literature about spontaneous polarization, only a few attempts have been made to determine the rotational viscosity. We set up a model for the electric response of a ferroelectric liquid crystal cell on application of an electric field. For the application of a triangular wave voltage we derive a relation between the rotational viscosity, the spontaneous polarization, the tilt angle, the maximum induced polarization current and the electric field strength. Experiments are carried out on several ferroelectric liquid crystals and the derived relation was used to determine the rotational viscosity. The relation between the rotational viscosity and the polarization on the one hand and the optical switching time on the other hand is discussed in some detail.     

丙烯酰胺微乳液聚合

由乳化剂聚乙二醇壬基苯基醚ＯＰ、聚乙二醇壬基苯基醚（ＴＸ—４）、丙烯酸胺、水和煤油组成微乳液时，体系中水相丙烯酸胶浓度及体系温度对乳化剂最小量有明显的影响；而ＯＰ、ＴＸ—４比例及油水比例的影响不大．本文研究了辐射引发微乳液聚合的动力学，得到如下表达式：聚合速率及聚合物特性粘数的表观活化能分别为５３．２ＫＪ／ｍｏｌ，－３３．２ＫＪ／ｍｏｌ．聚丙烯酸胺微乳液具有特殊的增稠性能，与聚电解质增稠剂相比，电解质对增稠效果的影响不大，而其他表面活性剂的影响较大．     

The effect of a magnetic field on the rheological properties of iron-aerosil-glycerol suspensions

The viscosity of a suspension consisting of iron nanoparticles, aerosil, and glycerol has been studied under the conditions of a rotational flow in the presence and absence of a magnetic field. At low shear rates, the viscosity is governed by the magnetic field and increases 40 times with a rise in the field strength; at high shear rates, the effect of the mechanical stress field prevails. In this case, the relative viscosity increases by 1.4 times, and its concentration dependence in the magnetic field is described by a curve passing through a maximum.     

Atom transfer radical polymerized PMMA/magnetite nanocomposites and their magnetorheology

We synthesized core/shell-typed magnetic nanoparticle composites using poly(methyl methacrylate) (PMMA) as a shell and magnetite nanoparticle (MN) as a core, in which the PMMA shell was prepared via atomic transfer radical polymerization (ATRP) method. Chemical structure and morphology of the synthesized MN–PMMA nanocomposite were investigated using FT-IR and TEM, respectively. Magnetorheological (MR) fluid was prepared by dispersing synthesized MN–PMMA in non-magnetic medium. Both shear stress and shear viscosity of the MR fluids as a function of shear rate were measured using a rotational rheometer with a magnetic field generator, exhibiting that a yield stress increased with an external magnetic field strength.     

Joule heating induced transient temperature field and its effects on electroosmosis in a microcapillary packed with microspheres

The Joule heating induced transient temperature field and its effect on the electroosmotic flow in a capillary packed with microspheres is analyzed numerically using the control-volume-based finite difference method. The model incorporates the coupled momentum equation for the electroosmotic velocity, the energy equations for the Joule heating induced temperature distributions in both the packed column and the capillary wall, and the mass and electric current continuity equations. The temperature-dependent physical properties of the electrolyte solution are taken into consideration. The characteristics of the Joule heating induced transient development of temperature and electroosmotic flow fields are studied. Specifically, the simulation shows that the presence of Joule heating causes a noticeable axial temperature gradient in the thermal entrance region and elevates a significant temperature increment inside the microcapillary. The temperature changes in turn greatly affect the electroosmotic velocity by means of the temperature-dependent fluid viscosity, dielectric constant, and local electric field strength. Furthermore, the model predicts an induced pressure gradient to counterbalance the axial variation of the electroosmotic velocity so as to maintain the fluid mass continuity. In addition, under specific conditions, the present model is validated by comparing with the existing analytical model and experimental data from the literature.     

Rheological properties and orientational distributions of dilute ferromagnetic spherocylinder particle dispersions. Part II. Analysis for the two typical magnetic field directions

We have investigated the orientational distributions and rheological properties of dilute colloidal dispersions, which consist of ferromagnetic spherocylinder particles. First, the governing equation of the orientational distribution function has been derived for the typical two cases of magnetic field directions: the direction parallel to the shear flow and the direction parallel to the angular velocity vector of the shear flow. The equation has been solved approximately by Galerkin's method. With these numerical solutions we have obtained the results of the orientational distribution and viscosity. The results obtained for the magnetic field in the shear flow direction are summarized as follows. In the case of a weak magnetic field, the particle tends to orient nearly toward the shear flow direction and its opposite direction. As the magnetic field increases, the orientation of the particle is restricted and the viscosity increases significantly. As the influence of the magnetic field becomes dominant, an overshoot in the viscosity curve appears. This is due to the fact that there is a maximum deviation of the averaged particle direction from the magnetic field direction. When the strength of the magnetic field increases significantly, the particle inclines close to the magnetic field direction and the viscosity converges to a constant value. Particles with a larger aspect ratio give rise to a larger increment in the viscosity since such elongated particles induce larger resistance in a flow field. We also have obtained results for the case of the magnetic field in the direction parallel to the angular velocity vector of the shear flow. When the flow field is dominant over both the rotational Brownian motion and the magnetic interaction, the particle rotates in the plane nearly perpendicular to the magnetic field direction. As the magnetic field increases, the particle inclines toward the magnetic direction. For this direction of field, the viscosity is independent of the magnetic field and is always zero.