Phase transitions,structures, and rheological properties of hydroxypropyl cellulose–ethylene glycol and ethyl cellulose–dimethylformamide systems in the presence and in the absence of a magnetic field

Phase transitions, structures, and rheological properties of hydroxypropyl cellulose–ethylene glycol and ethyl cellulose–dimethylformamide systems in the presence and in the absence of a magnetic field have been studied. The application of the magnetic-field results in increases in viscosity and supramolecular particle size in solutions of cellulose ethers. Concentration dependences of viscosity and particle size are described by curves with maxima.     

Effect of a magnetic field on the rheological properties of the systems hydroxypropyl cellulose–ethanol and hydroxypropyl cellulose–dimethyl sulfoxide

With the use of viscometry, the cloud-point method, polarization microscopy, the turbidity-spectrum method, and a polarization photoelectric apparatus, the relaxation pattern of the rheological behaviors, phase transitions, and structures of the systems hydroxypropyl cellulose–ethanol and hydroxypropyl cellulose–dimethyl sulfoxide are studied. The regions of existence of isotropic and anisotropic phases and the concentration dependence of the sizes of supramolecular particles are determined. It is found that a magnetic field increases the viscosities of solutions. The concentration dependences of viscosity and particle size are described by curves with maxima.     

Effect of magnetic field on the rheological properties of poly(ethylene glycol) and poly(dimethylsiloxane) mixtures with aerosil and iron nanoparticles

The effect of a magnetic field on the viscosity of magnetorheological poly(ethylene glycol)-aerosil-iron nanoparticle and poly(dimethylsiloxane)-aerosil-iron nanoparticle suspensions is studied. The magnetic field leads to an increase in the viscosity of the suspensions by a factor of 20–300. The concentration dependence of the effect of magnetic field on the viscosity of the systems is described by a curve with a maximum. The dependences of viscosity on shear rate upon loading and unloading do not coincide, thus indicating the relaxation character of the flow process.     

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.     

Effect of magnetic field on phase transitions in solutions of cellulose derivatives

LC transitions occurring in mixtures of cyanoethyl cellulose with DMAA or DMF and hydroxypropyl cellulose with ethanol, DMAA, or water in the presence and absence of magnetic field have been studied. With an increase in the polarity of solvent molecules and a decrease in the molecular mass of the polymer, the LC phase develops at higher concentrations and lower temperatures. Under application of magnetic field, the domain structure is formed in solutions and the temperature-concentration region of the LC phase widens. Cyanoethyl cellulose and hydroxypropyl cellulose solutions are found to possess memory: after the magnetic field is switched off, the orientation of macromolecules and the increased temperature of phase transitions are preserved for many hours. As the molecular mass of the polymer is increased, the ability of macromolecules to orient themselves in the magnetic field declines. The threshold mechanism governing the effect of magnetic field on LC transitions in polymer solutions has been discovered. The critical value of magnetic intensity that brings about a shift in boundary curves is consistent with the value of H _cr necessary for the cholesteric liquid crystal-nematic liquid crystal phase transition.     

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.     

Influence of magnetic interactions between clusters on particle orientational characteristics and viscosity of a colloidal dispersion composed of ferromagnetic spherocylinder particles: analysis by means of mean field approximation for a simple shear flow

We have theoretically investigated the particle orientational distribution and viscosity of a dense colloidal dispersion composed of ferromagnetic spherocylinder particles under an applied magnetic field. The mean field approximation has been applied to take into account the magnetic interactions of the particle of interest with the other ones that belong to the neighboring clusters, besides those that belong to its own cluster. The basic equation of the orientational distribution function, which is an integrodifferential equation, has approximately been solved by Galerkin's method and the method of successive approximation. Some of the main results obtained here are summarized as follows. Even when the magnetic interaction between particles is of the order of the thermal energy, the effect of particle-particle interactions on the orientational distribution comes to appear more significant with increasing volumetric fraction of particles; the orientational distribution function exhibits a sharper peak in the direction nearer to the magnetic field one as the volumetric fraction increases. Such a significant inclination of the particle in the field direction induces the large increase in viscosity in the range of larger values of the volumetric fraction. The above-mentioned characteristics of the orientational distribution and viscosity come to appear more significantly when the influence of the applied magnetic field is not so strong compared with that of magnetic particle-particle interactions.     

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.     

Rheological Properties and Orientational Distributions of Dilute Ferromagnetic Spherocylinder Particle Dispersions

We have investigated the rheological properties and the orientational distributions of particles of a dilute colloidal dispersion, which is composed of ferromagnetic spherocylinder particles, subject to a simple shear flow. The governing equation of an orientational distribution function has been derived from the balance of the torques acting on a particle in an applied magnetic field. After a spherical harmonic expansion, an approximate solution to the governing equation has been found by Galerkin's method. The results obtained are summarized as follows. The orientational distribution function has a sharper peak for a stronger magnetic field, and the position of the peak changes from the flow direction to the magnetic field direction as the magnetic field comes to govern the shear flow. Since the orientation of the particle is highly restricted in the field direction as the magnetic field becomes strong, the viscosity increases significantly. The particles with a larger aspect ratio lead to the larger increment in the viscosity, since they induce a larger resistance in a flow field. Copyright 2001 Academic Press.     

Diamagnetic susceptibility effects in NMR measurements of the properties of water in polymeric membranes

The NMR spectra of water adsorbed at various relative humidities on various cellulose ester membranes have been studied. Membranes of cellulose acetate (CA), cellulose triacetate (CTA), and cellulose acetate butyrate (CAB) were investigated. It was found that the resonance peak of a liquid imbibed in or adsorbed on membranes from high relative humidity is very sensitive to the angle between the membrane plane and the direction of the magnetic field, shifting 5–7 ppm to higher fields as the membrane plane is rotated from a perpendicular to a parallel position with respect to the direction of the external magnetic field. This phenomenon was found to be independent of the nature of the polymeric material (namely CA, CTA, or CAB), porosity of the membrane (varying from an “all bulk” dense sheet to an 80% porosity and 0.2 μm average pore size membrane), nature of the magnetic nuclei (H₂O or D₂O), intensity of the external magnetic field (60 Mcps or 100 Mcps), and nature of the liquid in the membrane (water, methanol, or n-hexane). It is therefore concluded that the dependence of the position of the resonance peak on the position of the membrane plane with respect to the external magnetic field, is a geometrical phenomenon due to the magnetic “bulk susceptibility” of the medium. Quantitative estimations of the magnitude of the diamagnetic susceptibility effect in a cylindrically rolled sheet are given. These estimates agree well with the experimentally observed “splittings.”     

Transport coefficients and orientational distributions of spheroidal particles with magnetic moment normal to the particle axis (Analysis for an applied magnetic field normal to the shear plane)

We have investigated the influence of the magnetic field strength, shear rate, and rotational Brownian motion on transport coefficients such as viscosity and diffusion coefficient, and also on the orientational distributions of rodlike particles of a dilute colloidal dispersion. The rodlike particle is modeled as a magnetic spheroidal particle which has a magnetic moment normal to the particle axis; such a particle may typically be a hematite particle. In the present study, an external magnetic field is applied in the direction normal to the shear plane of a simple shear flow. The basic equation of the orientational distribution function has been derived from the balance of torques and solved numerically. The results obtained here are summarized as follows. Although the orientational distribution function shows a sharp peak in the shear flow direction for a very strong magnetic field, such a peak is not restricted to the field direction alone, but continues in every direction of the shear plane. This is due to the characteristic particle motion that the particle can rotate around the axis of the magnetic moment in the shear plane, although the magnetic moment nearly points to the magnetic field direction. This particle motion in the shear plane causes negative values of the viscosity due to the magnetic field. The viscosity decreases, attains a minimum value, and then converges to zero as the field strength increases. Additionally, the diffusion coefficient is significantly influenced by such characteristic particle motion in the shear plane for a strong magnetic field.     

磁场对乙基纤维素胆甾型液晶相的影响

高分子胆甾型液晶相是一个在高分子和液晶领域都引起广泛兴趣的课题,不仅因为自然界中的多种生物大分子,如纤维素,多肽,DNA等可以形成胆甾型液晶相;而且由于胆甾相具有特殊的螺旋结构(如图1所示),能产生一些特殊的光学性能,如强烈的旋光性,圆二色性和选择性反射光性能等;并带来相应的应用,正是由于高分子胆甾相材料的性能和应用,使得高分子胆甾相液晶的相态转变和结构变化也一直倍受关注。     

Rheology and orientational distributions of rodlike particles with magnetic moment normal to the particle axis for semi-dense dispersions (analysis by means of mean field approximation)

We have considered a semi-dense dispersion composed of ferromagnetic rodlike particles with a magnetic moment normal to the particle axis to investigate the rheological properties and particle orientational distribution in a simple shear flow as well as an external magnetic field. We have adopted the mean field approximation to take into account magnetic particle-particle interactions. The basic equation of the orientational distribution function has been derived from the balance of the torques and solved numerically. The results obtained here are summarized as follows. For a very strong magnetic field, the magnetic moment of the rodlike particle is strongly restricted in the field direction, so that the particle points to directions normal to the flow direction (and also to the magnetic field direction). This characteristic of the particle orientational distribution is also valid for the case of a strong particle-particle interaction, as in the strong magnetic field case. To the contrary, for a weak interaction among particles, the particle orientational distribution is governed by a shear flow as well as an applied magnetic field. When the magnetic particle-particle interaction is strong under circumstances of an applied magnetic field, the magnetic moment has a tendency to incline to the magnetic field direction more strongly. This leads to the characteristic that the viscosity decreases with decreasing the distance between particles, and this tendency becomes more significant for a stronger particle-particle interaction. These characteristics concerning the viscosity are quite different from those for a semi-dense dispersion composed of rodlike particles with a magnetic moment along the particle direction.     

Transport coefficients and orientational distributions of rodlike particles with magnetic moment normal to the particle axis under circumstances of a simple shear flow

We have investigated the influences of the magnetic field strength, shear rate, and random forces on transport coefficients such as viscosity and diffusion coefficient, and also on the orientational distributions of rodlike particles of a dilute colloidal dispersion. This dispersion is composed of ferromagnetic spheroidal particles with a magnetic moment normal to the particle axis. In the present analysis, these spheroidal particles are assumed to conduct the rotational Brownian motion in a simple shear flow as well as an external magnetic field. The basic equation of the orientational distribution function has been derived from the balance of the torques and solved numerically. The results obtained here are summarized as follows. For a very strong magnetic field, the rodlike particle is significantly restricted in the field direction, so that the particle points to a direction normal to the flow direction (and also to the magnetic field direction). However, the present particle does not exhibit a strong directional characteristic, which is one of the typical properties for the previous particle with a magnetic moment parallel to the particle axis. That is, the particle can rotate around the axis of the magnetic moment, although the magnetic moment nearly points to the field direction. The viscosity significantly increases with the field strength, as in the previous particle model. The particle of a larger aspect ratio leads to the larger increase in the viscosity, since such elongated particles induce larger resistance in a flow field. The diffusion coefficient under circumstances of an applied magnetic field is in reasonable agreement between theoretical and experimental results.     

Sol-gel synthesis of titanium dioxide and titanium dioxide-hydroxypropyl cellulose hybrid material and electrorheological characteristics of their dispersions in poly(dimethylsiloxane)

Particles of titanium dioxide and titanium dioxide-hydroxypropyl cellulose organic-inorganic composite are synthesized by the sol-gel method. The effects of an electric field on the shear stress, viscosity, tensile and compression stresses of dispersions of titanium dioxide, the hybrid material, and titanium dioxide-hydroxypropyl cellulose mechanical mixture are investigated. It is established that the value of the electrorheological effect observed for hybrid material dispersions is higher than that for dispersions of titanium dioxide and mechanical mixtures of the materials.     

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.     

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.     

Nucleation Mechanism of Iron in an External Magnetic Field

In this work, the solidification of liquid iron with or without external magnetic field was investigated by using two molecular dynamics methods, namely direct cooling and two-phase simulation. The influence of external magnetic field on the solidification is characterized by the critical temperature and radial distribution functions. Our computational results show that under external magnetic field, the solidification point tends to decrease significantly. By further analyzing the diffusion coefficients and viscosity, we attribute the effect to the stronger fluctuation of liquid iron atoms driven by the external magnetic field.     

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.