ISSN:0148-6055    

DOI:10.1122/1.550627

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550628

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

We report a real time, two‐dimensional light‐scattering study of the nonlinear dynamics of field‐induced structures in an electrorheological (ER) fluid subjected to oscillatory shear. We have developed a simple kinetic chain model of the observed dynamics by considering the response of a fragmenting/aggregating particle chain to the prevailing hydrodynamic and electrostatic forces. This theory of the dynamics is then used to describe the nonlinear rheology of ER fluids, and it is concluded that light scattering allows one to indirectly probe the stress in an ER fluid.

ISSN:0148-6055    

DOI:10.1122/1.550629

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The rheogram, shear stress versus shear rate of magnetic suspensions composed either of monodisperse or polydisperse spherical particles, has been measured for different particle sizes. For small particles, where the ratio λ of magnetostatic energy to thermal energy is already much larger than unity (λ≊102–103), we find that the apparent yield stress is still strongly increasing with the size of the particles. It is only for very high values of λ (λ≊109) that we find experimental results which show no dependence on the size of the particles. These results are discussed with the help of a structural model based on the deformation of simple chains of particles, with an interparticle distance which depends on λ. It is shown that the experimental results can be explained by taking into account the fluctuations of the positions of the particles in a chain.

ISSN:0148-6055    

DOI:10.1122/1.550614

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The molecular Doi theory of rigid rod polymer rheology is used to describe the experimentally observed electrorheological effect of liquid crystalline polymer solutions. Two‐dimensional Doi theory has been modified by including the electric potential energy of a dipole in the energy term of the diffusion equation and in the free energy expression. Based on the same model, a linear theory of viscoelasticity has been developed to predict the storage and loss moduli under the influence of an applied electric field. Calculations indicate that molecular orientation induces viscosity enhancement, while the model attributes nonzero storage and loss moduli to the distortion of the orientation distribution function. Molecular parameters in the model can be estimated from either theory or experiment. Using a constant rotational diffusivity, model predictions of low shear electroviscosity and low‐frequency storage and loss moduli agree well with experimental data for poly(n‐hexyl isocyanate) solutions.

ISSN:0148-6055    

DOI:10.1122/1.550615

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The influence of temperature on the dc electrical properties (dielectric constant and conductivity) and electrorheology (complex moduli and static post yield flow stress) of corn starch/corn oil suspensions was determined in the range 5–70 °C. Both electrical properties increased with temperature. The complex moduliG′ andG″ determined with shear strain amplitude γ=0.08 and oscillation frequencyf=20 Hz decreased with temperature, whereas the quasistatic postyield stress τEfirst increased and then decreased. The conductivity of the particles, σp, and of the oil in the gap between the particles σf,gwas determined from the temperature dependence of the conductivity σ̄ of the suspensions. This gave σp≫σf,gfor the entire temperature range considered. The measured modulusG′ and the postyield flow stress τEwere compared with predictions of both a polarization model and a conductivity model for ER response. Although some accord existed for both models, better agreement occurred for the conductivity model. The ratio ofG″ divided by the viscosity of the suspension, η̄, increased with temperature, indicating that the decrease inG″ with temperature was less than that of the viscosity. This was attributed to the oscillation strain being beyond that for ideal viscoelasticity.

ISSN:0148-6055    

DOI:10.1122/1.550616

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

A model is presented for the electric current and attractive force between two slightly conducting spherical particles in a nonpolar liquid. The model includes the nonlinear conductivity of the liquid but does not require that the zone between the particles be divided into two distinct regions, nor is there a need to assume an equipotential condition at the particle surface. Further, the model gives the pertinent properties as a function of the separation of the particles. Numerical values are presented for the properties as a function of the conductivity ratio of the particles to the host oil (silicone oil), the electric field, and the separation of the particles. It is shown that good accord exists between the predicted yield stress and that measured on a suspension of zeolite particles in silicone oil. Some accord also occurs for the current density. Illustrated is the importance of the electrical conductivity parameterEcof the host oil on ER response. Empirical equations for the shear modulus and static yield stress are given in the Appendix.

ISSN:0148-6055    

DOI:10.1122/1.550617

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Electrorheological fluids can exhibit highly nonlinear behavior for large electric‐field strengths and strains. Oscillatory rheometry of these activated suspensions can, in some cases, result in the dynamics of the rheometer contributing to the measured shear stress within the ER material. Some experimental results are presented for an activated electrorheological fluid for a range of experimental conditions. These results show how the material behaves under an increasing electric field, for certain levels of strain and oscillatory frequencies. The results also show the effects of the inertia of the viscometer on the observed characteristic behavior of the material, and a method of removing this component from the data is discussed. A three element mechanical analogy of the ER material is presented, which together with the introduction of an inertial term models well the behavior seen in the experimental results. By taking into account some of the influences of the experimental apparatus, the proposed model is used to put forward some hypotheses for the characteristic physical response of an activated ER material.

ISSN:0148-6055    

DOI:10.1122/1.550618

出版商:The Society of Rheology    

年代:1995

数据来源: AIP