Jile Jiang

Structure parameter of electrorheological fluids in shear flow.

A structure parameter, Sn = η(c)γ/τ(E), is proposed to represent the increase of effective viscosity due to the introduction of particles into a viscous liquid and to analyze the shear behavior of electrorheological (ER) fluids. Sn can divide the shear curves of ER fluids, τ/E(2) versus Sn, into three regimes, with two critical values Sn(c) of about 10(-4) and 10(-2), respectively. The two critical Sn(c) are applicable to ER fluids with different particle volume fractions φ in a wide range of shear rate γ and electric field E. When Sn < 10(-4), the shear behavior of ER fluids is mainly dominated by E and by shear rate when Sn > 10(-2). The electric current of ER fluids under E varied with shear stress in the same or the opposite trend in different shear rate ranges. Sn(c) also separates the conductivity variation of ER fluids into three regimes, corresponding to different structure evolutions. The change of Sn with particle volume fraction and E has also been discussed. The shear thickening in ER fluids can be characterized by Sn(c)(L) and Sn(c)(H) with a critical value about 10(-6). As an analogy to friction, the correspondence between τ/E(2) and friction coefficient, Sn and bearing numbers, as well as the similarity between the shear curve of ER fluids and the Stribeck curve of friction, indicate a possible friction origin in ER effect.

A shear thickening phenomenon in magnetic field controlled-dipolar suspensions

A shear thickening phenomenon in dipolar suspensions of magnetorheological (MR) fluid is reported. The stress of the MR fluid abruptly decreases when the applied magnetic field increases to above a critical value under a small constant shear rate. It abruptly increases when the shear rate is higher than a critical value under a constant magnetic field, accompanied by a change in normal stress during shear thickening or unshear thickening processes. A shear-thickened structure is important for an MR fluid to obtain a high yield stress, which is beyond the prediction of a traditional dipole or multipole interaction model.

Reversible shear thickening at low shear rates of electrorheological fluids under electric fields

By shearing electrorheological (ER) fluids between two concentric cylinders, we show a reversible shear thickening of ER fluids above a low critical shear rate (<1 s(-1)) and a high critical electric field strength (>100 V/mm), which can be characterized by a critical apparent viscosity. Shear thickening and electrostatic particle interaction-induced interparticle friction forces are considered to play an important role in the origin of lateral shear resistance of ER fluids, while the applied electric field controls the extent of shear thickening. The electric-field-controlled reversible shear thickening has implications for high-performance electrorheological-magnetorheological fluid design, clutch fluids with high friction forces triggered by applying a local electric field, other field-responsive materials, and intelligent systems.

An experimental study on the normal stress of magnetorheological fluids

The dependence of the normal stress on the shear rate and magnetic field strength in the shear flow of magnetorheological (MR) fluids has been studied experimentally. An obvious normal stress could be observed when the applied magnetic field was higher than a critical value. The normal stress increases considerably with increase of the shear rate and magnetic field, and decreases suddenly and significantly upon the onset of shear thickening in MR fluids. The ratio of shear stress to normal stress, an analogue of the friction coefficient, increases with increase of the shear rate, but decreases with increase of the applied magnetic field. Along with the shear stress, the normal stress in MR fluids could provide a more comprehensive understanding of the MR effect, and the evolution of the particle structure in shear flow, and may have important implications for preparing high performance magnetostrictive elastomers with high force output along the magnetic field direction.

Shear thinning and shear thickening characteristics in electrorheological fluids

The electrorheology (ER) of suspensions based on polystyrene/polyaniline (PS/PANI) core/shell structured microspheres and those based on disk-like zeolite particles at different electric fields and particle volume fractions have been studied, respectively. Both types of ER fluids showed abrupt shear thickening under high electric fields and low shear rates, as well as shear thinning when the shear rate increased. A normalized method that considers the effects of electric field strength, shear rate and particle volume fraction was proposed to compare the rheological curves of the two ER fluids. The curves evaluated from the normalization method showed similar shear thinning at low shear rates and the hydrodynamic effect at high shear rates. Shear thinning represents the structure destroyed by shearing, and shear thickening at low shear regions indicates the dramatic structure change. The particle volume fraction and structure factor effects demonstrate that the mechanical contact between particles and the wall of the electrodes is crucial to the shear strength of ER fluids, indicating an electric/magnetic field modulated friction mechanism of the ER and magnetorheological (MR) effects.

Structure factor of electrorheological fluids in compressive flow

This paper examines the chain structure factor evolution of electrorheological (ER) fluids in compressive flow. The yield strength of ER fluids was modeled based on a single pair electrostatic interaction between particles and the structure factor, which includes all the effects except the single pair electrostatic interaction between particles presented by the local electric field strength between particles. Both the mechanical and electrical properties of ER fluids in compressive flow have been experimentally determined. The nominal shear yield stress of the ER fluid in compressive flow was derived by assuming that it was a transformed shear flow of a Bingham fluid. The single pair particle interaction strength is related to the measured electric current, which reflects the local electric field strength between particles. The structure factor evolution in compressive flow was derived by comparing the nominal shear yield stress and the single pair particle interaction strength. As expected, the calculated structure factor increased significantly using this method, much higher than that described by the many-body effect and the difference of dipole–dipole interaction and multi-dipole interaction between particles. Direct mechanical contacts and frictional forces between particles are thought to contribute significantly to the high structure factor and nominal shear yield stress of the ER fluid in compressive flow. This behavior might be similar in magnetorheological (MR) fluids.

The transient behavior of electrorheological fluid in tensile flow

Transient behaviors of (ER) fluids in tensile flow and applied stepwise voltages were experimentally studied. The transient tensile stress rises exponentially with time. The characteristic rising time of tensile stress is independent of the amplitude of the applied voltage and the tensile velocity, while the amplitude of tensile yield stress is significantly affected by the two factors. The transient tension applied as a stepwise voltage is different from a stable tension pre-applied at constant voltage in different particle chain structure forming processes. Because of the chain aggregation during an intermittent voltage on?off test, the achieved tensile yield stress showed an exponent of 2.75 to the applied electric field at low separation velocities (0.2? mm?s?1), higher than the square relationship predicted by traditional polarization models, and the exponent of 1.5 predicted by the conduction model. The results achieved in this study show that the mechanical properties of ER fluids are greatly affected by the method of applying the electric field, the strain rate, and the gap geometry between electrodes. These factors should be properly considered in the design and control of ER actuators.

Transient response of sheared magnetic powder excited by a stepwise magnetic field and its comparison with ER and MR fluids

The transient shear behavior of magnetic powder (MP) excited by a stepwise magnetic field in a rotational magnetic powder clutch (MPC) was experimentally studied. The experiment showed that the stable shear stress was approximately proportional to the applied magnetic flux density. The characteristic rising time of the shear stress was independent of the strength of the magnetic field and was affected by the shear rate. It took less than 0.1 s for the shear stress to rise to 63%.1 e 1 / of the stable value. The transient shear stress rising process consisted of two subprocesses: the chain forming process which was less than 100 ms, and the chain coarsening process lasting for dozens of seconds. Upon switching the field off, the shear stress fell rapidly to zero in 0.1 s. Control methods to improve the transient response time of the MPC were discussed and experimentally verified: applying a low voltage in advance; applying a high level voltage for the torque to rise to the target torque and then a desired voltage. These transient characteristics of MP were compared with those of electrorheological (ER) and magneto-rheological (MR) fluids and actuators. The study provides a better understanding of MP excited by a magnetic field and the implications for application. (Some figures may appear in colour only in the online journal)