We studied particle size, shape, molecular weight and temperature dependencies of ER effect of poly(ethylene glycol) (PEG) suspensions. The particles were dispersed in low-viscosity silicon oil. We obtained flow curves and dynamic yield stresses by increasing shear rate and compared these with static yield stresses in our previous study (obtained by increasing shear stress). The yield stress under a dc electric field increased by decreasing molecular weight and increasing particle concentration, and did not change with particle size or shape. In addition, significant differences between static and dynamic yield stresses disappeared in absolute value and electric field intensity dependence. And the static yield stress increased with temperature and the dynamic yield stress showed a maximum value at near room temperature. We also measured transient behavior of a PEG suspension at various temperatures and found that it took about 30 seconds to reach the constant viscosity on the application or removal of electric fields. It can be thought that this relatively long response time to create chain structures causes sudden flowing by increasing shear stress gradually and different results between static and dynamic yield stresses.
The gel type metallic gallium fluid is prepared by dispersing silica coated 30 to 50 nm diameter of FeNbVB particles (3 mass %) into liquid gallium. The rheological property by cone-plate and cylindrical type viscometers and the movement of fluid under magnetic field are investigated. The small increase of shear stress is observed at a low of shear rate under 0.09 T of magnetic flux density. The shear stress decreases at increase of shear rare. This phenomenon is caused by the slip and the space between metallic gallium and stainless steel plate of viscometer. The gallium fluid can be moved against the gravity force under magnetic field that would be applied for electric switch.
The flow curves of an electrorheological fluid, made from a suspension of corn starch particles, were examined under repeated applications of shear flow and dc electric field of magnitude of 2kV/mm. It was observed that a sample subjected to this repeated testing would show a considerable increase in field-induced shear stress after several repetitions. For example, at a shear rate of 25s-1, the shear stress increased after 10 repetitions to 690Pa from an initial value of 370Pa. In addition, after many repetitions there was an increased tendency towards “flow curves” where the shear stress decreased with shear rate. This behaviour was thought to arise from microstructural changes in the material induced by the repeated shearing and activation by electric field.
We investigated changes in surface profile of a dielectric liquid film put on a periodically aligned electrode array by imposing a voltage between adjacent electrodes. It is found that the surface of liquid film can be undulated by the electric field according to the periodicity of electrode array, and it rises on between adjacent positive and negative electrodes, on the other hand, it goes down at the central region on each electrode. The averaged height difference of undulated surface is proportional to the square of the interface electric field. It is also found that the averaged height difference increases with the width L of electrode for L < 1600μm, decreases with L for L > 1600μm, if the distance between adjacent electrodes is maintained constant. The averaged height difference does not change if the strength of interface electric field is adjusted to be the same for various thicknesses of liquid films. The time evolution of the height difference is also investigated using samples with various kinetic viscosities. It is found that the needed time t1/2 for the surface height difference to reach the half of the equilibrium one is proportional to the kinetic viscosity of the sample.
ER effects were studied in a suspension composed of starch particles and 5CB (4-n-pentyl-4'-cyanobiphenyl) having liquid crystalline nematic phase. To investigate the effects of dispersant 5CB, ER effects of starch/silicone oil and starch/5CB suspensions were explored, finding that the ER effect in starch/5CB suspension is about 3.5 times larger than that of starch/silicone oil suspension. It was also clarified that the magnitude of the ER effect is hardly affected by the difference in the phase, nematic or isotropic phase, of the dispersant 5CB, suggesting that the enhancement of the ER effect is not due to the orientational order of the liquid crystal but to the larger value of the dielectric constant of 5CB. In addition, dielectric properties were measured to make clear the relationships between the relaxation time of the polarization and the magnitude of the ER effect in these suspensions.
The flow behavior of nano-suspensions with particle volume fractions of φ = 2.4 and 4.4 vol% was investigated under no electric fields using a rotational rheometer with fixtures of parallel plates (or electrodes), the gap distances of which were narrower than several hundred micrometers. For the sample of φ = 4.4 vol%, the apparent yield stress was observed with gap distances of 50μm and 300μm. On the other hand, the flow behavior was strongly dependent on the gap distance for the sample of 2.4 vol%. These results were discussed in terms of the size of the microstructure and the gap distance between the parallel plates of the rheometer.
Passive-type force display systems have the merit of high safty, but there are some directions and link positions for which it is difficult to display the force by using only passive elements. To solve this problem, a method for improving the controllability by using redundant brakes had been proposed. This method, however, requires brakes with short response time; ER fluid brakes are suited for this purpose. In this paper, we develop a novel passive-type force display system with redundant brakes. We have developed new multi-cylinder ER fluid brakes which can be operated at relatively low voltage. For transmitting the force, a new linkage system is designed which does not require any belt-pully system. It has been demonstrated that this system can display forces in a wide direction by using our controlling algorithm. A potential application of this system to rehabilitation training is proposed.