Positive ER effect

The positive ER effect—occurs when the rheological properties of a fluid increase with applied electric field ... 

A few cycles of light on and off are shown in Figure 20 and a constant positive ER effect is observed- Note that the photo-induced ER effect is relatively small in comparison with the effect induced by an external electric field. 

An unique feature of ER suspensions is that mechanical strength can be continuously and reversibly adjusted from the liquid to solid stale by simply adjusting the applied electric field strength. The ER suspension is therefore a perfect electro-mechanical interface, stimulating a great deal of interest in a wide range of industrial areas. Since the positive ER effect was reported almost 60 years ago, ER devices have been designed mainly on the basis of the positive ER effect. The applications of the positive ER effect are thus focused upon in this chapter. 

A mixed liquid crystal fliat was developed for a strong dielectric liquid crystal display is an example of a low molecular weight liquid crystals [48]. This system exhibits a negative ER effect from direct current to alternating current of 10 Hz. Above this frequency, however, it shows a positive ER effect (see Fig. 10). From this result, the origin of the negative ER effect is seen as movement of the domain with long relaxation times. 

An example of polymer solutions is P(VDF/TrFE)/DMF solution [49] (Fig. 11). Poly(vinylidene-co-trifluoroethylene) (P(VDF/TrFE)) is known as a strong dielectric polymer. It possesses a permanent dipole moment in the main chain. The electric field frequency dependence of the relative viscosity was measured by a capillary rheometer. The result showed a positive ER effect at 10 Hz whereas it showed a negative ER effect at 1 kHz. 

Second-order rate coefficients for reaction (21) (X = I and OAc) were also reported by Abraham and Behbahany30 and are given in Table 20. Kinetic salt effects of added tetra-n-butylammonium perchlorate were studied for reaction (21) (X = I and OAc) both with solvent methanol and solvent tert.-butanol. Reaction (21) (X = 1) was accelerated in both solvents to about the same extent as was reaction (21) (X = Cl), and mechanism SE2(open) was therefore suggested. The reaction of tetraethyltin with mercuric acetate was subject to very large positive salt effects in methanol, perhaps due to anion exchange, but was unaffected by the electrolyte in solvent /er/.-butanol. Abraham and Behbahany30 considered that it was not possible to deduce the mechanism of the acetate reaction and that further work was necessary to decide between mechanism SE2(open) and mechanism SE2(cyclic). 

The photo-electrorheological (PER) effect—both the positive and negative ER effect can be enhanced by UV illumination in some ER systems. 

Work is still going on as to what the time domain response of the ER effect really depends on [105] or what is its meaning in terms of the fluid design. The initial and true ER effect in a valve, for example, is not the main concern it is also not the true time of the full pressure rise, the valve geometry, flow rate and other factors being involved. In the shear mode the position is similar yet the torque/input voltage response limit in response to small sine waves has been claimed to be as high as 1000 Hz. 

For a two-liquid mixture system, one liquid may form droplets, dispersed into another continuous liquid phase. Under an electric field, the liquid droplet may behave like a solid particulate, polarized and aligned along the direction of the electric field. A liquid-droplet bridge may form and connect two electrodes. Therefore, the viscosity of the whole system may be detennined by the viscosity of the fibrillated liquid droplet. If the viscosity of this liquid droplet phase is lower than that of the continuous phase, a negative ER effect is thus expected. Which liquid would form the droplet is dependent on the relative concentration of those two liquids. This is the reason that the viscosity and the concentration of two liquids are important for the system to show a positive or a negative ER effect. 

When the dispersed particle is photoactive, the ER effect could be enhanced by the UV illumination no matter whether the ER effect is positive or negative. This phenomenon is termed the photoelectrorheological (photo-BR) effect. In a US patent published in 1971 [27], photosensitive... 

Figure 19 The illustration of photo-induced positive and negative ER effect plotted as the shear stress vs. time. The number on each step is the electric field, kV/mm. Reproduced with permission from T. Hao, Adv. Mater., 13(2001)1847.

The ER effect is greatly enhanced if the particulate material is mixed with a liquid that is also ER activc.[134,135]. Polyhexyl isocyanate (PHIC) solution is a positive ER active material [136], and the viscosity of PHIC/p-xylene solution of concentration 23.3 vol% vs. the electric field is shown In Figure 63. When polymer resin particle dispersed in PHIC solution, the HR effect is greatly enhanced as shown in Figure 64. A similar phenomenon is also observed in the suspension of zeolite 3A particle dispersed in PHIC system. The enhancement may be related to the positive ER behavior of PHIC solution,... 

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