ER suspensions

Figure 8.1 (a, b) Schematic illustration of the chain-forming effect of an electric field E on an ER suspension. (Reprinted with permission from Kerr, Copyright 1990, American Association for the Advancement of Science.)... 

It is also found that the liquid phase is crucial in obtaining a good ER suspension and intensive ER effect. The role of silicone oils and other nonconductive Uquids should be examined from both experimental and theoretical aspects. 

It is found that the shear stress and yield stress of the PAG suspension are higher than those of the pure PANI suspension at the equal electric field strength. Furthermore, under electric fields, the shear stress of the PAG suspension shows a decline as a function of shear rate to a minimum value after the appearance of yield stress. The widely accepted flow model for ER suspensions, i.e., the Bingham fluid model cannot fit well the flow curves of the PAG suspension, especially in the low shear rate region (see Figure 14.8a). However, the flow curves of the pure PANI suspension maintain a relatively stable level, which can be fitted by the Bingham fluid model (see Figure 14.8b). This different flow behavior reflects that the PAG sheets possess a different ER response from the pure PANI particles under the simultaneous effect of both electrical and mechanical fields. In addition. 

Once the electric field strength is increased from 1.0 to 1.5 kV/mm, two thicker columns build up with much thinner chains parallel to them. They become much closer if the electric field is further increased from 1.5 to 2.0 kV/mm, though there arc still small amounts of much thinner chains available. Formation of the mechanically strong chains in ER suspensions is believed to be responsible for the sharp increase of the rheological properties of positive ER suspensions. 

Figure 1 The rceordcd shear stress of a positive ER suspension (zcolite/silicone oil) against time. The numbers on each step indicate the applied electric field strength (kV/mm). Note that after the electric field is switched off, the shear stress doesn t fully recover to the original value. Reproduced with pennission from T. Hao, Adv. Mater., 13(2001 )1847...

Aluminosilicate materials were found to have a very strong ER effect under water-free condition [6,7]. In 1991, a crystalline alumino-silicatc (zeolite, Linde 3A) powder of molecular formula KgNa.i[(A102)i2(Si02)i2] was dispersed into paraffin oil and the maximum stress of such an ER suspension was found to reach more tlian 100 kPa at 2.0kV/mm [8]. The yield stress under such a condition reaches 42.6 kPa. The maximum stress against strain amplitude is shown in Figure 5. 

Surfactant has two roles for an ER suspension Improving the particle sedimentation property and enhancing the ER effect 56. A maximum yield stress is observed when the surfactant concentration varies from 0 to 7 wt% for alumina/silicone oil suspensions with different water content (see Figure 7). When the surfactant concentration is low, the yield stress increases with the concentration increase of the surfactant. This may result from the surfactant-enhanced particle polarization, in which proton transportation rate... 

Particle sedimentation is often a problem in ER fluids containing the solid particle. As mentioned above, additive and surfactant are frequently used for enhancing both the stability of the ER suspension and the ER effect linally. One way to resolve this problem is to make a polymer coated microballoon particle, matching the density between the particle and the carrier liquid and thus reducing the particle sedimentation. An example is the poly(vinyl alcohol) (PVA) coated silica microballoon dispersed in the mixture of heptane and toluene (63]. The shear stress against the electric field is shown in Figure 11 for such a system. In both particle concentrations, 10 wt% and 30 wt%, the coated samples show a much better sedimentation property and much stronger ER effect, also. However, it may be hard to solely attribute the enhanced ER performance to the improved sedimentation property, as the PVA may act as an additive to enhance the ER effect. 

Since particulate-type ER suspensions have particle sedimentation problem, liquid materials are therefore used as dispersed phases. The liquid may be dispersed (immiscible) or dissolved (miscible) into the insulating oil. Various liquids are reported to show a remarkable ER effect, particularly the liquid crystal materials. 

The yield stress and the apparent viscosity of an ER suspension are largely dependent on the particle volume fraction. A linear relationship between the yield stress and the particle volume fraction was derived on the basis of the fibrillation model [103] and compared with the experimental data of the hydrated poly(mclhacrylalc) particle in a chlorinated hydrocarbon suspension obtained by Marshall [104]. The theoretical prediction was only valid in high particle volume fractions, and failed in low particle volume fractions, as stated in this paper and shown in Figure 41. 

The electric-tield-induced phase transition in an ER suspension was found to be different from that in general colloidal suspensions. Tao and Martin [55, 56] predicted theoretically tliat the bet structure has an energy lower than that of the fee (face-centered cubic) and other structures, based on dipolar interaction energy calculations. The dipolar interaction energy per particle for various crystal structures is shown in I able 3. The bet crystal structure is shown in Figure 6. 

Figure 16 Weiglit fraction of various paths in an ER suspension calculated with Fxjs. [46] and [47] under the assumption that functionality /"is equal to 8 and the extent of reaction a is equal to the particle volume fraction. Reproduced with permission from T. Hao and Y. Xu, J. Colloid Interf. Sci., 181(1996)581...

Similar to conventional colloidal suspensions, ER suspensions also show the shear tliickening and shear thinning behaviors. Figure 23 shows the viscosity of the 3-(methacryloxy propyl)-trimethoxysilane coated... 

In addition to the shear stress, the storage modulus (G ) and loss modulus (G") are two important parameters for characterizing ER suspensions. Linear viscoelastic behavior of ER suspensions is always addressed before the frequency sweep experiment. Typically at small strain amplitude both G and G" show an independence of strain amplitude, and then decrease with the increase of strain amplitude. Figure 30 shows strain dependence of both the storage modulus (O ) and loss modulus (G") for 1 pm in diameter silica/PDMS suspension with the particle volume fraction 17.1 vol% at various electric fields. Without an electric field, the loss... 

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