Non-equilibrium Structures Of Giant Electrorheological Fluids

The traditional dielectric electrorheological fluids (ER fluids) are suspensions composed of dielectric particles and the oil. Their shear stress or hardness can be continuously and rapidly controled with the external electric field. This strange property makes ER fluids have extensive and important application value. However, according the dielectric theory, the maximum yield stress is only about10kPa. This low value offen can’t meet the needs of practical application.Recently Wen Weijia and Sheng Ping first discovered giant electrorheological fluids (GER fluids). These particels have a urea coating, and size is about50nm. The yield stress is about130kPa, which make the GER fluids have more extentive application in large temperature range. Furthermore, a prominent feature of GER fluids is the near-linear dependence of the yield stress on electric field. Not only the upper bound of yield stress of GER fluids is about ten times larger than ER fluids, but also they have good resistance to retrogradation, and have thermal and chemical stability. All these speed up the practiclal application in industrial of GER fluids.So it’s quite important to study the mechanism and properties of the GER fluids. The GER fluids in work are often sheared, such as absorber, clutch, breaker and valve. So we should focus on the enhancement of shear stress instead of yield stress. At high shear rate, the viscosity will decrease, which is shear-thinng phenomenon. This phenomenon limits the widely application of GER fluids.Macroscopic physical properties of giant electrorheological (GER) fluids are crucially dependent on their internal particle microstructure. So the internal structure investigation is more important. Here, an experimental method has been developed to observe the lamellar microstructures of GER fluid, which fills two relatively rotating parallel disc electrodes (disc-disc geometry). Nevertheless, only microstructures in surface regions can be observed simultaneously via some experimental approaches in shearing process. To observe internal microstructures under shear in the whole region simultaneously, we further incorporate two phase dipole fluid model to GER fluids in shearing process. The internal microstructures in shearing GER fluids confined in the disc-disc geometry are studied by using experimental and theoretical methods. We find that there exist abundant internal lamellar microstructures whose patterns are caused by the variation of radial shear bands; and that there exist vertical shear bands which give rise to delamination perpendicular to the flow direction.