Structure Of Electrorheological Fluids Under Electric Field And Shear Flow: Experiment And Simulation

Traditional ER fluids are composed of micro-sized dielectric particles and insulating oil, whose yield stress is too small to satisfy the need of the industrial applications. In 2003, Wen et. al. first found that the urea coated barium titanyl oxalate ER material has giant yield stress (130kPa@5kV/mm), and wide temperature coverage. Those nano-sized ER materials are called Giant ER materials. Comparing with Traditional ER fluids, the yield stresses of Giant ER fluids increase from several hundred Pa to several hundred kPa. The sedimentation, hot stability, and chemical stability of GER fluids are also improved as well. All above make the industrial application of ER fluids possible. So the GER material synthesis and mechanism researches become the most critical contents.Several kinds of GER materials such as urea coated Ba_0.8(Rb)_0.4TiO₂(C₂O₄)₂, mesoporous cerium-doped TiO₂, and organic modified TiO₂ have been found. However, how those materials can be GER materials is still unclear. Further more, the stress of those materials decreases dramatically under shear flow from several hundred kPa to several kPa. Many efforts have been tried to increase the shear stress. For example, improvement of the electrodes is helpful to increase the shear stress but not enough. Under shear flow, the structure of ER particles is very unstable and the ER material overflows out of the apparatus, which make the shear stress decrease dramatically. So, the research of the ER fluids structure under shear flow is a key point to understand and solve the shear stress decrease of GER materials.In this paper, first we present a sol-gel method to synthesize the high yield stress ER materials. Second, the structure of ER materials under electric field and shear flow is studied both by experiment and theory. The detail contents are as follows:First, we present a simple sol-gel method to synthesize a kind of ER material which has high yield stress. The average size of Triethanolamine modified titanium dioxide particles is about 300nm measured by N4Plus Submicron Particle Size Analyzer and SEM. The infrared spectra of the ER particles showed that the TiO₂ is really modified by triethanolamine. The molecular dipole moment of modified material is considered as a crucial parameter. I synthesize a series of organic materials modified nano-sized TiO₂ particles with different molecular dipole moments. It is found that triethanolamine (the molecular dipole moment of triethanolamine is 3.48) modified TiO₂ has good ER effect. Under 5 kV/mm external electric field, the yieldstress is 32.6 kPa, which is about 50 times higher than that of pure TiO2 and the leaking electric current density is lower than 16 μA/cm². The sedimentation rate is about 98% in 20 days. So triethanolamine modified titanium dioxide particles are a kind of good ER materials.Second, under both electric field and shear flow, the mean cluster size of ER particles is studied by a suit of CCD equipped electro-rheometer apparatus which can observe the structure from different perspectives. The dynamic structure of nano-sized ER particles is too difficult to be observed now. In this experiment, the ER particles are micro-sized sulphonated polystyrene particles. The particle diameter ranges from 75 to 90mm, the particle density is close to the oil, and the dielectric constant and conductivity is suitable, which determine the sulphonated polystyrene particles are appropriate materials to observe the structure evolution under shear flow. In experiment, it is found that the mean cluster size of ER particles is proportional to the -0.26 power of shear rate and 0.64 power of electric field, which is coincident to the theoretical prediction. The results prove that the method and apparatus for ER fluids dynamic structure studies set up in this work is reliable.Third, through observation of ER particle chain structure under electric field and shear field, it is found that there are very large tilt angle between the electric field and ER particle chains. Under some shear flow, the chain tilt angle is even larger than that of theoretical predication. In experiment, it is observed that the ER particles rotate even in the particle chain, which influences the surface charge distribution, direction and magnitude of dipole moment. However, the previous theories such as the point-dipole (PD) model, the multiple point-dipole (MPD) model and multiple dipole-induced-dipole (MID) model do not include influence of the rotation and also can not explain the large chain tilt angle. Considering above effect, a modified Maxwell-Wagner model is present. The modified model can explain the existence of the large chain tilt angle. Further more, the relation between the dielectric constant and shear stress also can be got by the modified model. If the high-frequency limit of the dielectric constant is close to that of low-frequency limit, the shear stress decrease will be reduced dramatically, which can be a criteria for ER material.Fourth, the modified Maxwell-Wagner model is adopted in computer simulation. The structure of ER fluids under both electric and shear flow is simulated with molecular dynamic method. The simulation result gets a stable lamellar structure which consistent with the experiment data. In experiment, the ER fluid is composed ofSrCO₃ particles and silicon oil. First those particles are random distribution in the silicon oil, then the external electric field applied the ER particles form chains, and finally under both electric and shear flow the ER particles form a lamellar structure. Base on the modified Maxwell-Wagner model, the structure of ER fluids under both electric field and shear flow and the shear stress evolution are simulated by molecular dynamic method. During the process of simulation, the interaction between particles, Brownian force, the Stocks drag force from liquid, interaction between particles and walls, and the rotation effect to particles and images are included. The simulation results show that ER fluids form a lamellar structure under both electric field and shear field. The structure of a layer is discussed with the structure parameters. The results show that chains are the main structure instead of bct or fcc. Through simulation, we get the same results with experiment data which proves the modified Maxwell-Wagner model is a good model to understand the mechanism of ER fluids under both electric and shear flow.In Summery, It presents a method to get nano-sized ER materials with high yield stress. Considering the changes of ER particle dielectric and conductivity properties under shear flow, a modified Maxwell-Wagner model is present to understand the mechanism of ER fluids under both electric and shear flow. Further more, through the modified Maxwell-Wagner model; it is found that if ER particles have high frequency limited dielectric constant, the influence of shear flow will decrease dramatically.