Properties of ER Fluid under AC field Qiangguo Tang , Cong Li ,Lianxi Cheng, Jiping Huang and Luwei Zhou Phys. Dept., Fudan Univ., Shanghai, People’s Republic of China The character of ER fluid under AC field are studied. First , we investigate a new kind of material containing rare earth . Second , we research the lamellar structure of PMER fluid under AC field. Lastly, a theory model is established to understand the shear stress of PMER under ER fluid, some of the result is coordinate to the experiment of professor Lu. II. Effect of Rare Earth on ER Fluid I. Background Electrorheological fluid changes from normal fluid to solid-like states while applied an electric field on. This property is useful to industry application such as actuators, clutches, rehabilitation and so on. It was first discovered by Winslow at 1949. During 1980s the anhydrous ER fluid was invented.[1、2] It overcomes the unstable problem caused by water. In 2003, Sheng Ping made the giant electrorhological fluid with a core of urea molecule. Its shear stress reaches hundreds of KPa. [3]Although there are many great progress at ER fluid area, the mechanism is not sure. Character of ER fluid under AC field gives a perfect way understand ER field. III. Simulation Result of PMER under AC Field IV. Lamellar Structure under AC Field model       The interaction force decrease with Increase the frequency of external field, When the frequency is high, the PMER changes back to the ordinary ER fluid. V. REFERENCES [1] Block, H.; Kelly, J. P. Prog. IEE Colloq. 1985, 14, 1. [2] Filisko, F. E.; Radzilowski, L. H. J. Rheol. 1990, 34 (4), 539. [3] WEIJIA WEN， PING SHENG， KUNQUAN LU； The giant electrorheological effect in suspensions of nanoparticles； nature materials，2003