Chain/column evolution and corresponding electrorheological effect

We present an investigation about chain/column evolution and the correspond ing electrorheological (ER) effect performed with glass/oil ER fluid. Our r esults demonstrate that once the field applied to the ER fluids surpasses a certain time period, the particles begin aggregating to form chains. These chains then coarsen and eventually form columns in the direction of the ex ternal field. We found that different column structures can be obtained dep ending on how the electric field is applied to the ER fluid. Only a loose c olumn structure can be achieved if a square pulse field is applied to the E R fluid, yet a compact column is formed when the field strength is increase d slowly. We have measured the ER effect with a sensitive yield stress test ing device as the structure varies. The results indicate that there exist t hree increasing tendencies of interaction among particles corresponding to three processes of sequential transition between states; they are (1) rando m spatial configuration to chain, (2) chain to metastable column, and (3) m etastable column to stable column. (C) 1999 American Institute of Physics. [S0021-8979(99)07201-1].