Electrorheological (ER) fluid is a type of smart fluid whose shear yield stress relies on the exteal electrical field strength.The transition of ER fluid microstructure driven by the electrical field is the reason why viscosity changes.Experimentally,the transparent electrodes are used to investigate the column size distribution where an exteal electric field is applied to a colloidal suspension,i.e.,ER fluid is increased.The coarsening profile of ER suspensions is strongly related to electrical field strength,but it is insensitive to particle size.In addition,in a low field range the shear stress corresponding to the mean column diameter is studied and they are found to satisfy a power law.However,this dependence is invalid when the field strength surpasses a threshold value.