The visualization study on the microstructure of the Electro-Rheological (ER) fluid was carried out. The dense particle motion was difficult to visualize because of the severe light scattering by the particle itself. In this study, the laser light sheet was illuminated to visualize the dense particle condition, 10 wt%. As the ER fluid, the cholesterol particles (similar to 20 mm) were dispersed into the silicone oil (10 cS). The humidity of particle was controlled to be 4%. The particle density was set to be 10 wt%. Under this condition, individual particle could not be distinguished. However, the particle cluster motion could be clearly visualized. With the electrical field, the chains of the particles were formed. The ER fluid with the chain was recorded onto the video camera. With higher electrical field, the layer of the particle was formed at the top and bottom electrode. The layer caused the decrease of the fluid channel width, resulting in the resistance to be increased. The effects of electrical field on the layer formation were experimentally investigated. The Mason number, which is the ratio of shear force to electrical force, was found to be the key parameter of the layer formation.
