Electrostatic motion of polymer particles in silicon fluid

In an electrorheological fluid (ER fluid), a suspending small particle plays an important role. As the requirement for practical ER fluid is the large difference of apparent viscosities with and without applied electric field, strong particle chain formation is desired. From previous experimental work, the shape of suspending particle produces a predominant effect on the ER effect. Elongated particles form complex particle chains with which ER effect becomes much increased. Since it is extremely difficult to investigate the condition of chain formation with actual particles experimentally, rather large particles of mm order were used. Acrylic acid resin (PMMA) or poly-propylene particles of 3 mm long and oval cross section were put into silicone oil where parallel electrodes are placed. In order to increase the surface conductivity of a particle, an anti-static agent is coated on the particle. From the up-and-down motion of the single particle between the electrodes, charging characteristics and the velocity of the particle were measured with an oscilloscope and a high-speed video camera. When the particle is in motion, the current consists of the induced current by particle motion and the charging current during the particle contact with an electrode. Comparing the motion of the particle to the current waveform, it was found that the charging time changes drastically with surface conductivity of a particle. When two particles are placed in the system, two modes of motion were observed: one is attraction and the other is repulsion. Since the cause of two modes is due to the net charge attained by particles, the sign of charge has significant meaning. Two particles of rather low surface conductivity are sometimes joined together and move up-to-down simultaneously. For higher surface conductivity, the particles moves actively and the probability to form the particle chain increases