FIELD-INDUCED STRUCTURAL ORDERING IN ELECTRORHEOLOGICAL FLUIDS

Structural relaxations of an electrorheological fluid (ERF) due to cha nges in the applied electrical field strength or shear rate are observ ed on time scales 1 s < t < 40 000 s. Commercial ERFs consisting of me soscopic polyurethane particles in a silicone oil matrix were studied by three different experimental techniques in order to obtain and comp are the characteristic relaxation times. It is demonstrated that diele ctric spectroscopy, viscosimetry and light transmission experiments re present the same results concerning the structural relaxation phenomen a of ERFs when electrical fields are applied. The tendency of strong i nduced dipoles to align the particles in the direction of the field in creases the effective dipole moment and therefore Delta epsilon, the s hear viscosity eta and the amount of light transmitted along the field direction in an ITO/glass sandwich cell. The optical experiment is ca pable of resolving fast processes within the first 1 ms if large elect rical fields are applied. The effects of electrophoresis and shearing, which both counteract the field induced structures, are also addresse d.