DIFFUSE OPTICAL PROBES OF PARTICLE MOTION AND STRUCTURE FORMATION IN AN ELECTRORHEOLOGICAL FLUID

While severe multiple scattering precludes direct observation of the spatial arrangement of the particles suspended in most electrorheological (ER) fluids, both diffuse optical transmittance and diffusing-wave spectroscopy are useful noninvasive probes of particle motion in these materials. The temporal variation of the diffuse transmittance of a commercial ER fluid provides a measure of the time scale for field-induced structure formation. The response times observed with low static fields and with alternating fields are consistent with the expected induced-dipole attraction between particles. However, the field dependence of the response times measured with high static fields suggests that monopole forces play a role in the aggregation process in this fluid. The relevance of particle charge is confirmed by a novel application of diffusing-wave spectroscopy: detection of the oscillatory electrophoretic motion of the particles induced by alternating electric fields. Possible consequences of particle charge for the behavior and performance of electrorheological fluids are discussed.