Diffuse optical probes of particle motion and structure formation in an electrorheological fluid

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Abstract
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.