FERROFLUID VISCOMETER - TRANSIENT MAGNETIC BIREFRINGENCE IN CROSSED FIELDS

A ferrofluid viscometer turns the viscosity determination into an optical birefringence measurement. Some colloidal magnetic particles are diluted in the studied medium. In a constant field, these ferrofluid particles behave as magnetic dipoles and experience a couple which tends to align these along the field: as each particle is optically anisotropic, the medium becomes birefringent. If the field is switched off, birefringence relaxes to zero with a characteristic time proportional to solution viscosity and to the particle volume. New measurements in crossed fields (static H(c) and pulsed H(p)) are here presented. Various relaxation regimes are explored depending on respective hierarchies of the Brownian energy kT, anisotropy energy of particles KV and magnetic energy mu H(c). In these regimes the relaxation time becomes independent of particle size.