FERROFLUID VISCOMETER - TRANSIENT MAGNETIC BIREFRINGENCE IN CROSSED FIELDS

A ferrofluid viscometer turns the viscosity determination into an opti cal birefringence measurement. Some colloidal magnetic particles are d iluted in the studied medium. In a constant field, these ferrofluid pa rticles behave as magnetic dipoles and experience a couple which tends to align these along the field: as each particle is optically anisotr opic, the medium becomes birefringent. If the field is switched off, b irefringence relaxes to zero with a characteristic time proportional t o solution viscosity and to the particle volume. New measurements in c rossed fields (static H(c) and pulsed H(p)) are here presented. Variou s relaxation regimes are explored depending on respective hierarchies of the Brownian energy kT, anisotropy energy of particles KV and magne tic energy mu H(c). In these regimes the relaxation time becomes indep endent of particle size.