Abstract
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 μ H c. In these regimes the relaxation time becomes independent of particle size.
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