TED-AJ03-574 NEGATIVE VISCOSITY INDUCED IN A MAGNETIC FLUID

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Solutions, in which ferromagnetic colloidal particles are dispersed, are called magnetic fluids. The diameter of each particle is of order of 10 nm and each particle is composed of a single magnetic domain, the magnitude of the magnetic dipole moment of which is constant. The direction of the dipole moment changes with the rotational motion of the particle. Since the motion of each particle is affected by interactions with the molecules of the solvent, other ferromagnetic particles, macroscopic flow in the solvent and external magnetic fields, magnetic fluids show various interesting rheological and magnetic characteristics under different conditions. In this paper, we investigate the rheological and magnetic characteristics of a magnetic fluid, which is subjected to both ac magnetic and shear flow fields, by a nonequilibrium Brownian dynamics method. We analyze the effect of the amplitude and frequency of an external magnetic field and the shear rate on the rheological and magnetic features. We find that the negative viscosity effect is induced at a certain frequency range of the ac magnetic field. We also find that there are two main mechanisms responsible for the occurrence of negative viscosity; that is, (1) Resonance between the rotational motions of the dipoles of particles and the fluctuation of ac magnetic fields occurs when applied magnetic fields are weak compared to the shear rate, in which case particles can still rotate in magnetic fields. Beyond this resonance frequency, negative viscosity appears; (2) The magnetic dipole moments of particles are forced to stay in the direction of the magnetic field when strong magnetic fields are applied in relatively low shear flow fields. However, negative viscosity occurs when the frequency of external magnetic fields exceeds a critical value, in which case the dipoles rotate continuously in a shear flow without stopping. In both cases, the mean angular velocity of the particles becomes higher than that of the solvent. We also find that the apparent viscosity is affected by the change of mean effective forces between the ferromagnetic particles caused by the change of the particles' rotational motions.
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TED-AJ03-574 NEGATIVE VISCOSITY INDUCED IN A MAGNETIC FLUID

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