8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P9.42Tue 611:23-14:00Anisotropic microrheological properties of chain-formingmagnetic fluidAlenka Mertelj, 1 Andraz Resetic, 1 Saso Gyergyek, 1 Darko Makovec, 1 and MartinCopic 11 J. Stefan Institute, Jamova cesta 39, POB 3000 Si-1001, Ljubljana, SloveniaIn an external magnetic field, magnetic colloids transform from an isotropic fluid to an anisotropicviscoelastic material. Using passive microrheology we measured microrheological properties ofmagnetic fluid as a function of direction and magnitude of external magnetic field. The effectivemicroviscosity strongly depends on the magnitude of external field, while it is almost independentof its direction. The measured effective storage modulus varies significantly within the sample anddepends both on the direction and the magnitude of external magnetic field. It vanishes in zerofield, while in nonzero field it is by a factor of 2 to 4 larger in the direction along the field thanperpendicular to it. The non-zero value of the storage modulus, which indicates the formation ofa viscoelastic fluid, appears at magnetic fields at which the dynamic light scattering experimentsreveal the formation of elongated structures[1]. The results of our study support the idea presentedin [2], where the hydrodynamic theory for polydispersed ferrofluid with two timescales of relaxationof the magnetization are introduced. The fast relaxation corresponds to the small particlesthat do not form chains in external field, while the slow relaxation accounts for the chains. Atshort times the linear response of the system is viscoelastic with field dependent viscosity that isisotropic and can be attributed to small particles and with highly anisotropic shear storage modulusthat originates in the formation of the chains. The long time behavior of the system is viscous,with the effective viscosity in the direction along the field larger than in the perpendicular direction.[1] A. Mertelj, A. Resetic, S. Gyergyek, D. Makovec, and M. Copic, Soft Matter 7, 125(2011).[2] S. Mahle, P. Ilg, and M. Liu, Phys. Rev. E 77, 016305 (2008).42