Field-dependent anisotropic microrheological and microstructural properties of dilute ferrofluids

. We have measured microrheological and microstructural properties of a superparamagnetic ferrofluid made of Mn 0.75 Zn 0.25 Fe 2 O 4 (MZF) nanoparticles, using passive microrheology in a home-built inverted microscope. Thermal motion of a probe microsphere was measured for different values of an applied external magnetic field and analysed. The analysis shows anisotropy in magneto-viscous effect. Additional microrheological properties, such as storage modulus and loss modulus and their transition are also seen. We have also obtained microstructural properties such as elongational flow coefficient , relaxation time constant , coefficient of dissipative magnetization , etc., using the analysis given in Oliver Muller et al. , J. Phys.: Condens. Matter 18 , S2623, (2006) and Stefan Mahle et al. , Phys. Rev. E 77 , 016305 (2008) over our measured viscosity data. Our values for the above parameters are in agreement with earlier theoretical calculations and macro-rheological experimental measurements. These theoretical calculations consider an ideal situation of zero-shear limit, which is best approximated only in the passive microrheology technique described here and a first time measurement of all these parameters with passive microrheology. Graphical abstract