Colloidal dispersibility of fatty acid-capped iron nanoparticles and its effect on static and dynamic magnetorheological response

Herein we report colloidal dispersibility of concentrated suspensions containing fatty acid-capped iron nanoparticles and its influence on the static and dynamic magnetorheological (MR) response. The amount of fatty acid absorbed on the surface controls the non-field dispersion state as well as weakly and strongly flocculated structure of the nanoparticles. Both yield stress at B = 0.5 T of an applied magnetic field is ca. 14–16 kPa Because the flocculated structure has a negligible effect on the field-dependence yield stress, the non-field dispersion state mainly determines the turn-up ratio; the ratio of the yield stress at B = 0.5 T to shear stress (at 1 s−1) at B = 0. The dynamic measurements indicate that the two dispersion states are different in the field-induced aggregation structures. A viscoelastic linear region appears for a weakly flocculated suspension, and does not for a strongly flocculated suspension. Additionally, the colloidal stability for the two dispersion states is discussed in terms of pair potential energy.