Characterization of the structure in magnetic ink

The structure in magnetic inks was investigated through rheology and magnetic property measurements. The rheological measurements showed gel-like behavior indicating formation of a network structure of magnetic particles. The network was modeled as a floc-floc structure. A study of scaling behavior of viscoelastic properties showed that the linear viscoelasticity was dominated by the internal structure of the floes while the limit of linearity was determined by the weakest floc-floc interactions. The floc is relatively soft but has strong bonds, while the interfloc link is weak and brittle.; In shear flow, the magnetic inks showed shear thinning and yielding behavior. The results were explained by the three types of structure changes—(1) breakage of interaction between flocs, (2) deformation of the flocs, and (3) rupture of the flocs. The floc size in shear flow was calculated using the floc-floc model. A decreasing size at high shear rates was attributed to floc rupture.; The temperature dependence of rheological properties of magnetic inks was examined. The linear viscoelastic and non-Newtonian behavior of magnetic inks in shear flow is independent of temperature because of dominant temperature-independent magnetic interactions between the particles.; Equating the steady shear viscosity to the amplitude of the complex viscosity (Cox-Merz rule) for magnetic dispersion was examined. This rule has been used to exploit the relative ease of linear viscoelastic measurements over steady shear measurements in the regime of high deformation rates. It was not applicable for magnetic inks.