Magnetic field directed assembly of two-dimensional fractal colloidal aggregates

Abstract

The anisotropy of dipolar interactions can sometimes be a hindrance when assembling colloids, as it limits the diversity of structures that can be manufactured. Here we demonstrate that a mixture of paramagnetic and diamagnetic colloids in a ferrofluid can be used to create a variety of fractal aggregates in the presence of a field. These aggregates exhibit growth both parallel and perpendicular to the field, a distinct departure from the linear chains that are typical of dipolar assembly. The fractal dimension of these aggregates displays a parabolic character as the ferrofluid concentration is increased and varies between 0.94 ± 0.03 and 1.54 ± 0.03—a wider range than that which is seen when colloids are assembled using short-range forces. This behavior is explained by examining how the ferrofluid concentration affects the relative strength of the dipolar interactions between each type of particle. These dipolar fractal aggregates may find use in the study of gelation via long-range forces or the preparation of gels that can be activated using an external field.