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 shortrange
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
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