Magnetic field directed assembly of two-dimensional fractal colloidal aggregates

摘要

The anisotropy of dipolar interactions can sometimes be a hindrance when assembling colloids, as it limitsthe diversity of structures that can be manufactured. Here we demonstrate that a mixture of paramagneticand diamagnetic colloids in a ferrofluid can be used to create a variety of fractal aggregates in the presenceof a field. These aggregates exhibit growth both parallel and perpendicular to the field, a distinct departurefrom the linear chains that are typical of dipolar assembly. The fractal dimension of these aggregatesdisplays 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 shortrangeforces. This behavior is explained by examining how the ferrofluid concentration affects therelative strength of the dipolar interactions between each type of particle. These dipolar fractalaggregates may find use in the study of gelation via long-range forces or the preparation of gels thatcan be activated using an external field.