Soft Polymer Magnetic Nanocomposites: Microstructure Patterning by Magnetophoretic Transport and Self-Assembly.

A method to produce and pattern magnetic microstructure in a soft-polymer matrix is demonstrated. An externally applied magnetic field is used to influence the dynamics of magnetophoretic transport and dipolar self-assembly of magnetic nanoparticle clusters in the liquid precursor of poly-dimethylsiloxane (PDMS). Magnetic nanoparticles agglomerate by an interplay of van der Waals forces and dipolar interactions to form anisotropic clusters. These clusters are concentrated on a substrate by magnetophoresis, wherein they self-organize by dipolar interactions to form microscopic filaments. The polymer is cured in the presence of the magnetic field to preserve the microstructure shape. The externally applied magnetic field and its gradient are the two main control variables of interest when considering magnetic control during nanoparticle self-assembly. Their influence on microstructure geometry is investigated through correlations with the height of a characteristic self-assembled filament, fraction of the substrate area covered by the microstructure and its shape anisotropy. These relations enable a priori design.