  External magnetic fields are known to attract and orient magnetically
responsive colloidal particles. In the case of 2D microplatelets, rotating
magnetic fields are typically used to orient them parallel to each other in a
brick-and-mortar fashion. Thanks to this microstructure, the resulting
composites achieve enhanced mechanical and functional properties. However,
parts with complex geometry require their microstructure to be specifically
tuned and controlled locally in 3D. Although the tunability of the
microstructure along the vertical direction has already been demonstrated using
magnetic orientation combined with sequential or continuous casting,
controlling the particle orientation in the horizontal plane in a fast and
effective fashion remains challenging. Here, we propose to use rotating
magnetic arrays to control the in-plane orientation of ferromagnetic Nickel
flakes distributed in uncured polymeric matrices. We experimentally studied the
orientation of the flakes in response to magnets rotating at various
frequencies and precessing angles. Then, we used COMSOL to model the magnetic
field from rotating magnetic arrays and predicted the resulting in-plane
orientations. To validate the approach, we created composites with locally
oriented flakes. This work could initiate reverse-engineering methods to design
the microstructure in composite materials with intricate geometrical shapes for
structural or functional applications.
