Understanding and overcoming shear alignment of fibers during extrusion.

Fiber alignment is the defining architectural characteristic of discontinuous fiber composites and is dictated by shear-dominated processing techniques including flow-injection molding, tape-casting, and mold-casting. However, recent colloidal assembly techniques have started to employ additional forces in fiber suspensions that have the potential to change the energy landscape of the shear-dominated alignment in conditions of flow. In this paper, we develop an energetics model to characterize the shear-alignment of rigid fibers under different flow conditions in the presence of magnetic colloidal alignment forces. We find that these colloidal forces can be sufficient to manipulate the energetic landscape and obtain tunable fiber alignment during flow within even small geometries, such as capillary flow. In most conditions, these colloidal forces work to freeze the fiber orientation during flow and prevent the structure disrupting phenomenon of Jeffrey's orbits that has been accepted to rule fiber suspensions under simple shear flow.