  Cellulose nanocrystals (CNC) can be considered as model colloidal rods and
have practical applications in the formation of soft materials with tailored
anisotropy. Here, we employ two contrasting microfluidic devices to
quantitatively elucidate the role of shearing and extensional flows on the
alignment of a dilute CNC dispersion. Characterization of the flow field by
micro-particle image velocimetry is coupled to flow-induced birefringence
analysis to quantify the deformation rate--alignment relationship. The
deformation rate required for CNC alignment is 4$\times$ smaller in extension
than in shear. Alignment in extension is independent of the deformation rate
magnitude, but is either 0$^\circ$ or 90$^\circ$ to the flow, depending on its
sign. In shear flow the colloidal rods orientate progressively towards
0$^\circ$ as the deformation rate magnitude increases. Our results decouple the
effects of shearing and extensional kinematics at aligning colloidal rods,
establishing coherent guidelines for the manufacture of structured soft
materials.
