  Placing colloidal particles in predesigned sites represents a major challenge
of the current state-of-the-art colloidal science. Nematic liquid crystals with
spatially varying director patterns represent a promising approach to achieve a
well-controlled placement of colloidal particles thanks to the elastic forces
between the particles and the surrounding landscape of molecular orientation.
Here we demonstrate how the spatially varying director field can be used to
control placement of non-spherical particles of boomerang shape. The boomerang
colloids create director distortions of a dipolar symmetry. When a boomerang
particle is placed in a periodic splay-bend director pattern, it migrates
towards the region of a maximum bend. The behavior is contrasted to that one of
spherical particles with normal surface anchoring, which also produce dipolar
director distortions, but prefer to compartmentalize into the regions with a
maximum splay. The splay-bend periodic landscape thus allows one to spatially
separate these two types of particles. By exploring overdamped dynamics of the
colloids, we determine elastic driving forces responsible for the preferential
placement. Control of colloidal locations through patterned molecular
orientation can be explored for future applications in microfluidic, lab on a
chip, sensing and sorting devices.
