  The ability to control the movement and assembly of particles in liquid
crystals is not only an important route to design functional materials, but
also sheds light on the mechanisms of colloidal interactions. In this study we
place micron-sized particles with "Saturn ring" defects near a wall with hills
and dales that impose perpendicular (homeotropic) molecular anchoring. The
strong splay distortion at the wall interacts with the distortion around the
particles in the near field and favors their migration towards the dales via
the so-called "lock-and-key" mechanism. We demonstrate experimentally that the
lock-and-key mechanism can robustly localize a particle at specific
topographical features. We observe the complex trajectories traced by the
particles as they dock on the dales, estimate the binding energy, and explore a
range of parameters that favor or disfavor the docking event, thus exploiting
the capabilities of our experimental system. We extend the study to colloids
with homeotropic anchoring but with an associated point defect instead of a
Saturn ring and show that they find a different preferred location, i.e. we can
place otherwise identical particles at well defined sites according to their
topological defect structure. Finally, for deep enough wells, confinement
drives topological transitions of Saturn rings to dipoles. This ability to
tailor wall geometry to guide colloids to well defined sites within nematic
liquid crystals represents an important new tool in directed assembly.
