  We propose the use of topographic modulation of surfaces to select and
localize particles in nematic colloids. By considering convex and concave
deformations of one of the confining surfaces we show that the colloid-flat
surface repulsion may be enhanced or switched into an attraction. In
particular, we find that when the colloidal particles have the same anchoring
conditions as the patterned surfaces, they are strongly attracted to concave
dimples, while if they exhibit different anchoring conditions they are pinned
at the top of convex protrusions. Although dominated by elastic interactions
the first mechanism is reminiscent of the depletion induced attraction or of
the key-lock mechanism, while the second is specific to liquid crystal
colloids. These long-ranged, highly tunable, surface-colloid interactions
contribute for the development of template-assisted assembly of large colloidal
crystals, with well defined symmetries, required for applications.
