We study the conditions under which and how an imposed cluster of fixed
colloidal particles at prescribed positions triggers crystal nucleation from a
metastable colloidal fluid. Dynamical density functional theory of freezing and
Brownian dynamics simulations are applied to a two-dimensional colloidal system
with dipolar interactions. The externally imposed nucleation clusters involve
colloidal particles either on a rhombic lattice or along two linear arrays
separated by a gap. Crystal growth occurs after the peaks of the nucleation
cluster have first relaxed to a cutout of the stable bulk crystal.Comment: 4 pages, accepted for publication in Phys. Rev. Let