  The effect of boundaries and how these can be used to influence the bulk
behaviour in geometrically frustrated systems are both long-standing puzzles,
often relegated to secondary role. Here we use numerical simulations and "proof
of concept" experiments to demonstrate that boundaries can be engineered to
control the bulk behavior in a colloidal artificial ice. We show that an
antiferromagnetic frontier forces the system to rapidly reach the ground state
(GS), as opposed to the commonly implemented open or periodic boundary
conditions. We also show that strategically placing defects at the corners
generates novel bistable states, or topological strings which result from
competing GS regions in the bulk. Our results could be generalized to other
frustrated micro and nanostructures where boundary conditions may be engineered
with lithographic techniques.
