  At sufficiently low temperatures and high densities, repulsive spherical
particles in two-dimensions (2d) form close-packed structures with six-fold
symmetry. By contrast, when the interparticle interaction has an attractive
anisotropic component, the structure may exhibit the symmetry of the
interaction. We consider a suspension of spherical particles interacting
through an isotropic repulsive potential and a three-fold symmetric attractive
interaction, confined in circular potential traps in 2d. We find that, due to
the competition between the interparticle and the external potentials, the
particles self-organize into structures with three- or six-fold symmetry,
depending on the width of the traps. For intermediate trap widths, a core-shell
structure is formed, where the core has six-fold symmetry and the shell is
three-fold symmetric. When the width of the trap changes periodically in time,
the symmetry of the colloidal structure also changes, but it does not
necessarily follow that of the corresponding static trap.
