  Understanding the out-of equilibrium behaviour of point defects in crystals,
yields insights into the nature and fragility of the ordered state, as well as
being of great practical importance. In some rare cases defects are
spontaneously healed - a one-dimensional crystal formed by a line of identical
charged particles, for example, can accommodate an interstitial (extra
particle) by a re-adjusting all particle positions to even out the spacing. In
sharp contrast, particles organized into a perfect hexagonal crystal in the
plane cannot accommodate an interstitial by a simple re-adjustment of the
particle spacing - the interstitial remains instead trapped between lattice
sites and diffuses by hopping, leaving the crystal permanently defected. Here
we report on the behavior of interstitials in colloidal crystals on curved
surfaces. Using optical tweezers operated independently of three dimensional
imaging, we insert a colloidal interstitial in a lattice of similar particles
on flat and curved (positively and negatively) oil-glycerol interfaces and
image the ensuing dynamics. We find that, unlike in flat space, the curved
crystals self-heal through a collective rearrangement that re-distributes the
increased density associated with the interstitial. The self-healing process
can be interpreted in terms of an out of equilibrium interaction of topological
defects with each other and with the underlying curvature. Our observations
suggest the existence of "particle fractionalization" on curved surface
crystals.
