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In established theories of grain coarsening, grains disappear either byshrinking or by rotating as a rigid object to coalesce with an adjacent grain.Here we report a third mechanism for grain coarsening, in which a grain splitsapart into two regions that rotate in opposite directions to match two adjacentgrains' orientations. We experimentally observe both conventional grainrotation and grain splitting in 2D colloidal polycrystals. We find that grainsplitting occurs via independently rotating "granules" whose shapes aredetermined by the underlying triangular lattices of the two merging crystalgrains. These granules are so small that existing continuum theories of grainboundary energy are inapplicable, so we introduce a hard sphere model for thefree energy of a colloidal polycrystal. We find that during splitting, thesystem overcomes a free energy barrier before ultimately reaching a lower freeenergy when splitting is complete. Using simulated splitting events and asimple scaling prediction, we find that the barrier to grain splittingdecreases as grain size decreases. Consequently, grain splitting is likely toplay an important role in polycrystals with small grains. This discoverysuggests that mesoscale models of grain coarsening may offer better predictionsin the nanocrystalline regime by including grain splitting.

Grain splitting is a mechanism for grain coarsening in colloidal polycrystals