Defect coalescence in spherical nematic shells

We study coalescence of topological defects in nematic liquid crystals confined to spherical shells using both experiments and computer simulations. We observe that the four s = +1/2 defects that are present due to topological constraints imposed by the spherical geometry coalesce by pairs after changing the molecular orientation at the outer surface from tangential to perpendicular; the result is the formation of two single s = +1 defects. It is noteworthy that the speed of the coalescence process is peaked when the defects are at opposite points on the equator of the shell; this maximum results from the thickness inhomogeneity of the shells.