Topological transformations in bipolar shells of nematic liquid crystals.

We investigate how defects in bipolar shells of liquid crystal evolve when the boundary conditions for the nematic director n at the outer surface of the shell are changed from tangential to homeotropic. When the boundary conditions for n are tangential, these bipolar shells are characterized by the presence of two pairs of boojums, one pair on each spherical surface. However, when n is tangential at the inner surface and perpendicular at the outer surface, only the innermost pair of defects remains. Interestingly, there are two possible routes from one shell to the other and both involve the generation of a disclination ring that shrinks with time, eventually disappearing. Although the process is reminiscent of that encountered in bulk nematic droplets, in the case of nematic shells the two defects on the inner surface play a relevant role in the overall evolution process.