Four questions about triple lines

The identification of triple lines as one of a hierarchy of defects is presented. There are several distinct cases of triple junctions, and these are sorted into classes. Viewpoints about open questions and directions for future research are offered, including: (1) the effect of induced order on the structure and energy of a triple line; (2) considerations of interfacial (complexion) transitions; (3) suggestions of methods for the direct measurement of triple line energy; (4) observations of the possibility of triple line anisotropy.