Preface to the viewpoint set: Triple lines

A triple line is the site of common contact between three phases, be they solid, liquid or vapour. Triple lines are ubiquitous in everyday life but also have considerable importance in materials science. In metals and ceramics, triple lines are found where a liquid contacts a solid, where three crystals meet or where a liquid meets a grain boundary; at high temperature all these triple lines can move. In every one of these instances, the triple line is a site of complex phenomena that govern the processing, the microstructural development and the properties of many engineering materials. This complexity arises for a number of reasons: triple lines are dominated by capillarity, are thermodynamically and chemically complex, and furthermore are often sites of singularity - in elasticity, for example. Given their importance and ubiquity, triple lines are a recurrent research theme in the materials science community. The important subject of wetting, of both metals and ceramics by high-temperature liquids, has such importance in the processing of inorganic materials that it is now the subject of dedicated conferences and publications. Dihedral angles, of importance both in processes such as liquid phase sintering and in phenomena such as liquid metal embrittlement, are another subject that has driven a number of research groups to devote significant effort leading to intriguing results and important contributions. Where three grains meet, triple lines again appear; grain boundary triple lines exert an important influence in processes such as recrystallization and grain growth, particularly of fine-grained materials; here too important and interesting contributions have recently been made. One could continue the list of venues where the subject has been a rich and important focal point of research, such as the materials science of thin films, the mechanics of joints, or our understanding of nucleation and growth, both in solidification and in solid-state transformations.