The borders between great empires are often populated by the most interesting ethnic groups. Similarly, the interfaces between two forms of bulk matter are responsible for some of the most unexpected actions. Of course, the border is sometimes frozen (the great Chinese wall). But in many areas, the overlap region is mobile, diffuse, and active (the Middle East border of the Roman empire; disputed states between Austria and the Russians, or the Italians, …).

At a certain naive level, these distinctions can be transposed to physical interfaces between two different forms of matter.

1. (1) The hard frozen surfaces of metals, of ionic solids, or of semiconductors can be studied under conditions of high vacuum: this allows us to probe them – using electron beams, or other radiations which extract electrons from the surfaces; or even beams of neutral atoms. The net result is, in our days, a highly sophisticated knowledge of these sharp robust fortifications.

2. (2) The soft interfaces built from liquids, from polymers, from organic solids, or from detergents are much harder to probe. High vacuum is usually not acceptable. And even if it is, the probing beams can damage the interface. For many centuries, the main information on soft interfaces came from mechanical studies: adhesion, slippage, wear, … During the last fifty years, electrical properties have also been helpful – in particular for the electric ‘double layers’ at the contact region between water and a solid.