A hard-disc system: structures of a close-packed thin layer

The possible structural transitions between n and (n - 1) thin layers of hard discs are considered. The apparently obvious homogeneous transition postulated on the basis of n = 2 is shown by an analogue experiment not to be the general solution. It is argued that the maximum density is retained by a 'Δ-form' transition which minimizes the density deficit at the boundary walls, and can be referred to different sections of a two-dimensional close packing.