  Adhesive [e.g. van der Waals] forces were not generally taken into account in
contact mechanics until 1971, when Johnson, Kendall and Roberts (JKR)
generalized Hertz' solution for an elastic sphere using an energetic argument
which we now recognize to be analogous to that used in linear elastic fracture
mechanics. A significant result is that the load-displacement relation exhibits
instabilities in which approaching bodies `jump in' to contact, whereas
separated bodies `jump out' at a tensile `pull-off force'. The JKR approach has
since been widely used in other geometries, but at small length scales or for
stiffer materials it is found to be less accurate. In conformal contact
problems, other instabilities can occur, characterized by the development of
regular patterns of regions of large and small traction. All these
instabilities result in differences between loading and unloading curves and
consequent hysteretic energy losses. Adhesive contact mechanics has become
increasingly important in recent years with the focus on soft materials [which
generally permit larger areas of the interacting surfaces to come within the
range of adhesive forces], nano-devices and the analysis of bio-systems.
Applications are found in nature, such as insect attachment forces, in
nano-manufacturing, and more generally in industrial systems involving rubber
or polymer contacts. In this paper, we review the strengths and limitations of
various methods for analyzing contact problems involving adhesive tractions,
with particular reference to the effect of the inevitable roughness of the
contacting surfaces.

Soft Condensed Matter;Materials Science