  A hydrostatically stressed soft elastic film circumvents the imposed
constraint by undergoing a morphological instability, the wavelength of which
is dictated by the minimization of the surface and the elastic strain energies
of the film. While for a single film, the wavelength is entirely dependent on
its thickness, a co-operative energy minimization dictates that the wavelength
depends on both the elastic moduli and thicknesses of two contacting films. The
wavelength can also depend on the material properties of a film if its surface
tension has a pronounced effect in comparison to its elasticity. When such a
confined film is subjected to a continually increasing normal displacement, the
morphological patterns evolve into cracks, which, in turn, govern the adhesive
fracture behavior of the interface. While, in general, the thickness provides
the relevant length scale underlying the well-known Griffith-Kendall criterion
of debonding of a rigid disc from a confined film, it is modified non-trivially
by the elasto-capillary number for an ultra-soft film. Depending upon the
degree of confinement and the spatial distribution of external stress, various
analogs of the canonical instability patterns in liquid systems can also be
reproduced with thin confined elastic films.

Soft Condensed Matter