  A continuum-based computational contact model is employed to study coupled
adhesion and friction in gecko spatulae. Nonlinear finite element analysis is
carried out to simulate spatula peeling from a rigid substrate. It is shown
that the "frictional adhesion" behavior, until now only observed from seta to
toe levels, is also present at the spatula level. It is shown that for
sufficiently small spatula pad thickness, the spatula detaches at a constant
angle known as the critical detachment angle irrespective of the peeling and
shaft angles. The spatula reaches the same energy states at the jump-off
contact point, which directly relates to the invariance of the critical
detachment angle. This study also reveals that there is an optimum pad
thickness associated with the invariance of the critical detachment angle. It
is further observed that the sliding of the spatula pad is essential for the
invariance of the critical detachment angle.
