  Sticky diffusion processes on bounded domains spend finite time (and finite
mean time) on the lower-dimensional space given by the boundary. Once the
process hits the boundary, then it starts again after a random amount of time.
While on the boundary it can stay or move according to dynamics that are
different from those in the interior. Such processes may be characterized by a
time-derivative appearing in the boundary condition for the governing problem.
We use time changes obtained by right-inverses of suitable processes in order
to describe fractional sticky conditions and the associated boundary
behaviours. We obtain that fractional boundary value problems (involving
fractional dynamic boundary conditions) lead to sticky diffusions spending an
infinite mean time (and finite time) on a lower-dimensional boundary. Such a
behaviour can be associated with a trap effect in the macroscopic point of
view.
