  Perturbations of fluid media can give rise to non-equilibrium dynamics, which
may in turn cause motion of immersed inclusions. We consider perturbations
("activations") that are local in space and time, of a fluid density which is
conserved, and study the resulting diffusiophoretic phenomena that emerge at a
large distance. Specifically, we consider cases where the perturbations
propagate diffusively, providing examples from passive and active matter for
which this is expected to be the case. Activations can, for instance, be
realized by sudden and local changes in interaction potentials of the medium,
or by local changes of its activity. Various analytical results are provided
for the case of confinement by two parallel walls. We investigate the
possibility of extracting work from inclusions which are moving through the
activated fluid. Further, we show that a time-dependent density profile,
created via suitable activation protocols, allows for conveyance of inclusions
along controlled and stable trajectories. In contrast, in states with a steady
density, inclusions cannot be held at stable positions, reminiscent of
Earnshaw's theorem of electrostatics. We expect these findings to be applicable
in a range of experimental systems.
