  Recent studies demonstrated that an elastomer containing hygroscopic
inclusions absorbs moisture and swell. Here we show that a thin film of such an
elastomer bonded to a rigid substrate undergoes morphological instability upon
absorption of water, the wavelength of which increases linearly with its
thickness. As the driving force for such a morphological instability arises
from the difference of the chemical potential of water between its source and
that in the film, its development is slowed down as the salinity of the water
increases. Nonetheless, the wavelength of the fully developed morphology, but
not its amplitude, is independent of the salinity. We also demonstrate that if
a domed disk shaped adherent is attached to the hygro-elastomeric film before
moisture absorption, the elastic force generated during the morphological
transition is able to dislodge it completely without the need of any external
force. These patterns, once developed in pure water, is subdued when the
salinity of water increases or if it is exposed to dry air. They re-emerge when
the film is immersed in water again. Such an active response could be important
in fouling release when a ship coated with such a hygro-elastomer changes its
location during its long travel through sea, where salinity varies from place
to place.
