Adhesive Release via Elasto-Osmotic Stress Driven Surface Instability

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.