An embedded interface regulates the underwater actuation of solvent-responsive soft grippers

In this work, we report the role of an embedded interface between two polymer thin films in determining the overall folding and actuation characteristics of a bilayer system applied for gripping submerged objects. Along with the material properties and geometry of the individual films involved, the strength of the embedded interface governs the folding behaviour of the bilayer when exposed to a solvent. The concentration gradient developed across the film thickness when exposed to the solvent results in the deformation of the film. The evolution of concentration through the film thickness as a function of time is closely related to the interface strength. It affects various aspects of the deformation, such as the direction of folding, curvature attained, and actuation rate. In this work, we have varied the strength of the interface between solvent responsive chitosan and hydrophobic Poly(methyl-methacrylate) (PMMA) by treating the substrate (chitosan) with varying concentrations of silane before coating. Experimentally, the folding characteristics of the solvent responsive bilayer films have been investigated for four different interfacial strengths. A coupled diffusion-deformation model for the film and a cohesive zone model for the interface is developed to provide insights into the underlying mechanism behind the observations made. Finally, the application of the bilayer as a gripper for submerged objects for two different types of interfaces is demonstrated. Interestingly, in this approach, the medium where the object is immersed acts as a trigger for folding the grippers.