Smart biopolymer films that offer shape transformation under the action of external stimulus have numerous applications ranging from soft robotics to targeted drug delivery. The water-responsive biopolymer films exhibit bending deformation similar to the bimetallic strip when one of its surface is exposed to water. The primary mechanism for such a deformation is the through-thickness concentration gradient which induces eigenstrains in the film. However, when these films are immersed in water, both the top and bottom surfaces (through the thickenss) are exposed to water resulting no bending deformation thus making them not suitable for under-water applications. To overcome this problem we develop a bilayer thin film structure with a tunable interface. The bilayer structure is made of hydrophilic (Chitosan) and hydrophobic (PMMA) biopolymers. The hydrophilic polymer acts as the active layer and responds to water; on the other hand, the hydrophobic polymer acts as a passive layer. The desired curvature and the speed of folding (actuation speed) can be controlled through the interface of the hydrophobic and hydrophilic layers. In this work, four types of interfaces, namely, strong, weak, weaker, and no interface (weakest interface) have been designed by changing the concentration of the bonding agent. The strong interface has the highest interfacial strength and hence gives permanent folding. Based on the experimental observations, a finite element model is developed incorporating the coupled chemo-mechanical behavior to mimic the self-folding in the bilayer thin film structure. A cohesive zone model is used to study the interfacial behavior of bilayer thin film structure by varying the cohesive energy. The finite element model gives a better understanding of the underlying physics of water responsive bilayer thin film structures. The simulation and experimental results show good correlations with each other. The study can be used to design a bilayer foldable structure with desired curvature and actuation speed eventually leading the development of under water responsive soft grippers.