Oral: Understanding of Autonomous Motion of Humidity-Responsive Polymer Films

Humidity-responsive polymer films exhibit unique autonomous motions, such as curling-up, humidity-coupled oscillation, and flipping-over locomotion, under a humidity gradient. However, the principle of such complex motions is unknown. In this work, we develop an analytical hygoscopic beam model with temporally evolving boundary conditions to unravel the mechanism. Our results show that the coupling between the humidity-gradient driven bending of the film and redistribution of humidity due to the film deformation is the key to the autonomous motion. Our analysis further delineates the criteria and mechanism driving each motion type. Curling-up results from humidity gradient intensification by a self-shadowing effect. Humidity-coupled oscillations arise from the time-dependent evolution of the stable contact point driven by film morphology. Finally, flipping-over locomotion occurs when the stable contact point reaches the film’s boundary. Our model provides understanding to the autonomous motion of humidity-responsive polymer films attributed to the interplay between humidity diffusion and deformation, offering insights to design self-propelled soft robots.