Photodriven Self-Excited Hydrogel Oscillators

Stimuli-responsive materials are designed for self-sustained soft robots. Using a photothermally responsive hydrogel cantilever as a model system, this paper investigates the mechanism and energy flow in self-excited oscillation under a constant light source. Based on an analytical model, we show that a periodic photomoment, produced by nonuniform water concentration across the cantilever’s thickness driven by diffusion, is imposed on the cantilever by the ever-switching light incidence between the top and bottom surfaces. The synergy between the photomoment and oscillation ensures positive work input to the cantilever when the diffusion timescale is comparable to the period of free oscillation. When the input energy is higher than the damping energy, the oscillation amplitude increases, while, when the input energy is lower, the amplitude decreases. Based on dimensional energy analysis, we determine the stable oscillation amplitude and construct phase diagrams for the increase and decrease of the oscillation amplitude, which are further confirmed experimentally. A mass-spring-damper system subjected to a displacement-dependent excitation force is developed to investigate the features in generalized self-excited oscillating systems. This work lays a solid foundation for understanding self-excited oscillation and provides design guidelines for self-sustainable soft robots.