A photo-driven self-excited hydrogel oscillator

When a photo-responsive hydrogel cantilever vibrates under a fixed light source, a periodic photo moment is imposed on the cantilever by the ever-switching light incidence between the top and bottom surfaces. The photo moment is induced by a diffusion driven inhomogeneous distribution of water concentration through the cantilever’s thickness. Combining theory and experiments, we find that when the water’s diffusion time scale is comparable to the cantilever’s inertia time scale, net energy can be pumped into the cantilever to not only maintain a self-excited vibration by overcoming the damping, but also to increase the oscillation amplitude. Scaling analysis is conducted to understand the effect of material and geometric parameters on the self-excited oscillation, and phase diagrams for amplitude-increasing and decreasing oscillation is constructed. A mass-spring model is studied as a simplified epitome to understand the features observed in the hydrogel oscillator.