Enhancement and optimization of the receiving input power density is a key to many applications that requires maximizing the energy input, such as energy harvesting, small signal sensing, etc. Normal incidental illumination is the most direct way to maximize the input power density when the input energy is in a form of parallel electromagnetic wave. In nature, many plants, such as sunflowers, developed phototropism to spontaneously sense and track the light source and maintain their disk to be illuminated normally to the photonic input. By tracking the sun, the sunflowers are able to efficiently raise the temperature of their disk in the morning in time to attract more visits of the pollinators. Currently there is no synthetic material system can, omni-directionally, sense, track and harvest the input emissive energy. In this work, we report a soft material system that can self-adaptively track the energy source whichever the direction of the source goes. We proposed a physically symmetric system which a neat PNIPAM hydrogel that is evenly incorporated with Au Nanoparticles as photonic absorbers for specific frequency of input light. Input photonic energy shines on the geometrically symmetric hydrogel system and induce non-symmetric temperature gradient. When the temperature of part of the hydrogel is higher than the LCST of the PNIPAM hydrogel, the gel system will automatically bend toward the light source. The bending will be terminated when the top of the hydrogel points direct to the light source and block the light from shining on the side of the gel. The start and the termination of the bending, adding together, define a spontaneous tracking functionality of the input light source. We studied the tracking system with a simple mechanical model and FEM simulation. The hydrogel system has been optimized to achieve fast and omni-directional real-time tracking in all direction. The response time of tracking is within 10s of seconds and the tracking performance covers 360 degrees of azimuthal plane. The error of tracking accuracy is better than 1%. A demonstration of an omni-directional solar vapor generator has been presented in this work by employing a phototropic hydrogel pillar array. The rate of vapor generation is comparable between the normal incidence and an angular incidence near 90 degrees to normal, indicating a significant compensation of energy harvesting in the case of the angular incidence attributed to the tracking functionality. We believe that the proposed material system can be applied in many applications that require maximizing their energy input.
We would like to be considered for the joint session with SM08: “Autonomous Hydrogels for Soft Robotics”