Wettability-Controlled Directional Actuating Strategy Based on Bilayer Photonic Crystals.

Although the water-triggered bending behavior of bilayer films has been a wide concerned, there are few reports on wettability-controlled directional actuators with visible color changes. Using photonic crystals as carriers, bilayer directional bending structural color actuators were prepared based on the hydrophilic difference. Top inverse opal with strong hydrophilicity can promote water penetration and strengthen the effect of swelling. While, bottom inverse opal with weak hydrophilicity can inhibit water penetration and weaken the effect of swelling. When the bilayer structure is immersed in water, its wettability differences will produce different optical responses for visualization and will bring different swelling performances, resulting in directional bending. Infiltration differences are visualized as structural color red shifts or transparency. The mechanism of the design involves optical diffractions in the fabricated periodic nanostructures, differences in the surface wettability and swelling rate, uses the infiltration and capillary evaporation of water to realize the spectral diversity of reflectance, and the enhancement of bending by gradient infiltration. This work deeply analyzes the improvement of the photonic crystal structure on the optical and bending performance of the wettability-controlled actuator, provides a basic model for the design of bionic components, and opens an idea for the combination of bilayer photonic crystals and actuators.