Light-Operated Dual-Mode Propulsion at the Liquid/Air Interface Using Flexible, Superhydrophobic, and Thermally Stable Photothermal Paper.

The direct transformation of external energy into mechanical work by the self-propelled motor inspires and promotes the development of miniaturized machines. Several strategies have been utilized to realize the self-driven motion, but in some cases multiple power sources are needed, and this would complicate the operation in diverse environments. In this regard, the dual-mode self-propelled system based on a single power source is highly desirable. In this work, single-light-actuated dual-mode propulsion at the liquid/air interface is realized by using flexible, superhydrophobic, and thermostable photothermal paper made from flexible ultralong hydroxyapatite nanowires, titanium sesquioxide (TiO) particles, and poly(dimethylsiloxane) coating. The superhydrophobic surface enables the thermostable photothermal paper to float on the water surface spontaneously and significantly reduces the drag force. In the usual situation, the heat power produced by the photothermal effect is utilized to trigger the Marangoni propulsion. While the Marangoni effect is quenched in water containing the surfactant, the propulsion mode can be directly switched into the vapor-enabled propulsion mode by simply increasing the light power density. Particularly, the light-driven motion in a linear, curvilinear, or rotational manner can be realized by designing the self-propelled machines with appropriate shapes by using the processable photothermal paper. It is expected that the as-prepared dual-mode self-propelled, flexible, superhydrophobic, and thermostable photothermal paper-based devices have promising applications in various fields such as microrobots, biomedicine, and environmental monitoring.