Bulk-Cusp Microstructure for Controllable Multi-Directional Fluid Spreading

Controllable wetting of liquid on solid surface is meaningful for advanced science and engineering. Current researches about controllable fluid wetting are generally limited to unidirectional spreading. Achieving controllable multi-directional fluid spreading on micro-structured surface remains a significant challenge. Herein, we propose a novel type of bulk-cusp microstructure, exhibiting 0 to 4-directional self-driven spreading of droplet without external energy input. The bulk structure is assumed to have cross or square shape, implying relatively high and low coverage ratio of the precursor film, respectively. Owing to the drag effect of the precursor film, the cross-cusp microstructure facilitates controllable spreading of the main droplet, whereas the square-cusp microstructure just has guidance action on precursor film due to its low coverage ratio. Mechanism analysis reveals the capillary forces generated from the narrow gaps between adjacent cusps effectively separate the precursor film. The area coverage ratio of precursor film determined by the shape of the bulk influences the coupling or decoupling of the droplet body and precursor film. Such controllable multi-directional fluid spreading of the bulk-cusp microstructure enables its successful application in lubrication enhancement and smart evaporation cooling. This study may open a new avenue for design of microstructures for controllable multi-directional fluid spreading, promoting the development of next-generation smart surfaces.