Bulk-cusp microstructure for controllable multi-directional liquid spreading

Controllable wetting of liquid on solid surface is meaningful for advanced science and engineering. Current researches about controllable liquid spreading are generally limited to unidirectional modes, while achieving multi-directional spreading on microstructured surfaces remains challenging. Herein, we propose a novel type of bulk-cusp microstructure, exhibiting 0 to 4-directional spreading of droplet without external energy input. This behavior occurs not only under single-drop deposition but also under continuous liquid injection. 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 liquid spreading enables applications in lubrication enhancement and smart evaporation cooling.