Capillary interactions between droplets and ideal roughness: Attractive protrusion and repulsive trench

The interactions between liquid drops and ideal defects (protrusion or trench) on a contact angle hysteresis (CAH) free surface were studied by experiments and Surface Evolver (SE) simulations. The CAH-free characteristic was achieved by the slippery liquid-infused porous surface (SLIPS) and the ideal defects were constructed on such a CAH-free surface. Both hydrophobic (water) and lyophilic (hexadecane) systems were considered. The force (attraction and repulsion) between a drop and a defect was determined by the inclination experiments. Regardless of the surface wettability, the drop tends to adhere to the protrusion on a horizontal plane. A critical force corresponding to a critical tilted angle is required to detach the drop from the protrusion to overcome the attraction between them. In contrast, during the encounter of a drop with a trench, the pass of the drop is resisted by the trench, irrespective of the surface wettability. When the external force exceeds the critical value, the drop crosses the trench directly in the hydrophobic system but impregnates the trench in the lyophilic system. For all cases studied, the experimental results agree well with the simulation outcomes. This work provides insights into the influence of surface roughness on droplet wetting.