Morphology of drop impact on a superhydrophobic surface with macro-structures

Drop-surface interaction is predominant in nature as well as in many industrial applications. Superhydrophobic surfaces show potential for various applications as they show complete drop rebound. In a recent work, it has been reported that the drop lift-offtime on a superhydrophobic substrate could be further reduced by introducing a macro-ridge. The macro-ridge introduces asymmetry on the morphology of drop spreading and retraction on the surface. This changes the hydrodynamics of drop retraction and reduces the lift-offtime. Keeping practical applications in view, we decorate the surface with multiple ridges. The morphology of the hydrodynamic asymmetry is completely different for the drops impacting onto the tip of the ridges from those impacting onto the middle of the valley between the ridges. We show that the morphology forms the key to the lift-offtime. We also show that the outward flow from the ridge triggers a Laplace pressure driven de-wetting on the tip of the ridge, thus aiding the lift-offtime. At the end of this work, we propose a ridge to ridge separation that effectively reduces the lift-offtimes for impacts both at the tip of the ridge and offset from it.