Drop Impact on a Superhydrophilic Spot Surrounded by a Superhydrophobic Surface.

The spatial variation in the wettability of a surface can have a significant effect on the spreading and retraction behavior of an impacting droplet and hence the overall impact dynamics. Although composite surfaces have proven applications, there is a lack of understanding of droplet impact on surfaces with a sudden jump in wettability. Here, we study the behavior of a liquid drop impacting a composite surface having a superhydrophilic (SHL) spot surrounded by a superhydrophobic (SHB) region. We find that the droplet exhibits different regimes: no-splitting, jetting, and splashing, depending upon the spot size (β) and the Weber number (). At a smaller β, the behavior shifts from the stable to jetting regime and then to the splashing regime, with increasing . We find that by increasing the value of β, one can avoid the undesirable splashing and jetting regimes and attain a stable regime even at a higher . Our study reveals that β has a significant influence on the maximum spreading diameter β at a smaller but a negligible effect at a higher . We show that the dominance of capillary energy at a smaller and viscous energy at a higher underpins the phenomena. We employ an energy conservation approach to develop an analytical model to predict β on a composite SHL-SHB surface by considering the total energy of the system before the impact and at the maximum spread position. We find = (/) emerges as a key parameter in the model that accurately predicts the experimentally measured β. Our study reveals the existence of an inertia-viscous dominated regime at a smaller and an inertia-capillary dominated regime at a larger . The outcome of our study may find applications in stable and precise positioning of impacting droplets.