Impact of a droplet on a circular superhydrophilic region surrounded by a superhydrophobic region*

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 (b_s)  and Weber number (We). At a smaller b_s, the behavior shifts from the stable to the jetting regime and then to splashing regime, with increasing We. We find that by increasing the value of b_s, one can avoid the undesirable splashing and jetting regimes and attain a stable regime even at a higher We. Our study reveals that b_s has a significant influence on the maximum spreading diameter B_max at a smaller We but a negligible effect at a higher We. We show that the dominance of capillary energy at a smaller We, and viscous energy at a higher We underpin the phenomena. We employ an energy conservation approach to develop an analytical model to predict B_max  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  K  = (Re^1/2/ We) emerges as a key parameter in the model that accurately predicts the experimentally measured B_max. Our study reveals the existence of an inertia-viscous dominated regime at smaller K and an inertia-capillary dominated regime at larger K . The outcome of our study may find applications in stable and precise positioning of impacting droplets.