Spreading and migration characteristics of impacting droplets on hybrid-wettability surfaces

Surface wettability influences the droplet impact characteristics, especially for a droplet impacting with low inertia. The present work reports an experimental investigation of droplet impact on homogeneous and heterogeneous wettability surfaces for different Weber numbers. Droplet impact characteristics on surfaces with three homogeneous surface wettabilities, i.e., hydrophilic, hydrophobic, and superhydrophobic, and two heterogeneous surface wettabilities, i.e., hydrophilic-hydrophobic and hydrophilic-superhydrophobic, have been analyzed. The symmetric deposition, spreading, and recoiling on homogeneous surfaces are affected by the surface wettability gradient across the droplet on heterogeneous surfaces resulting in asymmetric behavior. Furthermore, hybrid wettability surfaces suppress the partial rebound, complete rebound, and complete rebound with droplet breakup observed in the homogeneous hydrophobic and superhydrophobic surfaces. The initial inertia force of the droplet significantly affects the asymmetric and droplet migration behavior. The average recoiling velocity of the droplet increases with the inertia of the droplet. The rate of increase in droplet migration is maximum for a Weber number of 12 for both surfaces with hybrid wettability. The analysis of asymmetric spreading and migration of impacting droplets on heterogeneous surfaces is important in enormous applications, such as microfluidic devices, self-transport of liquid, and water harvesting.