Effect of wettability on droplet impact: Spreading and splashing

Abstract

This study experimentally investigates the dynamic behaviors of water droplets over wide ranges of diameters and velocities (200 μm < D0 < 2600 μm, 50 < We < 3000) impacting on surfaces with various wettability (ranging from hydrophilic to superhydrophobic), and focuses on the effect of surface wettability on water droplet spreading and splashing. The experimental results show that the surface wettability contributes to the lift of the lamella, related to the cosine of the advancing angle (cosθa), that significantly influence the spreading and splashing. The lifting lamella promotes the spreading speed by reducing the surface friction and the viscous dissipation in the boundary layer. The mean spreading speed is linear to the impact velocity on each surface type, and the contact angle slightly affects the mean spreading velocity on the hydrophilic or hydrophobic surface. While, the mean spreading velocity on the superhydrophobic surface is much larger than the other surfaces due to the fewer viscous dissipation. The splashing behavior is dominated by the force from the air surrounding the lamella. The experimental results demonstrate that the splashing threshold K = OhRe1.25 is independent of the advancing angle for the case of θa < 90°, but is proportional to cosθa for the case θa > 90°. Furthermore, we put forward a prediction model of droplet splashing threshold, considering the effect of surface wettability, as K/K0 = 1 + αcosθa, which could quantitatively express the effect of surface wettability on water droplet splashing.