Hydrophobic wettability effects on low-Weber-number droplets morphology evolution

Abstract

The impact behavior of the droplets was significantly influenced by the substrate temperature, surface hydrophobicity, and tilt angle. To elucidate the underlying interaction mechanisms between the droplet and the surface, this paper presents an experimental investigation of the interaction between droplets impacting various heated metallic surfaces. The study utilized three distinct hydrophobic aluminum substrates and employed 4 wt% glycerol aqueous solution as the test liquid. The temperature of the metallic substrates was maintained between 80 °C to 260 °C, while the droplet impact velocity was kept constant at 0.884 m/s. Under low-temperature conditions, droplets exhibit a sequence of spreading, receding, and oscillation. In contrast, elevated temperatures induce atomization and the Leidenfrost effect; these elevated temperatures promote spreading, accelerate receding, and enhance droplet rebound. Hydrophobic surfaces inhibit maximum spreading diameter while simultaneously increasing receding velocity and rebound amplitude; stronger hydrophobicity results in a more regular rebound morphology. As the tilt angle increases, droplet spreading and rebound tend to occur in the direction of the tilt, causing changes in the trajectory, displacement, and shape of the droplets. Furthermore, the synergistic effect of high temperature and strong hydrophobicity intensifies the coupling between receding and rebound. Adjustment of the tilt angle can amplify or qualitatively alter the interdependencies among other factors. Ultimately, the macroscopic spreading characteristics are determined by the dynamic balance between the intrinsic contact angle properties and the extrinsic tilt angle.