Dynamics of impacting droplets on hydrophobic concave surfaces

The dynamic behaviors of droplets impacting hydrophobic concave substrates are investigated via high-speed photography. The effects of the Weber number (We) and the curvature (k) on the morphological evolution of droplets impacting hydrophobic concave substrates were observed. The maximum spreading arc length (Lmax), maximum jet height (Hmax), and rate variations during the spreading and retraction stage of impacting droplets were experimentally quantified. Meanwhile, the critical impacting parameters for producing satellite droplets on different curvature concave surfaces are also quantitatively expressed. Moreover, a prediction model for the Lmax of droplets impacting varied concave substrates was innovatively established based on energy analysis of impacting droplets, which was proven to present good accuracy by experimental data. The results revealed that concave surfaces suppress the spreading of the droplet while obviously enhancing the retraction stage. The Lmax and Hmax of the droplet decrease with the increase in k due to more energy dissipation. As the We increases, the Lmax increases, and the Hmax decreases. Furthermore, when the We is fixed at 90, as the k increases from 0.0606 to 0.1667 mm−1, the spreading rate of the droplet decreased from 3.1 to 2.6 m/s, and the retraction rate increased from 0.7 to 0.78 m/s. Additionally, it was observed that We promotes the formation of satellite droplets, while k suppresses the formation of satellite droplets. Simultaneously, the critical We and k parameters for generating satellite droplets were also provided. Some references are provided for designing functionalized special-shaped hydrophobic surfaces with controllable droplet wetting and dynamic behaviors.