It is of great significance to investigate the dynamic behavior of droplets impacting on curved surfaces with macro-ridges for enhancing equipment performance in fields such as anti-fogging and anti-icing. In this work, the curved surfaces with triangular macro-ridges are designed and fabricated by 3D printing technique. The dynamic behavior of droplets impacting these surfaces is captured via a high-speed camera. The effects of impact height, macro-ridge apex angle (30°, 60°, and 100°), and surface wettability on the dynamic behavior are systematically investigated, with a focus on the maximum spreading factor, maximum spreading time (the time to achieve maximum spreading factor), and detachment point. The results indicate that when a droplet impacts on the macro-ridged (MR) surface at a smaller Weber number (We), the droplet undergoes four stages: spreading - splitting at the top - liquid film spreading - liquid film retraction. Initially, under the action of the inertial force, the droplet spreads rapidly along the circumferential direction. Due to the viscous force between the droplet and the surface, the liquid film gradually wraps around the macro-ridge. Then, the liquid film is split into two parts by the macro ridge. Next, the liquid film expands to its maximum extent, the film gets thin, and the front remains in a thick state; After that, the liquid film begins to retract under the action of surface tension, and the maximum spreading length decreases. Thereafter, the liquid film continuously spreads and contracts under the influence of gravity and surface tension, eventually forming a relatively uniform liquid film. As the Weber number (We) increases, the initial kinetic energy of the droplet rises and the inertial force dominates the spreading of droplets on the surface, leading to an increase in the maximum spreading factor, while reducing both the maximum spreading and the detachment point. When the We number exceeds a certain critical value, the droplet splashing occurs, resulting in a reduction of the maximum spreading factor due to reduced droplet volume. Secondly, as the macro-ridge apex angle increases, the critical We number for the detachment of the droplet decreases, resulting in less liquid remaining on the macro-ridge surface. Moreover, a thinner liquid film forms on the surface, so that the maximum spreading factor becomes greater. Meanwhile, the maximum spreading time tc increases and the detachment position α rises. Additionally, when the droplet impacts on the superhydrophobic macro-ridged (SMR) surface, compared with the common macro-ridged surface, the critical We number for the droplet to undergo detachment is lower, the spreading speed is faster due to smaller viscous dissipation, and the maximum spreading factor significantly decreases. Finally, considering the effects of surface wettability and the macro-ridge apex angle, the relationship among the maximum spreading factor and maximum spreading time of the droplet and the We number, the macro-ridge apex angle, and surface wettability is obtained based on the energy conservation method, and compared with the experimental results, the two are in good agreement. The presence of macro-ridges and differences in surface wettability significantly alter the maximum spreading factor and maximum spreading time of impacting droplets. For the macro-rib surface used in this study, the We number is redefined considering the effect of the macro-rib apex angle, and the wettability of the surface is taken into account by introducing a wettability correction factor f(f,q). When a droplet impacts a macroridged surface, the maximum spreading factor of the droplet is βmaxµ f(f,q)0.2We0.4, and the maximum spreading time is tc/t0µ f(f,q)We-0.5.
Key words
droplet /
impact /
curved macro-ridged surface /
spreading /
detachment
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Funding
National Natural Science Foundation of China (52576166)
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