Impact dynamics of saline droplets on flat surfaces

Saline droplet impact on solid surfaces is widespread in nature and industry, yet existing research lacks systematic analysis over a wide salinity range. Herein, we investigate the dynamic characteristics of saline droplets (0–26 wt. %) impacting flat aluminum plates at different velocities (0.5–2.5 m/s) through experiments and theoretical analysis. The impact process is quantified by the spreading factor, height factor, characteristic times, and post-impact oscillation parameters. A higher impact velocity enhances inertial spreading, thus increasing the maximum spreading factor and reducing the minimum height factor. Increasing salinity suppresses spreading mainly by increasing viscosity, decreases the maximum spreading factor, and only weakly affects the spreading/retraction rates and the final equilibrium spreading factor. The post-impact oscillation cycle time increases with salinity, whereas the damping coefficient is positively correlated with salinity but nearly independent of impact velocity. A scaling law for the oscillation cycle time and a modified model for the maximum spreading factor and its corresponding time are established by accounting for salinity-dependent density, viscosity, surface tension, and dissipation pathways. The revised spreading model predicts the data over the whole salinity range at room temperature, with relative deviations of about −6% to 16%. The energy analysis shows that the increase in salinity mainly affects the droplet impact process through enhancing viscous dissipation. This study deepens the understanding of salinity's influence on droplet impact dynamics and provides references for related engineering applications.