Size-Dependent Impact Dynamics of Water Droplets on Hydrophilic and Hydrophobic Surfaces

The impact behavior of water droplets of varying diameters was systematically examined on the Poly methyl methacrylate (PMMA, hydrophilic) and polystyrene (hydrophobic) surfaces to reveal the role of the droplet size and wettability in spreading and retraction dynamics. High-speed imaging was employed to analyze the maximum spreading diameter, spreading velocity, recoiling velocity, and rebound behavior. The result showed that the maximum spreading diameter was found to increase with droplet size on both substrates, governed by the rise in initial kinetic energy. This effect was more pronounced on the hydrophobic surface, where spreading velocity scaled more sharply with droplet size. A predictive correlation, {d{₌₀ₗ}} W{{e}^{0. 5}}D₀^{0. 25}, was proposed by incorporating the wetting effects in terms of initial droplet size. The experimental spreading results were compared with the proposed correlation and four other existing models. The proposed model performed the best, keeping the errors below 10% in all tested conditions. Distinct retraction trends were observed on the two surfaces considered. On the PMMA surface, the spreading velocity remained nearly constant across droplet sizes, while the recoiling velocity decreased as the larger drops lost energy to adhesion and pinning. On the polystyrene surface, the spreading velocity increased with the droplet size as inertia dominated over adhesion, and the larger droplets recoiled more strongly, producing higher rebound and oscillations.