Spreading of droplets impacting different wettable surfaces at a Weber number close to zero

Six different surfaces ranging from superhydrophilic to superhydrophobic were fabricated. Droplet impact experiments were conducted on these surfaces with water droplets 2.25 mm in diameter at a Weber number close to zero to study the effects of surface wettability on the impact process and post-impact oscillation. Droplet impact on all surfaces, except for superhydrophobic surfaces, is dominated by a spreading stage and no evident retraction is observed. Using the inertia-capillarity time t_c = ρR₀³/σ as characteristic time, the spreading stage on all surfaces at We < 0.5 was calculated to be 2.25 ± 0.11t_c; further, it could be divided into inertial and viscous spreading stages at 1 ± 0.11t_c based on droplet height evolution. A semi-empirical correlation for calculating the evolution of spreading factors on different surfaces was fitted by a piecewise exponential function using experimental data, and a deviation of ±20% was observed between the fitted and experimental data.