Anisotropic spreading behavior of droplets impacting a concave cylindrical surface

Combining high-speed visualization experiments, numerical simulations using the Volume-of-Fluid (VOF) method coupled with a dynamic contact angle model, and theoretical analysis, this work systematically investigates the effects of the droplet-to-substrate diameter ratio ( D *), We , and contact angle on the anisotropic spreading scales and characteristic times in the axial and azimuthal directions, while deriving a correlation between the maximum spreading factors in the two directions based on energy conservation. Comparison with experimental data shows that the prediction error of the numerical model for the maximum spreading factor is <1%, and the prediction error of the theoretical correlation is <5%. The results show that, with decreasing D *, the maximum spreading factor and characteristic time in the azimuthal direction increase, while those in the axial direction decrease. For the condition θ sta , θ adv , θ rec = 90°, 100°, 80°, the ratios of the axial to azimuthal maximum spreading factors for D * = 0.055 and 0.44 are 1.03 and 1.47, respectively. Increasing the We increases the maximum spreading factors in both directions and reduces the spreading time. Under hydrophilic conditions, the D * has a more significant effect on regulating the spreading scale and time, at D * = 0.44, the maximum spread factor of the superhydrophilic wall is 1.46 times that of the superhydrophobic wall. The results elucidate anisotropic droplet spreading on concave cylindrical surfaces and provide mechanistic support for liquid film formation and gas–liquid separation in water separators.