Asymmetric splash and breakup of drops impacting on cylindrical superhydrophobic surfaces

Drop splash and breakup on cylindrical surfaces play an important role in a wide variety of industrial applications. In this work, water drops with a wide range of impact velocities (1.4 m/s–4.5 m/s) and cylindrical stainless steels with different diameters (1 mm–20 mm) are employed to investigate the asymmetric splash and breakup characteristics of drops impacting on cylindrical superhydrophobic surfaces. We identify two interesting phenomena, asymmetric splash and converging breakup. The splash behavior is found to be asymmetric in different directions, and the drops preferentially splash in the axial direction. Fundamentally, we propose two disparate splash thresholds, referring to the Weber number We and the diameter ratio D* = D/D0, in the azimuthal and axial directions, respectively. The converging breakup is caused by the much more rapid converging of the liquid rim in the axial direction than in the azimuthal direction. The aspect ratio βzmax/βxmax, governing the converging breakup, increases with We and decreases with D*. Fortuitously, the splashing angle θ is demonstrated to only depend on D* rather than We, and the relational expression of θ and D* is provided. Ultimately, we put forward universal relations between the mean diameter and velocity of secondary droplets, resulting from the converging breakup, and the dimensionless parameter We/D*. The results of this work are expected to provide valuable insights into anti-icing and microfluidics fields.