Quantitative Analysis of the Asymmetry of Inner Wall Film Flow in Oblique Droplet Impacts

During an oblique droplet impact onto a thin wall film, an asymmetric crown forms, but a quantitative description of this asymmetry is still missing. Crown ejection and splashing are driven by the inner wall film, making its asymmetry crucial in understanding the overall asymmetry. In this study, the asymmetry of the inner film flow is analyzed using direct numerical simulations (DNS) for impact angles of α = 50◦, 60◦, and 70◦ relative to the film surface. In addition, the Weber number, Ohnesorge number, film thickness, and wall boundary conditions are varied. The velocity field and the liquid’s mass and momentum distribution in azimuthal sections are extracted to characterize asymmetries. Lower impact angles increase asymmetry in both the amplitude and the shape of the distribution. A fluid tracking analysis over the redirection phase provides further insight into the origin of asymmetry. A one-parameter geometric model, based on an azimuthal cut of a sphere, accurately reproduces the momentum distribution for all investigated conditions, with the parameter depending solely on the impact angle. This study comprehensively explains the asymmetry of the inner wall film flow and provides a foundation for the overall asymmetry of oblique impacts.