Post-impact lamella evolution of drop impact on superhydrophobic cylindrical surfaces at high Weber number

Abstract

Drop surface interaction is omnipresent in nature and vital to many engineering applications. Most previous studies on drop impact dynamics on superhydrophobic cylindrical surfaces have focused on low-impact Weber (We) numbers, wherein the asymmetric bouncing behavior is the prominent outcome. However, it is observed that an impacting drop at higher impact We numbers (>100) exhibits more complex dynamics. The asymmetric post-impact lamella evolution in axial, and azimuthal directions are analyzed in detail. At higher impact velocities, the lamella expands in an azimuthal direction over the solid cylindrical surface, sweeping a certain angle followed by further expansion in air and over the solid surface until attaining the critical detachment angle or swiping angle, which is found to be a function of surface curvature and impact velocity. Thereafter, the expansion proceeds only in the air until the lamella completely shatters away, indicating the absence of a retraction phase in the azimuthal direction contrary to that during low-velocity impact. Lamella nucleation and film rupturing together, along with the ejection of satellite droplets, further add complexity. The present experimental study comprehensively evaluates the effect of higher Weber numbers (We up to 660) and surface curvature. Universal scaling relations are proposed for the lamella evolution in the axial and azimuthal directions based on the impact parameters to rationalize the same. A minimization of the surface energy approach has been hypothesized to predict the detachment angle utilizing the proposed scaling relations and is found to predict well with the experimental data.