Understanding of head-on coalescence of binary drops onto a cylindrical target

Abstract

In this paper, a numerical exercise in understanding the binary head-on collision of equal-sized vertically aligned drops on the cylindrical substrate is reported. Subsequently, radial spreading of the liquid bridge, impact, and detachment of merged drop from the solid target. Different dimensionless relevant factors, namely Weber number (We), contact angle θ, Ohnesorge number (Oh), Bond number (Bo), and diameter ratio Dc/Do are used to characterize the coalescence, and subsequent impingement onto the cylindrical substrate. Temporal deformation factor βf is predicted throughout the entire droplet cycle, and the maximum deformation factor βf,max is obtained just after the collision of drops because of the significant radial expansion of merged drop. A larger value of βf,max is achieved at a higher We for a constant θ, Dc/Do, and Oh. Interestingly, the ring drop is pinched off from the parent merged drop when the value of Dc/Do is lower at a higher value of We. Again, the possibility of the appearance of the ring is significantly higher when the target surface becomes non-wetting. A correlation for the maximum deformation factor is developed using computational data points, which fits well with the simulated data within ± 6 %. Lastly, a theoretical framework is also predicted to determine the maximum deformation factor.