Pinch-off dynamics in unequal-size droplets head-on collision on a wetting surface: Experiments and direct numerical simulations

Abstract

There is a growing interest in the optimization of spray systems to minimize reflexive separation and enhance droplet coalescence, which has the potential to greatly benefit industrial and agricultural applications. In this investigation, the pinch-off dynamics in head-on impacts of unequal-size droplets on a hydrophobic surface are explored, employing both experimental and numerical approaches. The study focuses on size ratios ranging from 1.0 to 5.0 and impact Weber numbers up to 208. The captured images from the high-speed camera are meticulously processed and analyzed in a detailed manner. Two distinct scenarios are observed in the experimental findings: (1) reflexive separation occurring without the formation of satellite droplets and (2) reflexive separation characterized by the presence of satellite droplets. Direct numerical simulations are also conducted to probe the underlying dynamics during droplet impact. The direct numerical simulation results closely replicate the experimental results, demonstrating excellent agreement with the dynamics of the pinch-off process. The simulated velocity field demonstrates the liquid's movement away from the neck region, leading to progressive thinning and eventual pinch-off. Furthermore, the study examines the evolution of the neck radius over time (τ), revealing a linear variation in log–log plots. Remarkably, the neck radius scales with τ2/3, even for different size ratios. A regime diagram in We–Δ space is reported.