Central uprising sheet in simultaneous and near-simultaneous impact of two high kinetic energy droplets onto dry surface and thin liquid film

Abstract

Droplet impact on both dry and wet surfaces is present in several applications, and often multiple droplets, instead of one single droplet, are involved. This paper focuses on the problem of two-droplet impingement on dry and wet surfaces with two Weber numbers (We) of 115 and 230, corresponding to two Reynolds numbers (Re) of 6100 and 8620, respectively. We study impact dynamics phenomena, compare simultaneous and time-delayed impact dynamics of two droplets, and investigate the time evolution of a central uprising sheet formed between the two droplets impinged on dry or wet surfaces, a problem that has been barely studied. A central uprising sheet forms between two impinging droplets at sufficiently high Re and We and short droplet to droplet spacing (high kinetic energy at the point of spread contact). Three different shapes for the central uprising sheet are observed for two droplet impact on a dry surface with various time delays: ordered two-dimensional (2D) semilunar shape (vertical and inclined), curved or C-shaped three-dimensional (3D) shape, and irregular splash. Our experiments show that the central uprising sheet undergoes splashing under conditions not predicted by existing correlations; also, during the early formation of the central uprising sheet, the effect of gravity force on the sheet evolution is negligible. Mixing and surface waves are also studied, taking advantage of liquids with three different colors.