The complexity of binary droplet collisions strongly increases in case of immiscible liquids with the occurrence of triple lines or for high energetic collisions, where strong rim instabilities lead to the spattering of satellite droplets. To cope with such cases, the Volume of Fluid method is extended by an efficient interface reconstruction, also applicable to multi-material cells of arbitrary configuration, as well as an enhanced continuous surface stress model for accurate surface force computations, also applicable to thin films. For collisions of fully wetting liquids, excellent agreement to experimental data is achieved in different collision regimes. High-resolution simulations predict droplet collisions in the spattering regime and provide detailed insights into the evolution of the rim instability. Another challenge is the numerical prediction of the collision outcome in the bouncing or coalescence region, where the rarefied gas dynamics in the thin gas film determines the collision result. To this end, an important step forward became possible by modelling the pressure in the gas film. With the introduction of an interior collision plane within the flow domain, it is now possible to simulate droplet collisions with gas film thickness reaching the physically relevant length scale.

Spray systems often operate under extreme ambient conditions like high pressure, which can have a significant influence on important spray phenomena. One of these phenomena is binary drop collisions. Such collisions, depending on the relative velocity and the impact parameter (eccentricity of the collision), can lead to drop bouncing, coalescence or breakup. This experimental and computational study is focused on the description of the phenomenon of drop bouncing, which is caused by a thin gas layer preventing the drops coalescence. To identify the main influencing parameters of this phenomenon, experiments on binary drop collisions are performed in a pressure chamber. This experimental system allows us to investigate the effect of an ambient pressure (namely the density and viscosity of the surrounding gas) on the bouncing/coalescence threshold. Keywords drop collision, breakup/coalescence, multiphase flow IntroductionUnderstanding the collisions of drops is of interest for many fields of application. They play an important role in raindrop formation [1,2] and in the modeling of fuel combustion [3,4]. Systematic experimental studies visualizing binary drop collisions provide information allowing to construct regime maps of the collision outcomes [5,6,7]. The mechanisms of bouncing, coalescence, stretching separation, reflexive separation, and shattering are observed. These mechanisms are determined by the kinetic and geometrical parameters of the collision, as well as the properties of the drop liquids and the surrounding gas. The main kinetic and geometrical parameters are defined in Figure 1, where D1 and D2 are the drop diameters and V is the relative drop velocity. B is the distance of closest approach of the drop centers, measured orthogonal to V at the instance of the collision. Dimensional analysis reveals that the collision Weber number We = V 2 D1ρ/σ, the Ohnesorge number Oh = µ/ σD1ρ, the impact parameter X = 2B/(D1 + D2) and the drop size ratio ∆D = D1/D2 determine the outcome of the collision, if the medium ambient to the colliding drops is not varied.