ABSTRACT
The impact of drops onto a dry or wetted surface can have great importance in technical applications and natural phenomena such as soil erosion, turbine blades corrosion, surface cooling, formation of charged droplets in a thunderstorm, dispersal of spores and micro-organisms, spray injection in diesel and gasoline direct-injection engines, liquid painting. The impact of liquid droplets onto a solid surface, either dry or wetted, has been studied for more than one century. However, because of the high complexity of the splashing and spreading processes, a detailed and satisfactory description of these phenomena is still not available in the scientific literature. A drop impinging on a surface can spread over it, rebound or form a crown (Fig. 1a-e). Several other phenomena may also occur. For example, as shown in Fig. 1b-c, during crown formation, some jets can be produced and then break, thus throwing away secondary droplets. In Fig. 1d a lamella breaks up during the receding contraction of a drop on a non-wettable surface. Finally, if the surface is covered by a liquid film or has a low wettability, a conical rebounding jet is formed which in turn can produce a few secondary drops as an effect of the elastic collision (Fig. 1e-f). The dynamics of the impact of a single liquid drop on a wall depends on the physical parameters of the impinging droplet, on the solid surface properties and on the near wall boundary conditions. In particular, the impact process is influenced by the drop dynamics (its diameter D, velocity V normal to the impacted surface, impact angle α), liquid properties (non-Newtonian or viscoelastic fluid, viscosity µ, density ρ, surface tension σ) and droplet internal conditions (circulation, presence of solid particles, non-homogeneous density or viscosity). The main surface parameters are its temperature, wettability, roughness, porosity, elasticity, charge, curvature and, if the surface is wetted, the film thickness, its waviness and chemical composition. The near-wall conditions can influence the impact dynamics mainly through the presence of an air boundary layer or a vapor film. Moreover, the im-pact frequency has to be considered as an independent
parameter, since its influence is not only the mere production of a liquid film of a given thickness on the solid surface. A time scale should be included in the dimensional analysis: its standard adimensional form is the “convective” time scale τ=Vt/D.
