Experimental and numerical analysis of the single droplet impact onto stationary ones

The present paper investigates experimentally and numerically the impact of a spherical water droplet onto a stationary sessile one lying onto a substrate. The experiments were performed with two different film thicknesses, three different We numbers and two surface contact angles (two substrates, aluminium and glass). For this purpose a CCD camera was used and the corresponding qualitative and quantitative characteristics regarding the time evolution of the phenomenon, such as the diameter and height of the evolving crown, were obtained by image analysis. The aforementioned investigation was extended applying also the V.O.F (Volume Of Fluid) numerical methodology for the prediction of the temporal evolution of the phenomenon, so as to identify important characteristics of the induced flow field, not easy to be measured. This permits the in depth understanding of the governing flow laws, which resemble to those in the case of a droplet impact onto shallow films. The governing Navier-Stokes equations are solved both for the gas and liquid phase coupled with an additional equation for the transport of the liquid interface. An unstructured numerical grid is used along with an adaptive local grid refinement technique, increasing the numerical accuracy along the liquid-gas interface with the minimum computational cost. The numerical model is validated against the corresponding experimental data showing a good agreement. The regimes of deposition and splashing are identified as a function of We number and of the maximum thickness of the steady film, which is affected by the surface wettability properties. Moreover, following an analysis of the controlling parameters describing the temporal evolution of the lamella spreading, the role of We and Oh numbers as also of the wetting contact angle were identified, providing analytical expressions for the main dimensions characterizing the phenomenon.