The Deformation of Single Droplets Impacting onto a Flat Surface 2002-01-2749

This paper presents an experimental study of the deformation of spherical liquid droplets impinging onto dry and flat surfaces making use of a CCD-camera with a high spatial resolution. The experiments consider different liquids (water and Diesel oil) and the effects of droplet velocity and diameter at the impact in a range of Weber numbers up to 1100 and Reynolds numbers up to 77400. Emphasis is put on the nature of the surface target. To consider this effect, two surface materials were used (perspex and aluminium) with surface roughness varying from less than 5μm (considered as a smooth surface) up to R_a=66,6μm. For the range of droplet diameters considered in the experiments, the corresponding dimensionless values of R_a/R_droplet vary from 1.5×10^-5 and 2.5×10^-2.

In a first step, the experiments focus on the spread of the liquid film and analysis of the results suggest that, provided that the Reynolds number of the droplet at the impact is large (Re>2000), the energy dissipated at the wall is not affected by the nature of the surface. The effect of surface roughness appears to be important for low Reynolds numbers, typically Re<1000.

Depending upon the physical parameters at the impact, the droplet may splash at the first contact with the surface (prompt splash) or the liquid film at the wall may break-up on secondary droplets during spread. In a second step, the experiments emphasize the effect of surface roughness on the onset of splash for different liquids. The experimental results are analysed in terms of the critical Weber number for which splash occurs, and compared with correlations reported in the literature accounting for the effects of surface roughness and droplet liquid and suggest the likely influence of, not only the nature of the surface (e.g. surface profile and material), but also of the liquid.