Dynamics of droplet impact on solid surface with different roughness

This paper reports an experimental investigation on the impact dynamics of droplets (water, decane, ethanol, and tetradecane) onto a flat stainless steel surface, using high-speed microphotography and with a particular interest in the effect of surface roughness on the impact dynamics. Results show that the impacting water droplet spreads on the surface in the form of a rim-bounded lamella and the rim contracts back after reaching the maximum spreading, while this contraction motion is absent for the fuel liquids. With the increase of Weber number (We) and surface roughness, splashing, evidenced by the ejection of secondary droplets, is favored. The droplet spreading, which is characterized by a normalized diameter β, is accelerated with increasing We, while the surface roughness and Ohnesorge number (Oh) tend to slow down the spreading process. Furthermore, the maximum normalized spreading diameter, β_max, depends primarily on the (We/Oh) and the increase in the surface roughness slightly reduces β_max. The transition from spreading to splashing is enhanced with increasing We or R_a or both. An empirical correlation of β_max as a function of the surface roughness was derived based on the present experimental data. In addition, the transition from spreading to splashing can be represented by a critical (We/Oh)^1/2, which was fitted as a function of the surface roughness. All the proposed empirical correlations show good agreement with literature data and are believed to be of importance for the spray/wall interaction modelling.