Experiments on splashing thresholds during single-drop impact onto a quiescent liquid film

The process of normal impingement of a single drop onto a quiescent liquid film was investigated experimentally to determine the thresholds of the early and late splashes. Seven liquids of different hydraulic properties were used and the liquid film thickness was varied within about 0.2–3 times the impacting drop diameter. It was found that the critical Weber number for the early splash to occur is dependent on the liquid properties (more specifically, the Ohnesorge number) but not on the liquid film thickness. On the other hand, the critical Weber number of the late splash was influenced by both the liquid properties and the liquid film thickness. It increased gradually with an increase in the liquid film thickness in a thin film range and jumped to a constant value when the critical film thickness was reached. The critical film thickness was 0.7 times the impacting drop diameter in average in the present experiments. Using the present experimental data, dimensionless correlations were developed for the thresholds of the early and late splashes. The predictive performances of the proposed correlations were tested against the experimental databases available in the literature. Due to the complexity of the drop impact process, noticeable disagreements remained between the experimental data and the predictions even in the simple situation explored in this work. Nonetheless, the present dimensionless correlations succeeded to express the overall trends of the splashing thresholds fairly well.