## 3 Results and Discussion
### Morphological Structures of Drop Impact on Liquid Films  
The impact of biofuel single droplet onto a liquid film resulted in six different observed phenomena: deposition, fingering, prompt-splash, crown splash, jetting, and bubble encapsulation. In Figure 4, it is possible to see two phenomena without the formation of secondary atomization: (a) spreading or deposition, when the droplet merges with the liquid film, occurring for low impact energies; (b) fingering, i.e., instabilities created in the outer rim of the liquid lamella and structures finger-like in shape grow. When the size of the fingers is larger, they tend to break up and form droplets by secondary atomization. The variable \(t\) expresses the time before and after impact--\(t=0\) the instant of impact. Prompt splash was also spotted (Figure 5) when the impact energy was high enough for the droplet to disintegrate in the first moments after impact. Tiny droplets eject from the liquid lamella periphery while the crown still rises or advances at its baseline. Since the sequence of images corresponds to an impact height of \(z_{3}=1\) m, the impact velocity is high \(U_{0}=4.06\) m/s). Therefore, the moment the droplet impinges onto the liquid, a film appears between subsequent impact frames. Figure 6 shows two high impact energy events: (a) the crown splash and (b) jetting or rebound. Crown splash occurs after the stage of maximum expansion and encompasses the breakup of the crown sheet and it is a frequent event during impacts of singles droplets with liquid films. In this phenomenon, the impact energy of the droplet impinging in the liquid film generates a momentum transfer to the cylindrical liquid sheet forming an uprising crown (\(t=1\) ms). Considering the timescale of impact measured from the droplet size and velocity, \(t_{i}=D_{0}/U_{0}=0.78\) ms, it evidences how inertia dominates the evolution of this hydrodynamic structure. From instabilities in the bounding rim, tiny droplets eject from fingers formed \(t=3\) ms after impact. During the crown collapse (\(t=14\) ms), the fingers stretch, and eventually break, generating secondary droplets. This phenomenon produces various sizes of secondary droplets, while prompt splash only produces tiny ones. Shortly after the crown collapse, a vertical extension of fluid (jet) rises from the center of the impact site--a phenomenon identified as jetting, or rebound in the cases where most of the impinging mass detaches from the surface after impact, which occurs more often in dry surfaces. The formation of this central jet was also spotted several times in these experiments, even without crown formation. As visualized on the right sequence in Figure 6, this jet often reaches its maximum height and breaks, ejecting one or more secondary droplets. The occurrence of deposition followed by jetting, as shown in the image sequence, only happened two times in the experiments performed. However, it was recurrently preceded by prompt and crown splash. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**[PERSON]** & **[PERSON]** & **[PERSON]** & **[PERSON]** \\ \hline
1411–16,889 & 103–1623 & 27,987–28,8101 & 1.863–9.593 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experimental ranges of dimensionless numbers. Figure 4: Image sequences: (**a**) the spreading of a single droplet in a liquid film for the 75% JF/25% HVO mixture (\(D_{0}=2.77\) mm, \(z_{1}=0.175\) m, \(\delta=1\)); (**b**) the fingering of a single droplet in a liquid film for the 75% JF/25% HVO mixture (\(D_{0}=2.47\) mm, \(z_{1}=0.175\) m, \(\delta=0.1\)). There was only one phenomenon left, the bubble encapsulation (see Figure 7). The occurrence of bubble encapsulation is seldom reported in the literature, and mostly for higher dimensionless thicknesses and impact energies [29; 30]. It just occurred for one set of impact conditions. After drop impact, one spots prompt splash, followed by the formation of a very thin and high crown ascending in an almost perfect vertical direction.