## 3 Results and Discussion
### Comparison of Secondary Atomization Methods  
To generalize the research findings and compare the methods of secondary atomization (disruption of liquid droplets as a result of their interaction with each other, with particles and substrates), collision regime maps for droplet-particle and droplet-droplet interaction were plotted in the We(Oh) coordinates (Figure 10). The generalization of the findings using the We and Oh numbers simultaneously allows for the surface tension, inertia and viscous forces, as well as the initial droplet size and velocities. The position of the agglomeration and stretching separation boundary was established on the interaction regime maps of liquid droplets colliding with a solid particle (Figure 10a), with each other (Figure 10b) and with a massive substrate (Figure 10c). The comparative analysis of regime maps presented in Figure 10a,b showed that an increase in the Ohnesorge number leads to an increase in the critical Weber number at which the stretching separation occurs when applying both methods of secondary atomization of droplets. For example, the maximum critical Weber number was recorded for the solution with the highest Oh, i.e., the glycerol solution (50 vol% glycerol, 50 vol% water). This suggests that irrespective of the method used to atomize a highly viscous liquid droplet, a similarity criterion like the Ohnesorge number is constant and corresponds to the range of 0.02-0.04. It was established that the Weber numbers necessary for stretching separation during the collision of highly viscous liquids with each other are almost 40% lower than those during the collision between droplets and a particle. Figure 9: Collision regime maps for liquid droplets interacting with solid particles (**a**) and ratios of free surface areas after (\(S_{1}\)) and before (\(S_{0}\)) the head-on collision with \(B=0\) (**b**) at varying interfacial tensions of liquid: 1—distilled water; 2—90 vol% diesel and 10 vol% water; 3—90 vol% diesel, 10 vol% water and 0.08 g/L emulsifier D\({}_{\text{c}}\)D; 4—90 vol% diesel, 10 vol% water, and 1.6 g/L emulsifier D\({}_{\text{c}}\)D.  
The comparison of the schemes of droplet-substrate collisions with those of droplet-droplet and droplet-particle collisions revealed that much higher Weber numbers are required for stretching separation (Figure 10c). This effect stems from the fact that, during a collision of a liquid droplet with a solid substrate, the stretching separation regime occurs consistently at much higher velocities of the initial droplet (almost 3-5 times as high as in its collision with a solid particle). The main difference is in the conditions and area of contact of liquid with a solid surface in the experiments with a substrate and particle. In the experiments with particles, the surface-active forces, the viscous friction and gravity forces cause the liquid to envelop the particle from all sides. The area of contact increases, a layer of liquid is entrained from the particle surface and is broken off to produce a group of child droplets. When a droplet collides with a substrate, the droplet spreads over the surface and the stain of contact increases but later decreases due to the effect of surface forces restoring the droplet shape on the substrate. This is followed by a slow gradual spreading of liquid on the surface. In the case of the atomization of droplets as a result of their interaction with each other and with a massive surface (a solid substrate), the change in the critical Ohnesorge number was similar. Based on the collision regime maps, approximations like \(\mathrm{We=a\cdot Oh+c}\) were obtained. These equations describe the transition boundaries between agglomeration and stretching separation.