## 3 Results and Discussion
### Effect of Liquid Surface Tension  
The images illustrating the effect of surface tension on the regimes and outcomes of the collisions of droplets with a solid particle are presented in Figure 6 and Video S2. Adding a surfactant to water reduces the surface tension almost twofold, which, in turn, increases the number of child droplets. In particular, adding a surfactant (0.5 vol% Tween-80) to distilled water reduces the surface tension from 72.69 to 47.4 mN/m. This increases the number of satellite droplets almost twofold. A reduction in the liquid surface tension causes inertia to dominate the surface tension forces of the liquid. Moreover, a decrease in the surface tension at the liquid-gas interface suggests a directly proportional reduction in the energy required to break up the specific surface. This effect leads to a more substantial transformation of the surfactant solution droplets compared to water droplets. When a surfactant solution droplet lands on a particle, it spreads over the surface and breaks up into a great number of child droplets. Similar conclusions were made in [8], where a decrease in the surface tension was shown to intensify the droplet spreading over the surface. When droplets broke up from a collision with a particle, there were no fragments of a spherical shape. Practically all the secondary fragments actively transformed during floating, assuming the shape of ellipsoids of revolution with variable amplitudes and temporal deformation cycles. It is clear in Figure 7a that the shift of the separation regime boundaries towards lower (by 15-25%) critical Weber numbers is caused by lowering the surface tension of the liquid almost twice. The results presented in Figure 7a indicate that a slight decrease in the liquid surface tension (from 0.0474 to 0.0361 N/m) almost does not affect the interaction regime maps (the changes being no more than 3%). At the same time, when the liquid surface tension becomes 35-55% lower, the time of the liquid droplet-particle interaction in the agglomeration regime decreases, i.e., the spreading rate increases. For example, the spreading time of a distilled water droplet over a particle is approx. 19 ms, that of the Tween-20 solution is 11 ms. A key specific aspect was identified. A further decrease in the liquid surface tension (from 0.0474 to 0.0361) has no significant impact on the spreading time of droplets over a particle (the changes do not exceed 5%). This suggests that a lower liquid surface tension significantly affects the duration of the droplet-particle interaction in the agglomeration regime: it reduces the interaction time and increases the spreading rate. The specified aspects are important to predict the conditions of changes (considerable and  
Figure 6: Images illustrating the collisions of liquid droplets with a particle in the stretching separation regime at varying liquid surface tensions. inconsiderable) on the droplet-particle collision regime maps when using liquids that have different rheological and surface-active properties. Figure 7b shows the ratios of the free surface areas after and before the atomization of liquid droplets as a result of their contact with a particle when varying their surface tension. Lower surface tension of liquid was found to increase the number of resulting fragments. In turn, this led to a higher ratio of \(S_{1}\) to \(S_{0}\) within the whole Weber number range. For instance, a reduction in the liquid surface tension from 0.07269 to 0.0361 N/m increases \(S_{1}/S_{0}\) by 30-45%. This is accounted for by the conversion of some energy spent on overcoming the surface tension forces to kinetic energy. These effects become weaker at a higher relative velocity of droplets, since the kinetic energy in the contact zone of droplets is already high. The experiments showed a significant surface transformation before the interaction (in the amplitude, rotation time and shear deformation) of solution droplets compared to water droplets.