## 3 Results and Discussion
### Velocity and Temperature Field for Sessile Water Drop after the Fall of Another Droplet of Water
The Effect of Surfactants on the Velocity Field Inside Sessile Drop after an Impact of Another Small Droplet  
Theoretical predictions [3, 49, 50, 51] for the free convection velocity inside sessile drop 2 of water or an aqueous solution with a low concentration of another liquid show that the numerical calculation overestimates the velocity value tens of times compared to the experiment [3, 52]. Multiple velocity reduction in the experiment is associated with the presence of contaminants in water, which cannot be eliminated [51, 53]. The exception is an aqueous solution of alcohol with a high concentration of alcohol [3, 50]. The suppression of a coffee-ring occurs at a certain form of micron-sized particles as a result of capillary forces [54]. It is assumed that the impurities reduce the surface tension gradient \(\sigma\) on drop 2 surface \(d\sigma/dl\), where \(l\) is the distance on the free surface of drop 2. Thus, even without adding the surfactant to the liquid, there is a significant impact of natural surfactants. There are many technical problems when the use of surfactants contributes to both the growth and reduction of the reaction rate. For example, the addition of low-concentration methanol to oil contributes to the control of hydrate plugs in pipelines [55, 56, 57] by lowering the hydrate formation temperature. Surfactants in the form of SDS (sodium dodecyl sulfate) significantly increase the rate of hydrate formation. The formation of micelles from surfactant molecules can increase the rate of hydrate formation 700 times [58] since micelles are active centers of hydrate formation. A small concentration of surfactants in the liquid suspension in droplet-gas suspension allows achieving small droplet sizes and prevents the merger of droplets and the growth of their diameters. To date, there is very little experimental data on the effect of surfactants on the velocity field inside drop 2. Most of the works are devoted to the study of physical and chemical properties and wettability, as well as the evaporation mode when adding surfactants [23, 24]. It is extremely difficult to study the hydrodynamics of two colliding droplets in a moving gas flow using PIV. This process can be studied by the example of falling of small droplet with surfactant additives on the sessile drop, located on a hot wall. Figure 8a-c present experimental data on the effect of surfactants of the following types: AF 9-12; 4% mass; OP-10; 1% mass; and Sodium DS; 0.1% mass. The falling droplet 1 consisted of a water + surfactant solution. Sessile drop 2 consisted of distillate. The surface \(Ma_{T}\) flow was directed from the center of drop 2 to its edge (Figure 5b,c), since \(\sigma\) increases in the direction of the edge. In 2.0-2.5 s after droplet 1 falling, the average velocity in the section of drop 2 enters the quasi-stationary mode, i.e., the velocities before and after the drop are equal. The surfaces of AF 9-12 and OP-10 did not change the contact radius of drop 2 (the contact line of drop 2 did not shift). The surfaces of SDS led to a jump in the contact line. An arrow in Figure 8c indicates the time at which the diameter of the contact line increased abruptly (approximately for 1 s) by 10-20%. The use of all three types of surfactants has shown a significant decrease in the average velocity jump. The use of AF 9-12 and OP-10 has led to about 1.8-1.9 times decrease in \(U_{\text{aver(max)}}\) (for pure water \(U_{\text{aver(max)}}\) = 7.5 mm/s (Figure 2b), and for AF 9-12 and OP-10 \(U_{\text{aver(max)}}\) = 4.0 mm/s (Figure 8a,b). The use of SDS has led to the maximum suppression of the convection velocity (the maximum jump of the average velocity \(U_{\text{aver(max)}}\) is 2.5 mm/s), i.e., \(U_{\text{aver(max)}}\) decreases about 3 times. A stronger decrease of \(\sigma\) for water + SDS (Figure 8c) has led to a non-equilibrium of the contact line of drop 2.