## 3 Results and Discussion
### Velocity and Temperature Field for Sessile Water Drop after the Fall of Another Droplet of Water  
Figure 2b shows experimental data for the case when the small cold droplet 1 of water falls vertically on the sessile drop 2 of water located on a heated wall. The fall of droplets in all experiments occurs approximately in 6-7 s after placing the large drop 2 on the heated wall. For the time \(t=\) 6-7 s, the velocity is approximately constant, and the temperature difference before falling between the wall and the interface is approximately \(\Delta T_{s}=\) 12-14 \({}^{\circ}\)C. As the graph shows, the interaction of the two drops has led to a sharp jump of the average velocity up to 7.2 mm/s. After about 2.5 s after the small droplet 1 falls, \(U_{\text{aver}}\) takes a value corresponding to the velocity before the fall, i.e., about 1-1.2 mm/s. Usually, estimates in calculations and experiments are given for the maximum velocity. From Figure 2b it can be seen that the maximum velocity \(U_{\text{max}}\) is several times higher than the average \(U_{\text{aver}}\). The value of the maximum velocity strongly depends on the method of its determination, i.e., there can be no clearly defined method for calculating the extremum due to significant unsteadiness of the process and random behavior for the instantaneous velocity vector. In these studies, the main focus is on \(U_{\text{aver}}\), since this velocity is used for qualitative and quantitative approximations (a relationship is established for free convection velocity, buoyancy and thermocapillary forces). However, the empirical coefficient has been obtained earlier for the average velocity in the drop \(U_{\text{aver}}\). To describe the Marangoni flow, it is important to know the change in the drop surface temperature \(T_{s}\) and the temperature difference \(\Delta T_{s}\) with time \(t\). Figure 3 shows experimental data on the change in the average temperature for the entire drop surface, which was determined over the entire thermal field at the time under consideration. The average temperature was determined by ten different circles drawn inside the drop interface. This algorithm corresponded to the software. The circles were drawn at a certain distance from the contact line of the drop to exclude the influence of the wall. Figure 4: Changes in average temperature difference \(\Delta T_{s}\) for the entire surface of drop 2 over time. Figure 3: Changes of average temperature for the entire surface of drop 2 over time. The average temperature was determined for each circle. The average surface temperature of the drop was calculated as the average value for ten circles. The error of averaging using this method was less than the measuring error of the thermal imager. The graph of changes in temperature \(T_{s}\) consists of two modes: rapid growth of \(T_{s}\) over time during the first 5-7 s after placing the drop on the hot wall, and a slow increase in the temperature of the free liquid surface for \(t>7\) s (quasi-stationary thermal mode). The fall of the cold small droplet 1 on the sessile hot drop 2 is realized at the very beginning of the quasi-stationary mode. Based on experimental data for the \(T_{s}\) field, a graph of the change in \(\Delta T_{s}\) over time is constructed (Figure 4). There the temperature difference \(\Delta T_{s}=T_{w}-T_{s}\) is defined as the average value of the temperature difference for the entire surface of drop 2 (the \(T_{s}\) values are taken from the graph in Figure 3). It is obvious that the modes of change of \(\Delta T_{s}\) coincide with Figure 3. Thus, based on the data in Figure 4, the maximum values of thermocapillary forces and buoyancy forces will correspond to the first few seconds after placing drop 2 on the wall.