## 3 Results and Discussion
### Velocity and Temperature Field for Sessile Water Drop after the Fall of Another Droplet of Water
One can also neglect changes in temperature due to deformation of the free surface of the drop caused by fluctuations. One dynamic factor remains: the change in the static pressure field and the organization of liquid rotation in the sessile drop 2 after droplet 1 falls. To date, there are no analytical and numerical solutions that would allow modeling all these factors. In addition, one can also add another factor: the organization of many vortices inside the drop due to the instability of free convection inside the drop 2. In connection with the above, experimental data on convection, as well as simple estimates that would allow assessing the role of key factors in the transfer of heat and momentum in the sessile drop, are of interest. It should be noted that this work is not theoretical and does not aim to obtain strict estimates. The basic idea of estimates is quite simple. As a result of the interaction of two drops, convective motion is realized due to the dynamic factor (discussed above) and the thermal factor: the appearance of a short-term local temperature difference on the surface of the large drop. If simplified simulation of convection due to a temperature jump in accordance with Equation (3) corresponds satisfactorily to the experimental data, then we can assume that the dynamic factor has a negligible influence at a set falling height of droplet 1 and the volume of the falling drop. Figure 5c provides characteristic velocities for the specified isothermal case (\(T_{02}=T_{01}=T_{\rm w}=20\ ^{\circ}\)C). As can be seen from the graph, after the fall of the small droplet 1, in large drop 2 there was a jump of \(U_{\rm aver}\) (\(U_{D}\)) only by 0.4 mm/s. The direction of the Marangoni force (\(Ma_{T}\)) in Figure 5b is shown before the droplet 1 impact. After the droplet impact, the direction of the \(Ma_{T}\) will change to the opposite direction (the coldest place of \(T_{s}\) will correspond to the center of drop 2). Then, the direction velocities from the dynamic and thermal factors will be the same. Thus, the total influence of thermal and dynamic factors leads to the velocity \(U_{\rm aver}=U_{MT}+U_{D}=7+0.4=7.4\) mm/s. The obtained calculated value coincides with the experimental one (7.2-7.5 mm/s in Figure 2b). At that, the role of the dynamic factor is only 5-6%. Since the predominant effect is exerted by the thermal factor, it is important to consider the features of the thermal field change on the surface of sessile drop 2 immediately after the fall of the second droplet 1. For this purpose, the thermal imaging measurements were carried out. It was important to determine how quickly the temperature gradient on the surface of the large drop 2 falls with time (after the fall of droplet 1). The maximum gradient \(\Delta T_{s}\) = 60 \({}^{\circ}\)C should appear immediately after the interaction of drops. Figure 6a shows the velocity fields (PIV measurement) at the interaction of two drops in isothermal conditions (\(t\) = 0 s corresponds to droplet 1 falling time). The non-circular horizontal section of drop 2 (Figure 6a,b) is caused by the appearance of \"blind spots\" due to the focusing of the laser sheet by the drop. Such zones were \"disguised\" in order to avoid the appearance of erroneous vectors in the final velocity field. As a result, the velocity field is asymmetric. The cross-correlation algorithm for calculating the velocity used in this work implies splitting the image into elementary regions (64 \(\times\) 64 pix regions were used) and finding the maximum of the correlation function in each region. At the same time, small local peaks of luminosity appeared periodically in the image of drop 2 due to the reflection of the laser from a large group of tracers. There were local peaks of velocity. When analyzing the results of experiments, such peaks were not taken into consideration. Figure 6b shows images of drop 2 before and after lightening the image in the image editor. In Figure 6b, the outline of drop 2 is clearly visible (highlighted with a red line).