# The Impact of Single- and Multicomponent Liquid Drops on a Heated Wall: Child Droplets
## 1 Introduction  
Nowadays, spray technologies are advancing towards better energy performance in atomization [1; 2; 3; 4]. Gas-vapor-droplet technologies most often involve heterogeneous sprays. Consequently, the priority research line is the enhancement of the secondary atomization of homogeneous and heterogeneous liquids. The most typical secondary atomization technologies are (_i_) collision between drops and drop impingement on a wall; (_ii_) explosive breakup of superheated liquid drops; (_iii_) drop acceleration leading to the catastrophic deformation of its surface and further breakup [1; 2; 3]. Drop impingement on a wall is the most cost-effective method [1; 2; 3]. However, there is still no agreement on the conditions for maximizing the number of child droplets. Therefore, it is expedient to analyze the latest achievements of the global scientific community in this area and to outline promising directions for future research [5; 6; 7; 8; 9; 10]. [PERSON] and [PERSON] [1] examined the interaction of a single free-falling water drop with a solid surface. The focus was on the effect of heat fluxes, roughness grade of the surface, as well as drop velocity and size on the impact characteristics. [PERSON] and [PERSON] [1] found out that the time of contact between the drop and surface in the case of rebound strongly depended on the thermophysical properties of the wall material. Moreover, the contact time decreased with an increase in the thermal conductivity of the material. They also established that the surface roughness affected the time of drop contact with the surface. In particular, the contact time of a drop noticeably increased with a growth of roughness. With an increase in the drop velocity up to 4 m/s, We = 67, the drop brokeup after impinging on the surface. If We \(<\) 30, drop rebound took place. At 50 \(<\) We \(<\) 80, the drop spread over the surface and the resulting lamella disintegrated to form secondary droplets. Moreover, [PERSON] and [PERSON] [1] demonstrated the snapshots illustrating details of the drop spreading over a heated surface. The contact time of a drop with a surface also depends on its hydrophobicity. For example, [PERSON] et al. [5] studied the behavior of a water drop when interacting with superhydrophobic surfaces in the presence of a cold airflow. Experimental studies were carried out at an ambient temperature ranging from \(-\)10 \({}^{\circ}\)C to 30 \({}^{\circ}\)C to investigate the effect of the airflow on a water drop. In the case of superhydrophobic surfaces, a decrease in the contact time of a drop with a surface was observed. [PERSON] and [PERSON] [2] and [PERSON] et al. [3] studied heat transfer mechanisms when a drop impinged on a heated surface. An increase in the impact velocity accelerated the spreading velocity during film boiling. In turn, this led to a decrease in the size of secondary droplets. However, the elevated viscosity contributed to an increase in the size of secondary droplets, while a reduction in the surface roughness decreased their size by more than 15%. Moreover, the factor of changing an impingement angle was examined. In particular, the size of child droplets did not depend much on the impingement angle, when it was \(\varphi>45^{\circ}\). However, at \(\varphi<15^{\circ}\), secondary droplets became much smaller. An experimental study by [PERSON] et al. [6] considered the effect of the surface inclination angle on the propagation dynamics of a drop over the surface after an impact. After the drop spreads and reaches its maximum diameter, reverse processes occur, i.e., a drop is formed from the film. For example, a surface slope angle of 67.4\({}^{\circ}\) provides maximum droplet spreading. Also, the Weber numbers influence the drop spreading diameter [6]. The maximum drop diameter increases with the Weber number and decreases when the Weber number reaches 2.7. With Weber numbers greater than 5, a drop bounces off a surface. [PERSON] et al.