## 3 Results and Discussion
### Analysis of Fuel Ignition and Combustion  
Previously, it was found that laser modification of surfaces makes it possible to intensify the processes occurring on them [15; 29; 30; 31; 32]. Among such processes are not only the flow of liquids [29], evaporation [30], boiling [31], and condensation [32], but also fuel ignition and combustion [15]. However, it has not yet been established how the configuration of the texture formed by laser radiation affects these processes, and also which configuration provides the most intense ignition and combustion of liquid fuels under conductive or a mixed heat supply. 3.2.1 Analysis of the Fuel Droplet Ignition and Combustion Characteristics under a Mixed Heat Supply  
When studying ignition and combustion of liquid fuels under a mixed heat supply, the temperature of the fuel droplet and the steel surface, on which the droplet was placed, corresponded to laboratory conditions. Under such conditions, the ignition and combustion characteristics are significantly affected by the fuel droplet/heating surface and fuel droplet/high-temperature environment interfacial areas. They are determined by the parameter of the interfacial interaction of a liquid with a solid body, which is the contact diameter (\(d_{\text{s}}\)) of spreading fuel droplets over the surface. Table 3 presents the contact diameters of fuel droplets (oil in the usual state, emulsion, and slurry) as they spread over surfaces P, AB, A2B, and 2 AB. The contact diameters on the AB, A2B, and 2 AB surfaces were measured in two directions due to microchannel configuration: parallel and perpendicular to the microchannels (longitudinal and transverse spreading of the droplet with respect to the microchannels). It was found that among three fuels used in experiments (oil in the usual state, emulsion, and slurry) with an identical droplet volume, the oil spreads over the steel surfaces with the largest contact diameter and the slurry with the smallest one (Table 3). This is due to the aggregation state of the dispersion medium and the dispersed phase of fuels. As can be seen in Table 3, the contact diameter of fuels in the longitudinal direction of spreading is 1.5-2.5 times larger than that for transverse spreading on the AB, A2B, and 2 AB surfaces. This is connected with the fact that, during transverse spreading, the microchannels act as energy barriers and prevent the movement of the contact line over the surface. Therefore, the droplet has an elongated shape along the microchannels. For the same reason, the spreading diameter in the longitudinal direction is larger on the AB, A2B, and 2 AB surfaces than on the polished surface P. However, in the transverse direction, the spreading diameter of the fuels on AB, A2B, and 2 AB is smaller than on the polished surface P, despite the fact that laser-modified steel surfaces demonstrate superhydrophilic  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Fuel**} \\ \cline{2-7}
**Surface** & \multicolumn{2}{c}{**Oil**} & \multicolumn{2}{c}{**Emission**} & \multicolumn{2}{c}{**Slurry**} \\ \cline{2-7}  & \(d\)\(\downarrow\)**, mm** & \(d\)\(\leftrightarrow\)**, mm** & \(d\)\(\uparrow\)**, mm** & \(d\)\(\leftrightarrow\)**, mm** & \(d\)\(\downarrow\)**, mm** & \(d\)\(\leftrightarrow\)**, mm** \\ \hline P & \multicolumn{2}{c}{10.2 \(\pm\) 0.2} & \multicolumn{2}{c}{9.2 \(\pm\) 0.1} & \multicolumn{2}{c}{7.6 \(\pm\) 0.1} \\ AB & 24.0 \(\pm\) 0.2 & 7.3 \(\pm\) 0.1 & 19.5 \(\pm\) 0.1 & 6.5 \(\pm\) 0.1 & 12.0 \(\pm\) 0.1 & 5.8 \(\pm\)  
and superoliophilic properties. The contact angles formed between water droplets, as well as oil droplets and steel surfaces after laser texturing, did not exceed 5-7\({}^{\circ}\)-. Changes in the surface energy and wetting properties (up to the extreme, in particular, superhydrophilic and superoliophilic) are associated with the destruction of existing bonds by laser radiation and the formation of oxides on the surface, which are highly polar [33].