# Secondary Atomization of a Biodiesel Micro-Emulsion Fuel Droplet Colliding with a Heated Wall
## 3 Results and Discussion
### Effect of Impact Velocity  
When estimating the quantitative parameters of the hydrodynamic breakup in a two-dimensional regime, we established the contribution of the variation in the droplet velocity \(U_{0}\) before reaching the substrate surface. Figure 7a shows the variation of \(d_{\text{s}}\)/\(D_{0}\) (its decrease indicates a decrease in \(d_{\text{s}}\), the mean child droplet diameter) depending on _We_ and the substrate surface temperature \(T_{0}\). The analysis of the results from Figure 7a brings us to the following conclusion:  
1. For the same \(T_{0}\) and \(U_{0}\) and growing _We_ only due to an increase in \(D_{0}\), the mean diameter of child droplets decreases moderately, in some cases by 50%. In general, for the current experimental conditions, a larger initial drop diameter provides finer child droplets.
2. \(T_{0}\) growth does not provide finer dispersion.
3. With increasing _We_, the difference in \(d_{\text{s}}\)/\(D_{0}\) for various \(T_{0}\) approaches a certain minimum.
4. An increase in \(U_{0}\) provides a finer dispersion of micro-emulsion fuel drops; in other words, a considerable growth of inertial forces (i.e., 2-2.5 times higher _We_) decreases \(d_{s}\) (up to 1.5-2.5 times in our conditions).  
Figure 7b shows how the interaction velocity of fuel micro-emulsion drops affects the number of child droplets depending on \(T_{0}\). The analysis of the results from Figure 7b revealed a number of features:  
1. A minor increase in \(U_{0}\) leads to a several-fold increase in \(N\). \(D_{0}\) also has a noticeable impact. Its growth causes \(N\) to increase, i.e., a micro-emulsion droplet of a larger initial volume disperses into more fragments of smaller dimensions (Figure 7a).
2. \(T_{0}\) growth has a moderate effect on \(N\). However, an important exception is observed at \(T_{0}\) = 500 \({}^{\circ}\)C. For almost every case from Figure 6b, there is a relative increase in \(N\) at this temperature. We link this to an intense film boiling breakup (Figures 5b and 6a), which is typical of this \(T_{0}\).  
The resulting conclusion is as follows: film boiling during the hydrodynamic breakup provides finer dispersion of a drop (Figure 6a), while forming more child droplets (Figure 7b).  
\(S_{1}\)/\(S_{0}\) is an integral parameter evaluating the effect of drop impact velocity variation on the hydrodynamic breakup characteristics. A minor increase in the impact velocity provides a consistent growth of the evaporation surface area (Figure 7c). The most prominent increase in the evaporation surface area is observed at the minimum examined temperature \(T_{0}\) = 300 \({}^{\circ}\)C. An increase in the substrate surface temperature has an overall negative effect on the evaporation surface area increase, although no major loss of the parameter occurs. Since it is a calculated parameter, the reasons for \(S_{1}\)/\(S_{0}\) increase are similar to those of the results in Figure 7a,b.  
The experimental results from Figure 7 demonstrate the dominant effect of drop impact velocity on child droplet characteristics for a wide range of _We_ and wall temperature variation. The analysis of the recorded images and data [35] indicates that an increase in the droplet velocity increases the inertia and intensifies the surface deformation of a moving droplet from a spherical shape. This extends the interaction area with the wall, which increases the number of child droplets and reduces their size.