## 3 Results
### Microscopic and Macroscopic Droplet Measurements
#### 3.2.3 Effects of Laser Fluence and Dye Concentration on the LIF/Mie Ratio  
The effects of laser fluence and dye concentration are presented in the following section based on E20 at reference conditions. The results are presented in Figure 8. For all investigations, representative droplet sizes of 30 \(\upmu\)m and 50 \(\upmu\)m (in an interval of \(\pm\)0.5 \(\upmu\)m each) were studied, to avoid extensive time-consuming measurements of the entire droplet range. Datasets covering the entire droplet size spectrum were already performed in a previous study except for the fuel ethanol in combination with the tracer eosin Y [15]. A reduced dataset was chosen to study the dependencies of the LIF/Mie ratio on laser fluence and dye concentration. Three laser fluences of 54 mJ/cm\({}^{2}\), 72 mJ/cm\({}^{2}\) (reference condition, corresponding to a pulse energy of 1.48 mJ), and 90 mJ/cm\({}^{2}\) were chosen for these investigations. The laser fluence showed only a small impact on the LIF/Mie ratio (see the inset at the upper-left side in Figure 8) which is in accordance with several other studies [15; 17; 20]. Basically, the LIF/Mie ratio does not vary with the laser fluence in the investigated range, as both the LIF and Mie signals showed linear behavior unless effects such as saturation occurred. This behavior is advantageous for spray measurements, as temporal and spatial fluctuations in laser fluence will play a minor role in such cases.  
The influence of the dye variation was studied using nile red concentrations of 100 mg/L and 200 mg/L in E20 at a constant laser fluence (72 mJ/cm\({}^{2}\)). The linear increase of the ratio with increasing dye concentration is shown in Figure 8 (see the insert on the upper right). For both droplet sizes, a doubling of the dye concentration led to a doubled LIF/Mie ratio. This measurement confirms the linear behavior of the LIF signal as a function of dye concentration, while the Mie signal should hardly be affected as long as laser extinction effects in the droplet were negligible. Additional uncertainties due to the saturation of the LIF signal were not observed. However, a partial evaporation of the fuel droplets at high or moderate ambient temperatures may lead to an increase of the dye concentration within  
Figure 8: Effects of the laser power and dye concentration on the LIF/Mie ratio for E20. The inserted diagrams show the effects of the variation of the laser power (**a**) and the dye concentration (**b**) on the LIF/Mie ratio, 293 K (nile red concentration: NR100 = 100 mg/L, NR200 = 200 mg/L; laser power: LF75 = 54 mJ/cm\({}^{2}\), LF100 = 72 mJ/cm\({}^{2}\), LF125 = 90 mJ/cm\({}^{2}\); SLF = simplified linear fit).  
the shrinking droplets. This will result in an increased LIF/Mie ratio and in an overestimated droplet size in spray experiments. Consequently, reliable droplet sizing measurements should preferably be conducted at moderate ambient conditions in combination with low evaporation rates. For conditions with increased evaporation rates in spray measurements, the variation of the dye concentration must be corrected. Another aspect is the varying droplet temperature during droplet heating and evaporation (of a spray), which will affect the LIF/Mie ratio. This source of uncertainty is discussed in the subsequent section.