## 3 Results  
### Spectral Fluorescence Characterization of the Dye  
In this section, the fluorescence spectra of the dye dissolved in Toliso and E20 are presented at various temperatures. This preliminary study was necessary for filter selection and signal interpretation in the imaging study using the droplet generator. The fuel-dye mixture was studied in a dye cell which was heated by a recirculating chiller (type: Julabo FP50). The micro cell was equipped with four windows (sapphire, diameter: 0.5 inch) with an optical access diameter of 9 mm each. A homogeneous temperature distribution within the micro cell was ensured by a built-in magnetic stirrer (8 mm \(\times\) 3 mm stir bar) and monitored by two thermocouples (type K, 1.5 mm, tc-direct GmbH, Moenchengladbach, Germany). The laser beam was generated by a 532 nm Nd:YAG laser (model 150-10, Spectra Physics, Santa Clara, USA). The LIF spectra were recorded under a detection angle of 90\({}^{\circ}\) using a photo spectrometer (model USB 4000, Ocean Optics, Largo, USA, wavelength range 495.9-831.8 nm, 3648 pixels, slit size 10 \(\upmu\)m, integration time 100 ms, average over 50 subsequent spectra for each measurement). Further details of the setup can be found in Reference [28]. The temperature-dependent fluorescence signals for the investigated fuel-dye mixtures are shown in Figure 2. The signals were normalized to the maximum intensity of E20. The chamber was cooled down to 263 K or heated up to 323 K, respectively. The fluorescence spectra were presented in the visible wavelength region 400-750 nm. In the UV region, there were also some absorption bands of toluene (240-270 nm) [46], but this range is not relevant for an excitation at 532 nm. The fluorescence of nile red in Toliso was characterized by continuous spectra with two individual peaks. The integrated fluorescence signal showed a temperature dependence of less than 10% in the temperature range of 263 K-323 K. The temperature dependence can partly be explained by the absorption behavior \begin{table}
\begin{tabular}{c c c c c c} \hline
**Property** & **Unit** & **Isooctane** & **Toluene** & **Ethanol** & **Gasoline** \\ \hline H/C-ratio & - & 2.25 & 1.14 & 3 & - \\ Boiling point @ 0.1 MPa & K & 372 & 383 & 351 & 298–483 \\ Density @ 293 K, 0.1 MPa & g/cm\({}^{3}\) & 0.69 & 0.87 & 0.79 & 0.72–0.78 \\ Dynamic viscosity @ 0.1 MPa, 293 K & mPa s & 0.501 & 0.587 & 1.145 & 0.6 \\ Surface tension @ 293 K & N/m & 0.0188 & 0.0285 & 0.0224 & 0.0216 \\ Heat of vaporization @ 293 K & kJ/kg & 305.4 & 409.3 & 938.2 & 380–500 \\ Stoichiometric air-fuel ratio & kg/kg & 15.2 & 13.4 & 9.0 & 14.7 \\ Lower Heating Value & MJ/kg & 44.6 & 40.6 & 26.8 & 40.1–41.6 \\ \hline \end{tabular}
\end{table} 
Table 1: Physical and chemical properties of the investigated fuels and gasoline for various ambient conditions. Data are extracted from references [40; 41; 42; 43; 44]. which showed a decrease with selevated temperatures (22% from 313 K-343 K) [28]. However, the fluorescence yield depends on temperature as well. The fluorescence spectra were slightly shifted to lower wavelengths with the increasing temperature. At higher temperature, the first peak of the emission spectrum (~555 nm) decreased more quickly in comparison to the second peak (~580 nm). The dependence of the fluorescence signal of nile red on the solvent becomes visible when Toliso is blended with ethanol. The peaks of the fluorescence spectra of E20 were shifted by approximately 50 nm towards larger wavelengths in comparison to Toliso. This behavior was already reported by [PERSON] et al. [39] in the case of nile red mixed with various solvents including ethanol and toluene. The emission spectra of E20 consist of one only peak, instead of two peaks for Toliso. Again, all emission spectra were slightly shifted towards lower wavelengths with increasing temperature. The E20 showed an increase of the (peak) intensity with higher temperature in contrast to Toliso, where a reduction with the increasing temperature was observed.