## 1 Introduction  
The addition of renewable biofuels, such as ethanol in fossil gasoline fuels, can contribute to the reduction of CO\({}_{2}\) emissions. Nowadays, direct-injection spark-ignition (DISI) engines are commonly used in the automotive sector. These engines show a higher efficiency than engines based on conventional port fuel injection. However, direct fuel injection requires a faster atomization and mixture formation, leading to a very complex combustion process. The liquid fuel atomization and the evaporation behavior govern the ensuing process steps including mixture formation, ignition, heat release, and pollutant formation as well as the IC (internal combustion) engine efficiency. These process steps of combustion are very intricate and not fully understood, and the utilization of alternative fuel blends leads to an even higher degree of complexity. An admixture of 20 vol. % ethanol (E20) to gasoline is common in the US and in Brazil, while E10 is internationally the most common blend. Ethanol admixture, however, may promote soot formation under certain conditions in DISI engines [1; 2; 3; 4], especially E20 which showed an increased soot formation in comparison to the base fuel, particularly for late injection timings in cold start conditions. The atomization and mixture formations are mainly governed by the thermo-physical properties of the fuel which essentially affect the soot formation [2; 3; 4; 5]. To meet the emission limits and efficiency requirements, the fuel-dependent spray-formation processes of DISI engines must be controlled, and their investigation with optical techniques may significantly contribute to better understanding of the spray structure which is necessary for a fuel-specific optimization of the injection. A suitable tool to characterize the spray formation is planar droplet sizing (PDS), as it allows for the determination of the droplet size (\(d\)) in terms of the Sauter mean diameter (SMD) over a plane defined by an illuminating laser sheet. The LIF/Mie ratio technique is based on the assumptions of a \(d^{3}\) dependence of the LIF signal and a \(d^{2}\) dependence of the Mie signal of droplets illuminated with a laser light source [6; 7; 8; 9; 10; 11; 12; 13; 14]. The qualitative LIF/Mie ratio can be calibrated using phase-Doppler anemometry (PDA) measurements [5; 6] or a droplet generator [9; 15; 16]. In principle, a LIF signal can be created by the excited aromatic fuel components (e.g., toluene, benzene, naphthalene) of a multi-component commercial gasoline fuel [17]. However, aromatic hydrocarbons show a distinct temperature and oxygen sensitivity to the fluorescence at the same time [18; 19]. Thus, the addition of a well-characterized fluorescence dye [5; 6; 20] can be favorably applied for PDS. For this purpose, a large number of fluorophores, mainly dissolved in water, were characterized in the past including dissolved sodium salicate [21], rhodamine 6G in water [9], and pyranine (\"solvent green 7\") [20]. Several studies dealing with droplet sizing in alcohol sprays already exist in the literature [5; 15; 16; 20; 22; 23]. The tracer Eosin Y, which has been used in previous studies [22; 23; 24; 25], cannot be dissolved in alkanes or gasoline fuels. Only a small number of tracers are soluble in alkanes, oils, diesel, and gasoline, e.g., pyrromethene and its derivatives (e.g., 597-8C9, 597-C8) [26; 27]. Nile red (C\({}_{20}\)H\({}_{18}\)N\({}_{2}\)O\({}_{2}\)) is another LIF-tracer which has already been employed in a previous study in commercial gasoline and diesel fuels [24] for the investigation of the primary breakup of sprays in the near-nozzle region. Its spectral absorption and fluorescence characteristics in gasoline-ethanol blends as well as aviation fuels (also blended with biofuels such as farnesane and HEFA (Hydroprocessed Esters and Fatty Acids)) was recently studied by our group [28]. To ensure that aromatic fuel components of a realistic gasoline fuel do not become excited by the laser, a visible wavelength is preferred.