## 1 Introduction
First of all, realistic droplet starting conditions need to be selected in a numerical simulation, preferably close to the atomiser nozzle. Therefore, detailed spray characteristics are captured at the first measurement position downstream of the atomiser. Once the droplets are correctly initialised in the simulation, the interaction with the gas flow is the key aspect for predicting droplet trajectories and evaporation. In particular, the relative velocity of droplets to the gas flow is of importance. This velocity is also called slip velocity and has a strong influence on droplet kinematics, secondary atomisation and evaporation [2]. The direct measurement or estimation of the gas velocity is especially challenging in the near nozzle region, where the spray affects the velocity field of the gas phase considerably. Moreover, the simultaneous measurement of droplet size in addition to droplet and gas velocity is necessary in order to deliver detailed validation data for turbulent spray dispersion models [30]. The combination of Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) is a well-known planar optical diagnostic technique for measuring droplet and gas velocity. However, successful phase discrimination and simultaneous measurement of droplet size are not straightforward, which may cause considerable measurement uncertainty [30]. An alternative way to realise the simultaneous measurement of droplet and gas velocity is the utilisation of the smallest detectable droplets as seeding particles of the gas flow. This methodology was successfully applied with the PDA for detecting evaporating [31] and burning droplets [32]. In these studies, droplets of a size smaller than 3-5 \(\upmu\)m were utilised to derive the gas velocity. The present diagnostics also offers the possibility to detect droplets of such small size, which is a distinctive advantage of microscopic imaging. An analytical and numerical study is presented, demonstrating that the average velocity of the gas can be approximated by the velocity of the smallest detectable droplets with sufficient accuracy. Furthermore, the discussion is extended to the statistical analysis of velocity fluctuations. High-spatial-resolution PIV is presented in literature as a promising technique for investigation of turbulent fluctuations in dispersed multiphase flows [33]. To the authors' knowledge, however, size-dependent information about the response of droplets to turbulent velocity fluctuations of the gas phase has not been retrieved using a shadowgraphy approach. In this study, it is demonstrated that information about the underlying turbulent fluctuations can be estimated with reasonable accuracy. Most importantly, the results can be utilised for validating the turbulent spray dispersion, since the droplet size is determined with high accuracy. Finally, it is shown that microscopic imaging may provide experimental validation data at different positions downstream of the atomiser, which can be used to study the evaporation characteristics of UWS under technically relevant flow conditions. To the authors' knowledge, similar measurements at such flow conditions and at comparable temporal and spatial resolution are not available.