## 3 Results and Discussion  
As an initial application of the TDLAS hygrometer, we investigated the drying process of an acoustically levitated droplet of pure distilled water. The drying experiment was performed at a drying temperature of 60 \({}^{\circ}\)C at a constant nitrogen flow rate of 1.0 nL/min. For the manual injection of the droplet into the acoustic levitator, the heating chamber had to be opened. In order to prevent, to the greatest possible extent, the ambient air from  
Figure 3: _Top:_ Typical H\({}_{2}\)O absorption line (measured data (circles) and fitted Voigt profile (solid line)), after removal of amplitude modulation and correction for offset and transmission fluctuations, for high (**left**) and low water vapor concentration (**right**), plotted as optical density (\(OD=-\ln(I/I_{0})\)) over a linear wavenumber scale. _Bottom:_ Deviation between measured data and fitted Voigt profile normalized to respective maximum values. entering the levitator and the MPC, the connection between the levitator chamber and the MPC was interrupted by a valve. After the droplet was successfully levitated, the valve was opened and the TDLAS measurements were performed at a laser scan frequency of 2 Hz. Figure 4 shows the results for the measured water mass flow from the exhaust gas of the droplet drying process and the normalized squared equivalent radius calculated from the initial droplet volume (top left), the measured water concentration in the Perspex chamber (bottom left) and images of the levitated droplets at different stages of the drying process (right). The steep rise of the water mass flow in Figure 4 marks the opening of the valve between the levitation chamber and the MPC. The mass flow increases within a few seconds until it reaches an almost constant level of approx. 10\(\cdot\)10\({}^{-6}\) g\(\cdot\)s\({}^{-1}\). The slight fluctuations at around 100 s, 200 s and also 280 s are due to the levitation process. The distance between the ultrasonic transducer and the reflector must be optimized from time to time to ensure a stable levitation process and to prevent the droplet from falling out. This process influences the shape of the droplet and, therefore, the size of the droplet's surface, which is immediately noticeable in the detected mass flow. The different shapes that a droplet can take during a drying process can also be seen in Figure 4 on the right-hand side in the top row. After about 290 s, a noticeable decrease in the mass flow and, therefore, a decrease in the drying speed can be observed. This constant decrease in mass flow continues until approx. 600 s, and then a second reduction in the mass flow takes place. From this point on, the droplet is completely evaporated, and the remaining water vapor is slowly flushed out of the levitator chamber (see Figure 4, bottom row). By integrating the measured mass flow over the drying time, an initial droplet volume of 4.17 uL can be calculated (as described in the section before) for the injected pure water droplet. Thus, the measured volume is about 4.3% higher than the injected volume. This slight deviation is due to the injection process, as the levitator chamber is opened during this process and the ingress of ambient air cannot be completely avoided. The water vapor concentration in the free space part in the Perspex chamber remains constant throughout the entire drying process. Starting from this initial droplet volume of 4.17 uL, the normalized squared equivalence radius \(r(t)^{2}/r(0)^{2}\) and its evolution over the drying time can be calculated (see green curve, Figure 4). Considering the evaporation process of the levitated droplet between 50 s and 550 s, it can be seen that this evaporation process can be adequately qualitatively described by the \(d^{2}\) law, which states that the normalized squared equivalent radius decreases linearly during the drying of single droplets [8].