## 2 Materials and Methods
The acoustic levitator is controlled by a function generator (GW Instek AFG-2005, New-Taipei City, Taiwan) and an amplifier (Keysight Technologies 33502A, Santa Rosa, CA, USA) and levitates the droplets at a working frequency of around 39 kHz, depending on the set drying temperature. During the single droplet drying, the heating chamber is flushed with a stream of nitrogen (Linde Nitrogen 5.0, Pullach, Germany) at a flow rate of 1 nL/min. The exhaust air from the levitation chamber is directed to the TDLAS hygrometer. To monitor the evaporation process, images of the drying droplets are acquired using a shadowgraphy setup, consisting of a light source (LS, SCHOTT KL1500 electronic, Mainz, Germany) for the back light illumination of the droplet and a CCD camera (CCD, JAI AG CV-M4, Copenhagen, Denmark). The central part of the hygrometer is the single mode fiber-coupled diode laser (NTT Electronics NLK1B5 EAAA, Yokohama, Japan) with its emission wavelength centered around the 110 \(\rightarrow\) 211 transition at 1396.97 nm. The distributed feedback (DFB) laser is mounted on a Peltier cooled 14-pin butterfly laser mount (Arroyo Instruments 205 TEC Butterfly LaserMount, San Luis, CA, USA), and is temperature-stabilized and powered by a combined laser driver (Arroyo Instruments 6305 ComboSource, San Luis, CA, USA). In order to tune the laser across the absorption line, a function generator (Fluke Calibration Type 271, Eindhoven, The Netherlands) supplies a triangular current modulation from 30 mA to 80 mA at 26 \({}^{\circ}\)C diode temperature with a frequency of 2 Hz. The emitted and fiber-coupled laser light is then split approximately into 90%, 7.5% and 2.5% fractions by a series connection of two 1 \(\times\) 2 fiber optical splitters (Thorlabs TW1430R2A1 and TW1430R3A1, Newton, NJ, USA). The 7.5% part is coupled into a fiber-Fabry-Perot interferometer (Etalon; Micron Optics FFP-I, Roanoke, VA, USA) and the transmitted light is detected by an amplified InGaAs-detector (PD3; Thorlabs PDA10 CF-EC, Newton, NJ, USA) to determine the dynamic tuning coefficient \(\frac{d\
u}{dt}\) of the laser. The remaining two fractions of 90% and 2.5% are guided into a sealed Perspex chamber, where they are transferred into the free space by two fiber collimators (C1, C2; Thorlabs CFC5-C (C1) and F260 APC-C (C2), Newton, NJ, USA). The Perspex chamber is continuously purged with nitrogen (Linde Nitrogen 5.0, Pullach, Germany), but it is almost impossible to keep it completely free of water. Therefore, the latter part, illustrated as orange line in Figure 1, is used as a reference signal \(I_{ref}(\
u,t)\) to monitor the remaining amount of water vapor in the free space region inside the Perspex chamber, allowing for a correction of the measured water mole fractions in the MPC by the corresponding amount. After an absorption length of 40 cm through the Perspex chamber, the light is detected by an amplified InGaAs-detector (PD2; Thorlabs PDA10 CF-EC, Newton, NJ, USA). The 90% fraction, illustrated as red line in Figure 1, is used to determine the amount of water in the exhaust gas coming from the acoustic levitator. To that end, the laser beam is guided by a combination of mirrors (M; PM) into an astigmatic multipass cell of the Herriott type (MPC; Aerodyne Research Inc. AMAC-36 LW, Billerica, MA, USA). This cell, with a mirror spacing of 20 cm and a volume of 300 mL, enables absorption lengths of 18 m or 36 m depending on the angle of incidence. In our case, with an angle of incidence of 2.6\({}^{\circ}\), an absorption length of 18 m is realized due to the total number of reflections being 90. The exiting laser light is focused analogously onto an amplified InGaAs-detector (PD1; Thorlabs PDA10 CF-EC, Newton, NJ, USA) via a parabolic mirror. The amplified signals from the three detectors (PD1-3) are fed via a connector block (National Instruments CB-68 LP, Austin, TX, USA) into an AD converter  
Figure 1: TDLAS hygrometer applied to the acoustic levitator.