Measurement of water mole fraction from acoustically levitated pure water and protein water solution droplets via tunable diode laser absorption spectroscopy (Tdlas) at 1.37 μm

Abstract

In order to understand the evaporation and particle formation processes of sprays in technical applications such as fuel injectors or drying processes in the food and pharmaceutical industries in detail, single droplet drying experiments, for example, acoustic levitation, are widely used as model experiments. We combined acoustic levitation and tunable diode laser absorption spectroscopy (TDLAS) to measure the absolute H2O concentration in the exhaust gas of a levitation chamber to investigate drying and particle formation processes from single droplets of pure water and protein–water solutions. To that end, we designed and developed a non‐invasive, calibration-free TDLAS‐based hygrometer to analyze the 1.4 μm overtone band. To increase the detection range of the developed hygrometer and to track the complete drying process of protein solution droplets even after the critical point of drying, the absorption length was extended to a path length of 18 m using an astigmatic multipass cell of the Herriott type. The setup was validated by drying pure water droplets, resulting in a determination of the water mole fraction in a range from 73 ppm to 1314 ppm, with a single scan resolution of 1.7 ppm. For protein solution droplets, the entire drying process, even beyond the critical point of drying, can be tracked and the different phases of the drying process can be characterized at different drying temperatures.