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.
Highlights
The metrological investigation of sprays is of great interest for a number of technical applications such as fuel injectors or drying processes in the food and pharmaceutical industries, including spray and freeze drying, with regard to the evaporation and particle formation process
We describe a novel approach to study the drying process of levitated single droplets of pure liquids as well as droplets of protein solutions using tunable diode laser absorption spectroscopy
We presented a novel approach of combining the techniques of acoustic levitation and direct tunable diode laser absorption spectroscopy (TDLAS) in order to investigate the drying process of single droplets of pure water and protein solutions
Summary
The metrological investigation of sprays is of great interest for a number of technical applications such as fuel injectors or drying processes in the food and pharmaceutical industries, including spray and freeze drying, with regard to the evaporation and particle formation process. Typical contact techniques, including the sessile droplet method [1] or suspension from a glass filament [2], bring some disadvantages such as the lack of free rotation, bubble formation or heat conduction between the droplet and the fixture To circumvent these problems, levitation methods such as optical levitation [3], electromagnetic levitation [4] and acoustic levitation via an ultrasonic field are applied. Levitation methods such as optical levitation [3], electromagnetic levitation [4] and acoustic levitation via an ultrasonic field are applied The latter method, where a liquid sample is levitated in one of the nodal points of a standing acoustic wave, which is established between an ultrasonic transducer and a reflector, has proven to be suitable and is frequently used for single droplet analysis [2,5,6]. This is due to its comparatively low acquisition and operating costs, as well as the fact that acoustic levitation does not require any special physical properties of the sample [7]
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