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The effect of process parameters on the droplet drying process, which is crucial for tailor‐made particle designs in spray drying processes, was investigated. Because of the challenging observability in spray towers single droplet experiments were carried out using acoustic levitation. The influence of the air humidity on the particle and solid layer formation of single mannitol‐water droplets and temperature profiles of the complete drying process were experimentally determined. Also, the influence of the air humidity on the solid layer formation and the droplet temperature evolution was investigated.

Summary: Droplet polymerization of N-vinyl-2-pyrrolidone (NVP) and sodium acrylate (NaAA) was carried out in an acoustic levitator for different ambient temperatures and relative humidities. The resulting particle morphologies were compared to particles of two crystalline systems (mannitol and ammonium sulfate) and two disperse systems (silica and styrene -butyl acrylate dispersion), all obtained under similar drying conditions. NVP was found to form a higher amount of crystals with lower temperature and relative humidity, contrary to the behaviour observed for mannitol and ammonium sulfate. The processes of both, polymerization and drying of NaAA lead to similar morphologies at low temperatures and humidities, probably caused by the precipitation of NaAA during polymerization.

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