Fundamental study on the disruptive combustion of precursor-laden single droplets for nanoparticle synthesis in spray flames

The synthesis of tailored nanoparticles is crucial for developing high-value applications across many industrial sectors. Flame Spray Pyrolysis (FSP) is a key approach for producing such complex functional materials. Here, a metal-containing precursor-solvent solution is atomized and combusted in a spray flame. Understanding the mechanisms of burning FSP droplets is essential for a scalable and controlled nanoparticle synthesis. In this work, the combustion of single micrometer-sized FSP droplets is studied to analyze the physicochemical mechanisms occurring during the droplet lifetime. Particular focus is placed on reproducible droplet disruptions (microexplosions and puffing), which enhance the mass transport from liquid to gas and are essential for the synthesis of homogeneous nanoparticles. High-speed imaging and acoustic emission measurements are used to analyze time and size scales, as well as the mechanisms and dynamics of droplet disruptions. The results demonstrate that the droplet lifetime on FSP scale is governed by droplet disruptions, promoting the gas-to-particle synthesis route for smaller droplet sizes. The disruptions are correlated to the formation of a surface shell, caused by thermal decomposition of the precursor. Flame and droplet-internal bubble dynamics indicate that the shell has viscoelastic properties, allowing it to restrain the internal pressure buildup until the disruption occurs. Implementing a novel heating-wire ignition enables the study of single burning droplets under elevated temperature conditions and the detection of disruption-related acoustic emissions. By coupling high-speed imaging with acoustic measurements, a transition from microexplosions to puffing was quantified based on an acoustic index and correlated to the precursor concentration. This enables the development of a mechanistic disruption model. Droplet burning in spatially confined reactors is analyzed and shows departures from the classical quasi-steady burning.