Laser-induced emission from iron oxide nanoparticles in spray-flame synthesis: in situ high-speed microscopy

Spray-flame synthesis uses low-cost precursors dissolved in organic solvents to produce functional metaloxide nanoparticles. In the spray flame, the precursor-laden droplets show frequent and intense thermally-induced disruption, so-called puffing and micro-explosion. This process is often correlated with high uniformity of particle sizes. Whether puffing and micro-explosion are also directly associated with the formation or release of iron oxide nanoparticles is not clear. Also, the spatiotemporal evolution of nanoparticles in the turbulent flow field of the flame is largely unknown from experiments. We performed simultaneous high-speed microscopic imaging of droplet shadowgraphs at 360 kHz as well as elastic light scattering (ELS) and laser-induced emission (LIE) of nanoparticles at 40 kHz. Comparing ELS and LIE images allows distinguishing signals from droplets, flame, and nanoparticles, as only the nanoparticles will appear in images from both methods. ELS and LIE show nanoparticles as thin narrow filaments, presumably following the local flow. Nanoparticle filaments are found at a height of 50 mm and more above the burner in the spray flame. The filaments show increasing LIE signal and higher confinement with increasing height above the burner. The appearance of LIE and thus nanoparticles does not directly correlate with the presence of droplets or their disruption.