Visualization and image analysis of droplet puffing and micro-explosion in spray-flame synthesis of iron oxide nanoparticles

Abstract Combusting metal precursor-laden droplets, required in spray-flame synthesis of nanomaterials, are known to undergo a rapid and disruptive disintegration, i.e., puffing micro-explosion. In this work, imaging with high spatiotemporal resolution image-analysis routines were developed investigate droplet disruption oxides. Droplet shadowgraphs imaged on high-speed camera. The solvent was mixture 35 vol% ethanol 65 2-ethylhexanoic acid which (in some cases) mixed 0.2 mol/l iron(III) nitrate nonahydrate precursor. Photometric morphological processing identified in-focus features, estimated their size, velocity, circularity, discriminated regular, spherical droplets from disrupting ones. While solely regular found the spray flame pure solvent, precursor/solvent mixture, addition droplets. Disruption events phenomenologically classified into puffing, comprising deformation local eruption, micro-explosion, violent disintegration multiple fragments. Puffing occur much more frequently than micro-explosions. Disrupting had 32% smaller Sauter mean diameter indicating that disruptions beneficial for evaporation. At 40 50 mm heights above burner, about 8 6%, respectively, per millimeter axial distance. Thus, throughout lifetime flame, all expected experience disruption. Graphical abstract