Acoustic signatures of single disrupting FSP droplets in a heated oxygen atmosphere

Abstract

Puffing (weak disruptions) and μ-explosions (strong disruptions) play an important role in the synthesis of nanoparticles via flame spray pyrolysis (FSP), as they increase the mass transport of the liquid precursor-solvent system into the gas phase, thus contributing to the formation of homogeneous nanoparticles. Therefore, the targeted experimental study of droplet breakup using single droplet experiments is an essential step in gaining fundamental knowledge about time- and concentration-dependent mechanisms of these disruptions. Here, we study the disruptive combustion of single droplets containing a commonly applied Tin-based precursor with initial Sn molar concentrations ranging from 0.05 mol L−1 to 1 mol L−1. High-speed imaging techniques have been combined with high sample-rate acoustic measurements to detect, distinguish, and quantify droplet disruption phenomena. It has been demonstrated that precursor concentrations up to 0.25 mol L−1 are more prone to μ-explosions, whereas higher Sn molar concentrations show puffing. This shift in the disruption modes between different precursor concentrations is characterized by a boundary that is acoustically identified by quantifying the strength of a disruption using a disruption strength index. Droplet sizes at the instant of the breakup are suggested to play a central role in causing the different droplet breakup characteristics. While the initial droplet sizes of the different precursor solutions are almost identical, they vary at the instant of disruption, resulting in distinct breakup characteristics. Thus, puffing is detected at the instant of disruption for droplet sizes above 55 μm, whereas μ-explosions occur for smaller droplets. Using high-speed measurements to investigate a fixed precursor concentration with two different initial droplet sizes confirms instantaneous droplet-size-dependent effects on the disruption mode.