Spherical Particle Formation Mechanism in Pulsed Laser Melting in Liquid under Controlled-Pulse-Number Irradiation using a Slit Nozzle Flow System

A detailed mechanism of pulsed laser melting in liquid (PLML) has remained a controversial issue because of the difficulty associated with tracking and observing particles irradiated with a specific number of pulses in conventional batch-style irradiation. In a previous study, we developed a flow irradiation technique with a new slit nozzle for PLML mass-production. The slit nozzle can precisely control pulse numbers irradiated onto particles flowing through the slit nozzle. In the present study, this unique feature of the slit nozzle was used to clarify the PLML mechanism by a specific pulse number irradiation. According to a numerical analysis of the flow with the slit nozzle, 79% of the particles flowing through the slit nozzle were irradiated with two or three pulses and other particles that were irradiated with three or more pulses upon one passage through the slit nozzle at a 1.1 mL s(-1) volume flow rate for a particle suspension in 60 wt % glycerol aqueous solution. Sub-micrometer-sized hollow spherical particles were formed via few-pulse irradiation of aggregates. After hollow particles were formed, the void in the hollow particles disappeared with increasing laser pulse number. Thus, an initial process of spherical particle formation and morphological transition of the particles with a specific irradiated pulse number were observed for the first time using the slit nozzle. The reduction of Fe3O4 particles via laser pulse irradiation was also tracked as a function of flow passage.