Bimodal size distribution of primary particles in the plasma of welding fume: Coalescence of nuclei

•The subject of study is the thermal dusty plasma of welding fume, in which the particles are the result of vapor condensation.•The nucleation and growth of the nuclei are considered simultaneously with their coalescence.•As a result, at the moment of phase change, there is the bimodal distribution of primary particles.•The spherical primary particles with the average size of about 1 nm, and the spherical primary aggregates with the average size of about 10 nm.

Nucleation and growth of the nuclei in the thermal plasma of the ionized metal vapors of welding fume are studied. The coalescence of liquid droplets formed by the vapors' nucleation is calculated. There is a high flow rate of condensable atoms from the gas phase into the condensed phase caused by the nucleation and association of nuclei with the aggregated droplets. The fast association of the nuclei with aggregated droplets leads to a decrease in the condensable vapor supersaturation and to the termination of nucleation and start of the bimodal coalescence. As a result, in the beginning of phase transition, there is bimodal size distribution of the primary particles: the small-size primary particles formed by association between the nuclei (intramodal coalescence only), and the primary large-size particles formed by association between the aggregated droplets and the nuclei (intramodal and intermodal coalescence).