Bubble dynamics and crystallization of ammonium perchlorate from its aqueous droplets during impacting on heated surface

The evaporation and crystallization characteristics of ammonium perchlorate (AP) aqueous solution droplets during impact on heated stainless-steel surfaces have been experimentally explored using a high-speed imaging method. It aims to elucidate the effect of experimental conditions on the quality of core–shell AP@Al particles prepared by the fluidized bed drying method. By varying the surface temperature (20 ∼ 400 °C), AP concentration (1 ∼ 10 wt%), and Weber number (10 ∼ 70) of the solution, their effects on bubble nucleation, growth, rupture, and solute precipitation have been systematically analyzed. Results reveal that the elevated surface temperatures enhance the bubble growth rates and secondary droplet atomization. Spatial distribution and morphology of AP crystals during evaporation were characterized through different digital imaging methods supported by scanning electron microscopy (SEM). Three typical distributions and morphologies were observed: the regular AP shells formed at lower temperatures, coffee-ring AP distributions at higher temperatures with low concentrations due to capillary flow, and multi-shell morphologies emerging at high concentrations due to intensive bubble generation and concurrent AP crystallization. The crystallization start time of 1 wt% AP droplet decreased from 477.6 s at T_w = 40 °C to 24.0 s at T_w = 120 °C. At T_w = 40°C, the crystallization start time decreased from 477.6 s for 1 wt% AP to 239.8 s for 10 wt% AP droplet. Both the increase in surface temperature and AP concentration lead to faster and earlier crystallization of AP and thereby eliminate the stick–slip phenomenon during droplet evaporation. Quantitative analysis shows a linear reduction in crystallization start time with increasing AP concentration. Increased Weber numbers shortened evaporation time by altering droplet contact diameter and area.