Numerical analysis of binary molten glass microspheres droplets co-directional collision behavior

The co-directional collision behavior of molten glass microspheres plays a crucial role in determining their sphericity and physical properties. Previous studies have predominantly examined head-on collisions of counter-moving droplets. However, the mechanisms governing co-directional collisions remain inadequately understood. To bridge this gap, a three-dimensional numerical model was developed using the open-source solver Basilisk, which incorporates the volume-of-fluid (VOF) method with adaptive mesh refinement (AMR), to simulate molten glass microsphere collisions. In comparison with experimental observations, the effects of velocity ratio and radius ratio on co-directional collision dynamics were systematically investigated through detailed analyses of droplet morphology evolution, velocity fields, and pressure distributions. Five distinct collision regimes are identified: (Ⅰ) bouncing, (Ⅱ) coalescence, (Ⅲ) reflexive separation without satellite droplets, (Ⅳ) reflexive separation with a single satellite droplet, and (Ⅴ) reflexive separation with an ejected satellite droplet. The results demonstrated that increasing the velocity ratio shifts collision outcomes from bouncing or coalescence to reflexive separation. Satellite droplet formation was observed only at high velocity ratios when the radius ratio approaches unity, whereas a significant droplet size disparity promoted coalescence. Additionally, the transformation between kinetic energy (KE) and surface energy (SE) before and after collision was analyzed. The findings revealed that KE was initially converted into SE upon impact and subsequently partially transformed back into KE during axial stretching. A regime map was constructed to classify collision outcomes, highlighting the pivotal role of velocity and radius ratios in regime transitions. This study provides theoretical insights to support the optimization of molten glass microsphere production processes.