Head-on impact-driven coalescence and mixing of drops of different polymeric materials

In this paper, we employ direct numerical simulations for studying the dynamics of two different, but miscible polymeric drops undergoing impact-driven coalescence with one of the drops making a head-on collision on an underlying drop. We consider two separate cases: (1) a PMMA (poly-methylmethacrylate) drop impacting a PVAc (polyvinyl acetate) drop and (2) a PVAc drop impacting a PMMA drop. For each case, three separate Weber numbers (We = 0, 1, 10) are considered. The interplay of finite impact speed and the specific wetting characteristic of the underlying drop leads to fascinating fluid dynamics. For example, for the case of the PMMA drop impacting on the PVAc drop, the strong wetting behavior of the PVAc drop ensures the impact-driven formation of an accelerated disk consisting of separate layers of PMMA and PVAc. Also, there is the generation of significant capillary waves at the drop-air interface, which in turn causes significant undulations of the drop-drop interface created due to such head-on collision. On the other hand, for the case of the PVAc drop impacting on the PMMA drop, the relatively weaker wetting behavior of the PMMA drop ensures that there is no formation of an accelerated disk; rather (for large We), the PVAc drop flows on top of the PMMA drop, forms a "canopy-like" structure, and wets the solid forming a transient core-shell structure (with PMMA core and PVAc shell). Our results raise the possibility of fabricating highly complicated multi-polymeric-material components by rapid curing of the drops in their post-impact configurations.