Numerical Investigation of the asymmetric coalescence of spreading dissimilar microscopic drops

Interaction of dissimilar drops is encountered across various fields of science with a wide range of applications including additive manufacturing, photonic crystals, lab-on-a-chip, etc. Despite the ubiquitous nature of such interactions, the dynamics of such multi-material coalescence of non-Newtonian polymeric drops is not well explored. Understanding such multi-material coalescence - which often involves spreading, coalescence, and mixing- is crucial in understanding the post-deposition kinetics of droplet-based multi-material direct ink writing processes. In this study, we probe the coalescence dynamics of two different polymeric, shear-thinning, power-law obeying micrometric drops using Direct Numerical Simulation (DNS). We observe non-monotonic migration of the liquid-liquid mixing front of the miscible coalescing drops over time which is influenced by the spreading, diffusive, and Marangoni effects. We quantify the growth of the mixing front and the mixing region which shows distinct regimes depending on whether the mixing is diffusion-limited or Marangoni convection-driven. These results would provide critical insights for the design of miscible multi-material systems.