Hydrodynamic Behavior of Droplet Pair Collision in Confined Shear Flow, and the Role of Physical and Geometric Parameters.

Gaining a comprehensive understanding of the dynamics involved in the collision of droplets under confined shear flow holds great significance for a diverse array of industrial processes and natural phenomena. The interplay between physical parameters and system geometry exerts a substantial influence on the stability and various attributes of the droplets, including their deformation, trajectory, and final state. We investigate computationally the mutual influence of the density ratio (60 to 800), viscosity ratio (24 to 60), initial offset, and confinement on the coalescence behavior of droplet pairs within a confined shear flow. The free-energy-based lattice Boltzmann method was employed, aiming to identify new regimes beyond the scope of earlier studies. The findings highlight the pivotal role played by density ratios and viscosity ratios in generating inertia and viscous interaction forces, which have profound implications for the resulting coalescence consequences. Meanwhile, the confinement imposed by walls and the initial vertical offset of droplets are shown to be critical in either promoting or suppressing different collision modes. In continuation, we have expanded the simulation to include compound droplets, where one droplet encapsulates another, serving as a model for core-shell structures. This has also yielded some enlightening preliminary results. The findings are expected to offer novel insights into the collision dynamics of droplet pairs and their underlying conditions.