Collision dynamics and deformation behaviors of multi-core compound droplet pairs in confined shear flow

In this study, we numerically investigate the collision dynamics and deformation behaviors of double-core compound droplet pairs in confined shear flows using a lattice Boltzmann method. Although droplet–pair interactions have been widely studied for simple droplets, the collision behavior of compound droplets, especially those containing one or more core droplets, remains largely unexplored. We address how the interplay between physical parameters (i.e., density ratio, viscosity ratio, and Capillary number) and geometric parameters (i.e., initial offset between droplets) influences the collision process and its outcomes. Our findings reveal that inner droplets strongly affect the deformation and stability of shell droplets, and the collision outcomes of both shell and core droplets. We identify several distinct collision outcomes: (i) shell–droplets' coalescence with separated core droplets undergoing planetary-like rotation, (ii) shell–droplets' pass-over with separated cores showing both rotational and translational motion, (iii) core–droplets' coalescence while shell droplets pass each other, and (iv) shell–droplets' pass-over accompanied by coalescence of core droplets. We show that transitions among these outcomes are governed primarily by Capillary number and initial offset. Channel confinement also plays a decisive role by strengthening wall-induced hydrodynamic coupling, prolonging the near-contact between the droplets and promoting coalescence of both shell and core droplets even at relatively large offsets. Under stronger confinement, transient post-merger oscillations may also arise because of intensified shear and restricted flow circulation. In addition, increasing the density and viscosity ratios from unity to higher values consistently leads to shell–droplet pass-over, while the core droplets remain separated and experience rotational motion.