Dynamics of a Deformable Compound Bubble within a Microchannel

Microfluidics has enabled the synthesis of uniformly sized compound bubbles with precise control over the gaseous core size, liquid shell thickness, and the number of encapsulated cores. These compound bubbles find applications in various fields, including diagnostics and therapeutics, microextraction, and the fabrication of gas-filled capsules and porous particles used in catalysis and material science. With the increasing interest in compound bubbles, this study presents, for the first time, a numerical analysis of the dynamics of a deformable compound bubble within a microchannel under different inflow conditions: (i) steady Poiseuille, (ii) oscillatory, and (iii) pulsatile flows. An in-house solver, based on the Level Set method, is employed to model this three-phase system. At low Strouhal numbers, St, the compound bubble experiences significant deformation, accompanied by enhanced mixing due to the presence of high-magnitude positive and negative velocities, resulting in a shorter residence time within the channel. In contrast, a higher St promotes a more plug-like flow with minimal deformation and mixing. Another novel aspect of this study is the numerical investigation of dual and triple-core compound bubble dynamics. When cores are positioned at the channel center, they migrate to the leading edge of the shell. Conversely, cores located away from the centerline follow the shell fluid streamlines and ultimately become trapped at the rear of the shell. Further, the study reveals that the presence of an outer shell in compound systems mitigates the effects of core properties, resulting in similar dynamics for compound droplets and bubbles under both steady and pulsatile inflows at smaller core holdup. This suggests that, despite substantial differences in density and viscosity, compound bubbles and droplets exhibit similar core and shell shapes, translational velocities, and residence times.