Dynamics of a deformable compound droplet under pulsatile flow

Abstract

The motion of a deformable compound droplet flowing within a confined channel in response to a pulsatile flow is numerically investigated. Toward this, an in-house solver based on the level-set method is developed. It is shown that a low Strouhal number leads to significant oscillation in flow velocity and a greater deformation of the compound droplet, driving it periodically along the streamwise direction resulting in an increased residence time, as compared to steady imposed flow. Conversely, high Strouhal number results in smaller amplitude oscillations in flow velocity leading to smoother droplet motion and lower deformation. Expectedly, a decrease in the pulsatile flow amplitude yields smoother and more uniform flow with reduced droplet deformation. Importantly, hitherto unreported cross-stream migration of an off-centered compound droplet under pulsatile flow is investigated. An off-centered compound droplet migrates toward the center of the channel, with its core being eccentric toward the wall it was initially close to. The influence of other key parameters such as density ratio, size ratio of the core, and the viscosity ratio of the shell in shaping the dynamics of the compound droplet is systematically explored. It is revealed that the eccentricity of the core can be manipulated by tuning inlet flow and density of core fluid. A more viscous shell results in a marginally higher film thickness near the wall and a higher droplet residence time within the channel. A higher Bond number results in a larger droplet deformation, resulting in a streamlined shape and significantly higher droplet speed.