Understanding the transport of confined drops through constricted microcapillaries is important for a variety of applications ranging from oil recovery to blood flows to microfluidics. A confined droplet flowing through a conduit can either be arrested at the constriction or squeeze through it. The dynamics of the trapped and squeezed states are expected to depend on capillary number, drop size, viscosity ratio and the conduit cross-section. However, comprehensive investigations of the dynamics of transport of a confined drop in constricted capillaries are lacking.
In this work, we studied trapping and squeezing of confined drops through a constriction using volume-of-fluid based numerical simulations. Two different geometries with circular and square cross sections were examined. Simulations were conducted for both constant flow rate and constant pressure inlet boundary conditions. In order to understand the physics behind these processes, a wide range of capillary number, viscosity ratio and drop size were investigated.
Our results show that both the trapped and squeezed states can exist in the circular and square cross-section channels. When constant inlet flow rate is used, the pressure upstream of the droplet increases, the magnitude of which depends on the capillary number. We find that when this pressure rise exceeds the Laplace pressure at the back-end of the droplet, it squeezes through. Counter-intuitively we observe that high-viscosity drops even though less deformable than low-viscosity drops, they squeeze through at a lower capillary number in both square and circular geometries. To probe the underlying mechanism, we analyze the stress distribution and kinematics both within the drop, the lubricating films and the gutters. Finally, we conduct experiments to test the key results of the numerical simulations.
See more of this Group/Topical: Engineering Sciences and Fundamentals