Experimental investigation of immiscible drop formation and transport in collapsible tubes at intermediate Reynolds numbers

Immiscible drop transport in compliant, thin-walled vessels is relevant to a range of cardiovascular and biomedical applications, yet the behavior of drops at moderate Reynolds numbers within collapsible geometries remains poorly understood. This study experimentally investigates the formation and transport of neutrally buoyant immiscible drops in both undeformed and deformed elastic tubes across Reynolds numbers spanning 101–103. High-speed imaging and particle image velocimetry were employed to visualize flow regimes, drop trajectories, and velocity fields. In undeformed tubes, the Weber number (Wei) governs the dripping-to-jetting transition while the Capillary number (Cao) controls drop size, which follows a reciprocal decay relationship with the flow rate of the continuous phase. Drop velocity exhibits a nonlinear dependence on Cao and cannot be directly inferred from the mean velocity of the continuous-phase flow. At low Wei and high Cao, unsteady helical drop trajectories emerge as smaller drops sample higher-velocity regions of the parabolic velocity profile. Vessel deformation fundamentally alters drop dynamics: the collapsed geometry enables dual-channel drop transport in the dripping regime, enhances Rayleigh–Plateau instability and accelerates drop pinch-off in the jetting regime, and promotes wall adhesion and channel clogging at low Cao where surface tension dominates. A distinct plug flow regime is identified at high Wei and low Cao in the collapsed vessel, where the initial liquid plug occludes one channel and diverts the jet toward the remaining open channel. These findings provide physical insight into drop transport in flexible vascular conduits, with implications for venous thrombosis, microgravity physiology, and liquid embolization procedures.