Experimental Investigation of Immiscible Liquid Drop Dynamics in a Collapsible Tube

The formation and transport of immiscible liquid drops in large, thin-walled vessels are crucial for cardiovascular flow phenomena, such as thrombus transport, and endovascular treatments like portal vein embolization. Physiologically, thin-walled vessels are highly flexible and can buckle or collapse when subjected to imbalances in transmural pressures, such as muscle and hydrostatic pressure changes. Despite extensive research on microfluidic droplet formation, the dynamics of large drops in collapsible vessels under naturally constricted or collapsed conditions remain underexplored. This study experimentally investigates the formation and transport dynamics of immiscible liquid drops in deformable thin-walled tubes. A co-flow liquid-in-liquid injection system generated immiscible silicone oil drops within a continuous phase of glycerin and water mixtures, ensuring neutral buoyancy as drops flowed through distended and constricted tube cross-sections. High-speed camera footage captured drop motion, while pressure and flow rates were monitored. Results indicate that drop size, spacing, and the transition from dripping to jetting regimes depend on both the external Capillary number (ratio of viscous force to surface tension) and the internal Weber number (ratio of inertial force to surface tension). Vessel deformation and collapse further constrict drop transport, altering drop geometry, average diameter, and transport efficiency.