Presentation: 2025 ND EPSCoR Annual conference 

October 21, 2025, NDSU Memorial Union, Fargo, North Dakota

Neutrally Buoyant Immiscible Drop Dynamics in Collapsible Vessels: An Experimental Study

The formation and transport of neutrally buoyant, immiscible drops in collapsible vessels play a critical role in cardiovascular therapies, biomedical procedures, and engineering applications. The flexibility of biological vessels makes them prone to deformation, such as buckling or collapse, when pressure imbalances occur across the vessel wall. In the human body, collapse can arise from posture changes, gravity induced fluid shifts, muscle contractions, or aneurysm bulging. Studying neutrally buoyant drop dynamics in collapsible tubes is also relevant to space physiology, where fluid transport must be understood under buoyancy free conditions. Applications span drug delivery, diagnostic devices, endovascular embolization, thrombus and cell transport, spacecraft fluid management, nutrient delivery in bioreactors, and cryogenic fluid handling. While microfluidic studies of drop dynamics are extensive, challenges related to larger drops in deformable vessels remain underexplored. In this work, we experimentally investigated immiscible drop formation and transport in a thin-walled tube model, focusing on intermediate Reynolds numbers 〖(10〗^1 to 10^3), characteristic of human venous flow. A neutrally buoyant system was achieved using ECO-702 oil in a 73:27 water–glycerin mixture (density ratio ~1:1, viscosity ratio 21:1). High-speed imaging, ultrasound flowmetry, and pressure transducers were employed. Results show that in undeformed tubes, the internal Weber number (Weᵢ) governs jet length and drop volume, while the external Capillary number (Caₒ) controls drop size and transport speed. Dripping regimes appeared at low Weᵢ and jetting at higher Weᵢ, with Caₒ promoting smaller, faster drops and chaotic behavior. Collapsed tubes significantly altered drop dynamics, with p