Collapsible tubes provide a revealing substitute for real veins

How immiscible liquid droplets form and move is relevant for vital medical applications like treatment of venous thrombosis in spaceflight physiology, or targeted embolization, in which a liquid agent is injected to block tumor blood supply. Yet most of what scientists know comes from rigid microfluidic channels operating at very low flow speeds, a poor stand-in for real veins that can flex, buckle, and even collapse.

Resan et al. used high-speed imaging and particle image velocimetry to track immiscible drops forming and traveling in a clear silicone tube meant to replicate physiological flow in large vessels. They built an experimental setup with a steady flow loop, a pressure-controlled test section, and a syringe pump and dispersed-phase fluid injection. To capture the effects of changing blood vessel shape, they conducted experiments in both a normal circular tube and one collapsed into a dumbbell shape.

“Drop formation in the normal tube is controlled by the balance between the liquid momentum, surrounding flow, and surface tension,” said author Yan Zhang. “Injected flow governs whether droplets simply drip off the needle or stretch into a jet, while the carrier flow sets droplet size, following a neat reciprocal decay as flow speeds up.”

The researchers observed entirely new transport behaviors when the vessel acquired a collapsed shape and adhesive forces at the wall boundary begin to dominate. They saw drops splitting between two separate channels, channel clogging, and a partial plug-flow regime with a large liquid plug blocking one side of the vessel, forcing flow through the other.

“Vessel compliance can’t be treated as a footnote,” said Zhang. “The flexibility of the conduit is a first-order variable.”

Source: “Experimental investigation of immiscible drop formation and transport in collapsible tubes at intermediate reynolds numbers,” by Nafis Saad Resan, Yechun Wang, and Yan Zhang, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0341019 .