Uncovering Taylor bubble interfacial flow with high spatial resolution velocity measurement

The dynamics of Taylor bubbles is investigated in a counter-current water flow, where the downward liquid flow in a vertical pipe counterbalances buoyancy, allowing extended steady-state observations. Using a combination of Particle Tracking Velocimetry (PTV), Laser-Induced Fluorescence (LIF), shadowgraphy, and dynamic masking, we unveil interfacial flow features, including tiny capillary waves. Velocity fields are measured for Taylor bubbles of varying lengths (1.2 D–14.9 D) in D= 12 . 4 m m diameter pipe. Upstream of the bubbles, the liquid flow is laminar with R e = 1270 whereas the bubble behavior is characterized by E o = 21 and N f = 4830 . In the liquid film region, the flow accelerates rapidly reaching up to about eight times the bulk velocity for the longest bubbles, yet no transition to turbulence is observed. The liquid film stabilizes at its final thickness and terminal velocity is reached for bubble lengths at about 5 D. Our measurements show that the bubble wake region hosts a strong primary toroidal vortex accompanied by a weaker, oppositely rotating secondary vortex. A particularly interesting is the emergence of stationary capillary waves at the bottom of the annular liquid film, likely associated to a localized widening of the liquid film region, reminiscent of a hydraulic jump where rapid flow abruptly decelerates. Velocity profiles, captured with a µ 20 µ m spatial resolution, expose significant interfacial shear, observed exclusively downstream of the stationary capillary waves while remaining negligible elsewhere. This increased shear not only explains the observed phenomena but also suggests local flow motion within the gas bubble itself.