Probing the effect of water-soluble fluorescent surfactant on complex multi-phase annular flows using structured planar laser-induced fluorescence*

We employ a novel optical diagnostic technique, structured planar laser-induced fluorescence (S-PLIF), developed in-house, in conjunction with a bespoke capacitance probe, to study the flow characteristics of downwards air-water annular flows. The film thickness, temporal characteristics of interfacial waves, and gas entrainment in thin annular films are examined. Simultaneous application of S-PLIF and the capacitance probe provides an opportunity to study the subtle effects of surfactants on the interfacial dynamics. The role of Marangoni stresses brought about by gradients in the surfactant interfacial concentration is highlighted, and the mechanisms underlying the observed phenomena elucidated.