Drainage-induced spontaneous film climbing in capillaries

This study reports the ability with which surface tension gradients are formed plainly by the drainage in a capillary containing a surfactant-laden liquid slug initially held in place by a vacuum. Provided that a thin film forms on the walls transverse to the downward flow, the drainage induces a surfactant gradient (i.e., Marangoni effect), which then leads to a film-climbing event against gravity. The overall climbing effect is limited by the capillary rise height (i.e., propensity for infiltration due to surface tension) and the surfactant gradient formed postmeniscus drainage, thus revealing a twofold role of surface tension in gravity-oriented capillaries hitherto unexplored. The interplay of mechanisms influencing the climbing films include surfactant kinetics, diffusion, and advection of surfactants.