We have experimentally examined the effects of a common soluble surfactant on gas
bubbles in liquid flows in inclined tubes. Air bubbles of known size (λ = 0.8, 1.0, 1.5)
are held stationary under minimum flow conditions in tubes inclined at fixed angles
(ω = 25°, 45°, 65°, 90°). Sodium dodecyl sulphate (SDS) is infused into the bulk flow
at two bulk concentrations (C = 10% or 100% critical micelle concentration (CMC)).
In addition to recording pressure and flow waveforms, we capture video images of
bubbles before and during exposure to the surfactant. Modification of the interfacial
properties by the surfactant results in extremely dynamic bubble behaviour including
interfacial deformation, deformation plus axial translation, and bubble detachment
from the wall plus translation. We measure the corresponding time-dependent pressure
gradient within the tube. The surfactant mediated responses observed are dependent
upon the interrelated effects of C, λ and ω. A high bulk concentration of surfactant
may produce more rapid modification of bubble shape and influence wetting, thus
increasing the potential for bubble detachment. The likelihood that detachment will
occur increases further as bubble volume in increased. In both vertical tubes in which
contact forces are absent and in non-vertical tubes, the infusion of surfactant may
result in axial translation either in the direction of, or opposite to, the direction of
the bulk flow. Critical to the translation and/or detachment of the bubble is the
surfactant-mediated modification of contact line mechanics. Contact line velocities
corresponding to rates of shrinkage of dewetted surface area are extracted from
experimental data. We also explore the potential effects of surfactants on interfacial
remobilization. This investigation demonstrates the potential use of surfactants to
be used for dislodging dewetted gas bubbles by the intentional manipulation of
interfacial properties.