Bubble detachment by diffusion-controlled surfactant adsorption

The effect of the diffusion-controlled adsorption of the surfactant Triton X-100 on detachment of gas bubbles adherent to the wall in pipe flow is studied. Air bubbles of known volume (scaled bubble dimension lambda = 0.8, 1.0, 1.5) are maintained in tubes inclined at 25degrees, 45degrees, 65degrees, or 90degrees. Triton X-100 is infused into the bulk flow to achieve a final mixed bulk concentration of either 10% or 100% of the critical micelle concentration. Pressure and flow waveforms are recorded, and video images of bubbles before and during exposure to the surfactant are captured. Surfactant-induced reductions of the surface tension and contact angle lead to distinct regimes of dynamic bubble behavior including static and oscillatory interfacial deformation, deformation with axial displacement, and complete bubble detachment from the wall. The surfactant-mediated responses depend on the inter-related effects of Triton X-100 concentration, bubble size, and tube angle. A high bulk concentration of Triton X-100 produces rapid changes in bubble shape and promotes wetting, increasing the potential for bubble detachment. Bubble detachment occurs more readily with larger bubble volume. For vertical tubes (i.e. no contact forces present) and non-vertical tubes (i.e. contact forces present), the bubble may be displaced axially in either the direction of or opposite to the bulk flow direction following introduction of Triton X-100. Bubble axial motion and detachment result from the surfactant effects on contact line mechanics. For moving contact lines, the contact line velocities corresponding to rates of shrinkage of de-wetted surface area (or, alternatively, rates of gas-liquid interfacial dilation) along with time-dependent contact front length are deter-mined from the experimental data. Potential effects of Triton X-100 on interfacial remobilization are described. Soluble surfactants have the potential to be used for dislodging gas bubbles adherent to a pipe wall by intentional manipulation of interfacial shape and wetting properties. (C) 2003 Elsevier B.V. All rights reserved.