04. Boiling and Multi-Phase Flow

It is well known that millimeter-size bubbles show two velocity curves depending on the purity of water, i.e., they draw a fast-velocity curve in clean water while a significant velocity reduction takes place by addition of surfactant. The fully-contaminated condition is usually defined as the condition for which the rise velocity does not change any more even with further increase in the surfactant concentration C, and that state has been thought to be reached at concentrations below CMC (critical micelle concentration), e.g., even with a slight change in the surface tension, sigma, the stagnant-cap state formed due to high Peclet number makes the velocity of a spherical bubble reduced to that of a solid particle. A question arises: whether a further increase in concentration beyond CMC results in a lower velocity even after the significant velocity reduction. We carried out experiments on deformed bubbles in water contaminated with Triton X-100. The terminal velocity and aspect ratio of bubbles were obtained by processing high-speed images. The terminal velocity was largely reduced at C = 0.10 and 10 mmol/m3 (< CMC (= 240 mmol/m3)), and the velocity curve agreed with the well-known curve for contaminated system. However, with C beyond CMC, the terminal velocity was further decreased, while the velocities at C = 250, 10,000 and 20,000 mmol/m3 were the same, meaning that the bubbles were in their fully-contaminated state. On the Re-Eo plane, the Eötvös number of which was calculated by sigma for CMC, the bubble Reynolds numbers in the fully-contaminated state agreed with those in a higher viscous/lower surface tension system with the Morton number same as that of water calculated with sigma(CMC). Therefore, bubbles were considered to be fully covered by surfactant and behaved as if they were in the higher Mo system.

Prof.

Shigeo Hosokawa