Abstract
To study the details of surface renewal of a clean and a film-covered liquid surface, a new technique using weak radiotracers has been developed. A small slug of radioactive Na 2SO 4 ( 35S) is injected below the surfaces. The β-particles have a range of 300 μm in water, so a Geiger counter positioned just above the surface records only the 35S which arrives (and departs) within this depth of surface layer. Measurements were obtained for mass transport by a slow (0.017 m s −1) water stream directed vertically on to an air-water surface from below. When a surface film was present, the appearance of tracer in the surface layer was significantly delayed compared to the transport to a clean water surface. The results show that the clean surface was influenced predominantly by surface renewal, whereas at the monolayer covered surface diffusion effects were much more significant. Capillary rippling was found to enhance transport, with this effect progressively suppressed by increasing strengths of surfactant, which also suppressed convective transport in unstirred systems.
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