An investigation has been made of a technique in which bubbles of nitrogen gas were formed simultaneously at adjacent orifices in aqueous solutions of potassium chloride, ethanol and n-octanol. The bubble coalescence time, defined as the time between the first touching of the bubbles until they coalesce, was measured by high speed cinephotography or video tape recording. The film formed between the bubbles increases in radius steadily with time. It was found that coalescence was markedly affected by solute concentration, gas flow rate and temperature. Excellent reproducibility in coalescence times was observed. With potassium chloride solutions, it was found that the coalescence time increased with increasing concentration until a fairly sharply defined concentration (500 mol/m ) was reached, above which the bubbles detached from the orifices without coalescing. With ethanol solutions, coalescence time increased with increasing concentration. Coalescence was not completely inhibited at the lower gas flow rates at any concentration. With n-octanol solutions, coalescence time increased sharply with increasing concentration above 7x10-3 mol/m3, and coalescence was completely inhibited above a concentration of 11x10-3 mol/m. The experimental observations have been discussed in terms of a two-stage mechanism of coalescence. In the first stage rapid stretching of the film formed between the bubbles, produces a surface tension gradient, at the periphery; stretching is arrested when this surface tension gradient is just balanced by the pressure in the film. For non-volatile solutes, in the second stage the film continues to grow in area, but the fresh film -formed at the periphery becomes progressively thinner because the proportion of liquid which escapes by viscous flow increases with increasing film radius. However, for volatile solutes after the stretching process has halted, the coalescence time of the film is determined by the supply of solute to the surface by gas phase diffusion. Decrease in coalescence times at elevated temperatures is due to decreases in surface tension, viscosity and an increase in vapour pressure.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:635858 |
| Date | January 1983 |
| Creators | Ahmed, M. |
| Publisher | Swansea University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.0092 seconds