Paul Stevenson

Measurement of bubble size distribution in a gas–liquid foam using pulsed-field gradient nuclear magnetic resonance

Pulsed-field gradient nuclear magnetic resonance, previously used for measuring droplet size distributions in emulsions, has been used to measure bubble size distributions in a non-overflowing pneumatic gas-liquid foam that has been created by sparging propane into an aqueous solution of 1.5g/l (5.20mM) SDS. The bubble size distributions measured were reproducible and approximated a Weibull distribution. However, the bubble size distributions did not materially change with position at which they were measured within the froth. An analysis of foam coarsening due to Ostwald ripening in a non-overflowing foam indicates that, for the experimental conditions employed, one would not expect this to be a significant effect. It is therefore apparent that the eventual collapse of the foam is due to bubble bursting (or surface coalescence) rather than Ostwald ripening. This surface coalescence occurs because of evaporation from the free surface of the foam. An analytical solution for the liquid fraction profile for a certain class of non-overflowing pneumatic foam is given, and a mean bubble size that is appropriate for drainage calculations is suggested.

The Influence of Stirring upon the Adsorption of a Cationic Surfactant onto Activated Carbon Particles

The adsorption rate of the cationic surfactant cetyl trimethylammonium bromide (CTAB) onto particles of activated carbon has been determined from measurements of the changes in the conductivity of the supernatant solution. This technique exhibited a high sampling frequency which enabled detailed analysis of the adsorption kinetics. It was demonstrated that the adsorption rate was generally positively correlated with the stirring rate, although the effect was small. The adsorption rate could be modelled by 1.3-order kinetic equations; first-order kinetic treatment only approximately described the process, but gave a better description than second-order kinetics. The hypothesis that adsorption was controlled by the intra-particle diffusion mechanism was not supported by the data. In addition, the technique enabled the construction of adsorption isotherms of ionic surfactants onto particles. In this case, the adsorption equilibrium was described by a Langmuir isotherm.