Stability of microbubbles prepared by co-axial electrohydrodynamic atomisation.

Previous studies have indicated that microbubbles prepared by co-axial electrohydrodynamic atomisation (CEHDA) are less stable than those prepared by other methods such as sonication and microfluidic techniques. The aim of this investigation was to determine the reasons for this observation and how this might be addressed in future work. Microbubbles were prepared by CEHDA using (i) a glycerol-air system, (ii) a glycerol-Tween 80-air system and (iii) a glycerol-zirconia-air system and also by simple agitation of (i) and (ii), in order to compare the effect upon the dissolution rate of microbubbles of different materials and processing methods. Both theoretical examination and the experimental results indicated that all three quantities were important in controlling the rate of microbubble dissolution, namely: surface tension at the gas/liquid interface, the effective diffusivity of gas through this interface and the initial concentration of gas dissolved in the surrounding liquid. However, it was the difference in gas concentration in the surrounding liquid that was indicated as the primary reason for the differences in stability observed with different processing methods. It was concluded, therefore, that improved stability could be achieved for microbubbles prepared using CEHDA by saturating the collecting fluid with gas and/or maintaining a high concentration of microbubbles during collection.