An examination on the validity of the assumptions commonly made in dynamic surface tension measurement using a pendant bubble

The assumptions commonly made in dynamic surface tension (DST) measurement using the pendant bubble method are examined. A pendant bubble changes its shape in response to DST due to surfactant adsorption. The shape change induces motion in the bulk fluid during the relaxation of surface tension. Three assumptions motivated by which are: (i) negligible influence of the bulk fluid motion, (ii) uniform surface concentration along the bubble surface, and (iii) negligible convective effects in surfactant mass transfer. The first two are related to the validity of the use of the Young-Laplace equation for DST measurement, so that DST measurement can be conducted under the conditions of keeping a bubble nearly hydrostatic and of a constant surface tension at each instant in time. The last assures the transport process to be diffusion-dominated, so that genuine kinetics can be examined without being mediated by the bulk convection. This study invokes a dimensional analysis to inspect these assumptions via identifying relative importance among relevant effects. Experimental data are provided for justification. Criteria are not only established in line for validating these assumptions, but also guide in the choice of appropriate experimental conditions for conducting DST measurement.