An analysis of the sensitivity of pendant drops and liquid bridges to measure the interfacial tension

Abstract

Drop shape techniques, such as axisymmetric drop shape analysis, provide accurate measurements of the interfacial tension from images of pendant drops for a wide variety of experimental conditions. However, these techniques are known to fail when dealing with nearly spherical drop shapes, which may occur, for instance, when working with interfaces between liquids of similar densities and/or under microgravity. We analyzed the advantages of using liquid bridges close to the minimum volume stability limit instead of pendant drops to measure the interfacial tension under different experimental conditions. First, the sensitivity of both configurations to a variation of the interfacial tension is studied numerically as a function of the volume for several Bond numbers B. The results indicate that a liquid bridge close to the minimum volume stability limit is generally more sensitive than a pendant drop of the same volume, especially for small values of the density difference across the interface and/or gravity. This suggests that the use of liquid bridges may extend the range of applicability of drop shape techniques. To explore this possibility, synthetic images of both pendant drops and liquid bridges were generated and then processed by TIFA-AI. The results demonstrated that the use of liquid bridges enhances the range of Bond numbers for which drop shape techniques work satisfactorily. More specifically, similar accuracy is obtained from both configurations for B ∼ 10−1, while the use of liquid bridges yields much better results for B ∼ 10−2. Finally, experiments were conducted to partially validate the analysis based on synthetic images. Good agreement was found between the values determined from the real and synthetic images.