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.

Measurement of surface and interfacial tension using pendant drop tensiometry

“…The concept of measuring contact angle and interfacial tension simultaneously is appealing for many applications, particularly where fluids are being selected for (micro) fluidic handling applications. Recent work has extended the pendant drop technique to enable measurement of contact angle, by the introduction of a secondary solid surface in the form of a flat substrate [23][24][25] or particle [26]. In certain circumstances, this has the added benefit of applying a secondary force that allows measurement of interfacial tension at low or even zero Bond numbers [26].…”

“…More recently, the pendant drop method has been further developed to include a wider variety of drop configurations, from capillary bridges between two parallel plates [23][24][25] to compound pendant drops [26] formed when a spherical particle is attached to a pendant drop. These configurations have provided a significant extension to pendant drop tensiometry as they enable the method to accurately measure interfacial tensions for Bond numbers as low as zero [26].…”

Effects of surface-active impurities on the liquid bridge dynamics

“…When working with liquid bridges the experimenter has at his disposal an additional adjustable parameter, the slenderness, to enlarge the range of experimental conditions. In addition, the liquid bridge shape remains sensitive to the surface tension value even in the minimum volume stability limit, while drops become spherical for small enough volumes (Ferrera et al 2007). When illuminated with backlight, millimeter liquid bridges act like optical lenses that deviate the light beam.…”

Laplacian drop shapes and effect of random perturbations on accuracy of surface tension measurement for different drop constellations

“…Depending on specific purpose or given experimental constraints, different experimental constellations are considered: pendant drops (PD) [1][2][3][4][5][6][7][8], sessile drops (SD) [9][10][11][12][13][14], constrained sessile drops (CSD) [15][16][17][18][19], captive bubble (CB) [20][21][22][23] and liquid bridges (LB) [24,25] are of interest. The most commonly used of these are pendant drops and unconstrained sessile drops (SD).…”