An Examination on the Accuracy of Interfacial Tension Measurement from Pendant Drop Profiles

Abstract

Interfacial tension (IFT) γ of a fluid-fluid interface is determined by using a video-enhanced pendant drop tensiometry. Fitting the image data of the x and z coordinates of a drop edge to the theoretical drop profile of the Young-Laplace equation yields four parameters : the location of the apex X 0 and Z 0 , the radius of curvature at the apex R 0 , and the capillary constant B. The IFT is then determined by γ = ΔρgR 0 2 /B, where Δρ is the density difference between fluids. The accuracy of IFT measurements using the pendant drop shape technique is carefully studied by examining two systems : air-water and n-tetradecane-water-diethylene glycol monohexyl ether, corresponding to air-liquid and liquid-liquid interfaces, respectively. It is found that the measured IFT is drop size dependent. The results of analysis show that a drop or bubble with a smaller absolute value of capillary constant may have larger measurement error due to the limitation of accuracy on the edge detection of a drop image. Three ways of improvement for accurate measurement of interfacial tension are recommended.