A laminar-jet-discharging method for measuring the interfacial tension of deformable surfaces

Abstract

We propose a laminar-jet-discharging method to measure the interfacial tension of deformable surfaces such as soap bubbles. This method avoids the need to measure the small pressure difference inside and outside a soap bubble in a static state. By allowing the air in a soap bubble to discharge in a laminar flow state through a long tube, we show that the surface tension of the soap bubble is proportional to the rate of change of the fourth power of the bubble radius. Experimentally, we verify that this linear relationship is valid over a long period and thus can be measured with cameras at slow recording rates. Our method offers a straightforward and accurate way to measure the surface tension of soap bubbles using easy-to-obtain devices. In addition, we propose a new development of the pendant drop method based on the silhouette of the drop, which does not require any pressure transducer or computation of the second-order derivatives of the drop profile and hence is easier to implement and less sensitive to the accuracy of the drop profile determination. For pure liquids, results from the new pendant drop method compare well with standard values. This method is thus used to verify the laminar-jet-discharging measurements.