Design and implementation of an experimental device by the pendant drop method for measuring surface tension of pulmonary fluid

Abstract The quantitative measurement of pulmonary fluid surface tension in many lung diseases including respiratory distress syndrome (RDS), pneumonia, COVID-19 and acute respiratory distress syndrome (ARDS) has great physiological and clinical significance. The conventional techniques like film balances and bubble methods, however, are hard to use in the clinical practice due primarily to a limitation of small sample volume. In this work, we developed an experimental device based on the pendant drop method to clinically measure surface tension of pulmonary fluids in ARDS patients. We provided a detailed discussion of the theoretical background to the design of hardware and software as well as validation of measurement precision. The measured surface tensions of the two test liquids, pure water and dimethyl silicone oil, are 72.35±1.00 mN⁄m and 21.38±0.61 mN⁄m, respectively, in good agreement with the corresponding literature values of 72.02 mN⁄m and 21.10 mN⁄m at room temperature of 25℃.Using this experimental setup, we tentatively measured surface tensions of four pulmonary fluid samples from several mechanically ventilated ARDS patients. The results showed that the measured surface tension at 25±1℃ is within the range of 23.8～53.9 mN/m, comparable with reported values in the literature. Further, the calculation of the Worthington number showed that the data obtained are sufficiently accurate. We came to the conclusion that the pendant drop tensiometry is a powerful tool in the studies of pulmonary fluid surface properties due to a small sample size, high degree automation and freedom from contamination.