Captive Bubble Tensiometer for Evaluating Time-Dependent Interfacial Behavior of Polymeric Pulmonary Surfactants

The pulmonary surfactant system plays a critical role in maintaining alveolar stability by reducing the air-liquid interfacial tension during respiration. Recently, synthetic surfactants made from amphiphilic polymers have emerged as promising pulmonary surfactant candidates. For the development of advanced polymeric surfactants, a reliable platform for quantitatively evaluating their time-dependent interfacial behavior is needed.

In this study, a captive bubble tensiometer was developed to assess the time-dependent interfacial behavior of polymeric surfactants. A captive bubble was formed in a cuvette at a fixed bubble volume, and its shape evolution was continuously recorded using a camera. Time-resolved image frames were extracted and analyzed with OpenDrop to obtain interfacial tension and surface area data. From these data, the surface area compression ratio was calculated to quantify the degree of interfacial compression. Dynamic interfacial tension was further analyzed as a function of surface age to characterize the relaxation of the surfactant monolayer at newly formed or shrunken interfaces.

Model polymer surfactant nanoparticles (NPs) were prepared from a PS-PEG diblock copolymer via the Equilibration-Nanoprecipitation (ENP) method, and NP size was controlled by adjusting the co-solvent to solvent ratio. Because the PS core of these NPs is kinetically frozen, monomer-micelle exchange does not occur, enabling precise size control from a single polymer system. Given that the structural deformation of interfacially adsorbed NPs governs surface tension relaxation at dynamic interfaces, NP size may influence the time-dependent interfacial tension response after bubble contraction. The developed platform provides a quantitative approach for comparing the dynamic interfacial properties of polymeric surfactant systems and for evaluating their potential in pulmonary surfactant applications.