Lung surfactants are critical for mammalian respiration, reducing surface tension at the alveolar air-liquid interface to prevent collapse. While most studies use Langmuir trough (planar geometry) to probe surfactant behavior, alveoli are inherently spherical with typical radii of ~ 100 µm. Here, we investigate the adsorption dynamics and phase morphology of Survanta, a clinical replacement lung surfactant, on micrometer-sized air bubbles using a custom microtensiometer and high-speed confocal fluorescence microscopy. By simultaneously tracking surface tension changes and interfacial morphology during adsorption, we identified coexisting liquid-ordered (LO) and liquid-disordered (LD) phases in the absence of cholesterol—an observation that, to our knowledge, has never been previously reported. We confirmed a curvature-driven circle-to-stripe transition in LC domains on bubbles (R < 100 µm) and validate a scaling law that relates average stripe width w, to bubble radius R, line tension λ, and Young’s modulus Y, consistent with theoretical predictions after factoring the anisotropic bending energy cost into the net free energy change, w ~ R^(4/5)(λ/Y)^(1/5). These findings provide new insights into understanding monolayer softness, particularly for lung stability.
