The captive bubble method for the evaluation of pulmonary surfactant: surface tension, area, and volume calculations.

For measuring the properties of lung surfactant, we provide formulas for calculating the surface tension, area, and volume of captive air bubbles in aqueous media. Only measurements of bubble height (h) and diameter (d) are required. Data processing has been automated using standard video capture hardware and software, and our own image-processing and -analyzing programs. Our polynomials in h/d describe the ratios of actual bubble surface area and volume to that of a spherical bubble having the same diameter. For surface tension, a polynomial in h/d describes the ratio of the surface tension of a flat semi-infinite bubble to the surface tension of the measured bubble of the same height. Coefficients for the area and volume polynomials were obtained from h and d, and measured areas and volumes of revolution of calibrating bubbles. Coefficients for the surface tension polynomial were obtained analytically from a published polynomial in h/d [3]. Results using these polynomials agree satisfactorily with those obtained independently [4] using bubble perimeter measurements.