Optical Issues for the Rapid Determination of Surface Tension using Captive Bubbles

Abstract

Bubbles and droplets provide several advantages for studies on interfacial films, particularly at the very low surface tensions achieved by pulmonary surfactant in the lungs. The captive bubbles commonly used to study pulmonary surfactant, however, also suffer from a major disadvantage. The bubbles float against a concave surface molded into an agarose gel, which obscures the location of the upper air/water interface. Methods that calculate surface tension in real time use the height of the bubble, which requires the position of the upper surface. Experiments that use feedback to maintain constant surface tension, and fixed thermodynamic conditions, require measurements in real time. The studies reported here considered how a series of optical issues affect measurements of a bubble's height and surface tension. As long as the imaged intensities remained within the dynamic range of the camera, total light intensity had no effect. Longer wavelengths were scattered less by the gel and surfactant suspensions, which increased their transparency. Misalignment of the camera and agarose gel relative to the gravitational frame of reference produced subtle changes but important errors. More collimated light generated narrower edges and more accurate dimensions, but darkened the concave surface of the gel, which further obscured the location of the air/water interface. With optimized optics, a single threshold intensity can accurately locate all sections of the interface. This interfacial grayscale allowed accurate measurements in real time that extended over the full range of surface tensions during compression of a solid film, and over a broad range of volumes during isobaric compression of a collapsing film.