Optical factors in the rapid analysis of captive bubbles

Bubbles and droplets offer multiple advantages over Langmuir troughs for compressing interfacial films. Experiments, however, that manipulate films to maintain constant surface tension (γ) present problems because they require feedback. Measurements of bubbles and droplets calculate γ from the shape of the interface, and calculations in real time based on finding the Laplacian shape that best fits the interface can be difficult. Faster methods obtain γ from only the height and diameter, but the bubbles and droplets rest against a solid support, which obscures one section of the interface and complicates measurements of the height. The experiments here investigated a series of optical variables that affect the visualized location of the different surfaces for captive bubbles. The pitch of the support and camera as well as the collimation of illuminating light affected the accuracy of the measured dimensions. The wavelength of illumination altered the opacity of turbid subphases and hydrated gel used to form the solid support. The width of all visualized edges depended on the spectral width and collimation of the illuminating light. The intensity of illumination had little effect on the images as long as the grayscale remained within the dynamic range of the camera. With optimization of these optical factors, the width of all edges narrowed significantly. The surfaces away from the solid support approached the infinite sharpness of the physical interface. With these changes, the grayscale at the upper interface provided the basis for locating all surfaces, which improved real-time measurements based on the height and diameter.