A microscopic investigation of bubble formation nuclei

Numerous experiments suggest that bubble formation in aqueous media is initiated by stable gas nuclei. Although attempts have been made both to detect and to describe these entities, their very existence is still controversial. This paper reports a detailed investigation using light and electron microscopes. The objects identified as nuclei are found in both distilled water and gelatin, and they resemble ordinary gas bubbles. Radii are on the order of 1 μm or less and can be three orders of magnitude smaller. The number density decreases exponentially with increasing radius. A gas filling is implied by the observation that nuclei expand when the pressure decreases and contract when it rises. The occurrence of nuclear clusters and of binary or osculating nuclei suggests that stabilization is achieved via surfactant films. The monolayer thickness of these films, estimated from the thicknesses of bilayer septa, is (20±7) Å. Many nuclei are embedded in reservoirs of surface‐active material made visible by osmium–tetroxide staining. Electron microscope sections are hardened by infiltrating gelatin with epoxy. Reservoirs, encased in epoxy, form microbubble chambers in which the coalescence and bursting of nuclei can be studied during extended exposures to the electron beam.