Membrane Method for Producing Air Nanobubbles Using an Anopore Membrane with a Pore Diameter of 20 nm

A new simple method is proposed for producing bulk air nanobubbles (NBs) in water by filtration through a dry Anopore membrane (Anodic Aluminum Oxide, AAO) with a pore diameter of 20 nm. Unlike the case in traditional membrane approaches, where a gas is passed through a membrane, in this method water is forced through a dry membrane. It is shown that filtering through hydrophilic polyethersulfone membranes with the same nominal pore size does not lead to formation of bubbles. Properties of the resulting nanodispersions are investigated using the dynamic light scattering (DLS), ultramicroscopy, and nanoparticle tracking analysis (NTA) methods. It is found that NBs with a low initial concentration and a hydrodynamic radius of ~50 nm are formed immediately after filtering. Within 10–15 min, the NB concertation increases to a stable level of about 108 particles/mL, and their average size becomes larger. Thirty minutes after generation, the size distribution measured by the NTA method has a maximum at a radius of ~78 nm. Dispersion demonstrates a considerable time stability: after 24 hours the NB concentration remains at a level of ~107 particles/mL. The proposed method is a simple reproducible laboratory technique of producing stable nanobubble dispersions.