Methods have been used to generate air bubbles beneath a planar air-water interface, stabilized by partially hydrophobic quasi-spherical silica particles (primary diameter of 20 nm) in pure water. Particles tended to aggregate at the planar interface, and all the silica dispersions had low foamability. However, those bubbles that were formed (with radii of 5-200 mum) were completely stable to disproportionation for several days, in contrast to similar bubbles stabilized by the best protein foam stabilizers, which typically shrank and disappeared in 1 or 2 h.