Bulk nanobubbles or not nanobubbles: that is the question

Bulk nanobubbles are a novel nanoscale bubble system with unusual properties which challenge our understanding of bubble behavior. Because of their extraordinary longevity, their existence is still not widely accepted as they are often attributed to the presence of supramolecular structures or contaminants. Nonetheless, bulk nanobubbles are attracting increasing attention in the literature, but reports generally lack objective evidence that the observed nano-entities are indeed nanobubbles. In this paper, we use various physical and chemical analytical techniques to provide multiple evidence that the nano-entities produced mechanically in pure water by a continuous high-shear rotor-stator device or acoustic cavitation and spontaneously by water-ethanol mixing are indeed gas-filled domains. We estimate that the results presented here combined provide conclusive proof that bulk nanobubbles do exist and they are stable. This paper should help close the debate about the existence of bulk nanobubbles and, hence, enable the scientific community to rather focus on developing the missing fundamental science in this area.