Bulk Nanobubbles or Not Nanobubbles: That is the Question

Abstract

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.

Proving and interpreting the spontaneous formation of bulk nanobubbles in aqueous organic solvent solutions: effects of solvent type and content

“…These results seem to suggest, therefore, that a pure organic solvent acts as a gas sink removing any excess gas from the solution and, consequently, it does not form nanobubbles. Hence, since BNBs exist in pure water, as reported by a number of scientific reports including our own, 5,6,17,[48][49][50] then they should disappear at some organic solvent-water ratio, which is confirmed by the present results.…”

“…In the case of methanol, ethanol, propanol and acetone, the molecules of these solvents adsorb on the nanobubble interfaces and provide stability to the BNBs. 5,18 In the case of DMSO, it seems that the chemical structure of this solvent plays an even more important role in stabilising BNBs which seems to more than compensate for its comparatively low relative gas solubility. The exact mechanism by which this stabilising effect takes place in the case of DMSO is not a priori clear, however, and necessitates further study which is beyond the scope of this paper.…”

“…This result confirms that the molecules of these solvents do adsorb on the nanobubble interfaces and provide long-term stability to the BNBs. 5,18 For the solvents which produced stable nanobubbles, we monitored the BNB suspensions with the maximum bubble number density in 20 mL air-tight glass vials at room temperature. After a period of three months, the bubble number density and the bubble size distribution were unchanged within experimental error (AE 1 to 5% depending on the solvent and its concentration).…”

“…Their existence has been reported in a number of recent experimental studies [1][2][3][4] and strong indirect evidence has been provided to show that such nano-entities must be gas bubbles. 5,6 Nonetheless, considerable speculation and controversy still exist about the existence and nature of BNBs, their origin and their extraordinary longevity which contradicts predictions of the diffusive gas transport based on the Epstein and Plesset theory. 7 Those who dispute the existence of BNBs tend to attribute them to supramolecular structures, solvent or oil nanodroplets or simply impurities and contamination, based on either questionable experimentation or sheer speculation.…”

An Improvement on Selective Separation by Applying Ultrasound to Rougher and Re-Cleaner Stages of Copper Flotation

“…This phenomenon is the so-called bridging effect or secondary-collector role of the nanobubbles, which induces an increase in the particle-bubble attachment efficiency. These bubbles can be generated by applying ultrasonication due to the hydrodynamic cavitation and hold an extensive lifetime on the solid surface [73]. More detailed information can be found elsewhere [74].…”