Stable Air Nanobubbles in Water: the Importance of Organic Contaminants

Abstract

Nanobubbles, surprisingly stable submicrometer gas bubbles in water, appear to be common in water and biological fluids and are of great interest in technical applications ranging from ultrasound contrast agents to flotation in the mining industry. Nanobubbles on surfaces have been more researched than freely floating bulk nanobubbles, and the reason for their stability appears to be better explained. The stability of bulk nanobubbles is less well explained, several theories exist, and even their existence is sometimes questioned. In the present study, an attempt was made to generate nanobubbles through hydrodynamic cavitation as well as through vigorous shaking in test tubes, and it was found that none of these methods generated a detectable concentration of possible bulk nanobubbles if pure water was used, with or without a small addition of NaCl, the equipment was cleaned properly, and certain plastic materials were avoided. These results indicate that trace organic contaminants are necessary for nanobubble stabilization. Experiments were also made with the dissolution of a high concentration of inorganic salts, which generated bubbles by creating air supersaturation. Light scattering submicron particles were found in all solutions and appeared to be actual gas bubbles in at least one case. However, in many cases, these light scattering particles were unaffected by vacuum and pressure and appear, therefore, to be something else other than air bubbles. It is concluded that, in future research on nanobubble stability, it is very important to avoid contamination, as well as to ascertain that light scattering objects really are bubbles and not oil droplets or particles.

Does salting-out effect nucleate nanobubbles in water: Spontaneous nucleation?

“…Such a compressible behavior of the suspension indicates the gas-filled nano-entities. In contrast, employing a vacuum to the nanobubble sample [38] , mixing degassed water with nanobubble sample [39] exhibit disappearance of nanobubbles. Direct irradiation of ultrasound in the nanobubble sample also leads to the disappearance of nanobubbles, as shown in Fig.…”

Generation and Stability of Size-Adjustable Bulk Nanobubbles Based on Periodic Pressure Change

“…In the past decade, diverse methods have been used to produce bulk nanobubbles including cavitation, electrolysis, ultrasonication coupled with Pd-coated electrodes, temperature gradients, and so on 15–20 , in which cavitation and electrolysis were used more often. Cavitation, mainly referring to hydrodynamic cavitation, usually occurs in flowing fluid where the local pressure is lower than the critical value.…”

Dynamics of aqueous peptide solutions in folded and disordered states examined by dynamic light scattering and dielectric spectroscopy

“…S3, ESI †) and mention that similar phenomena were reported in the literature for bulk water and solutions created using different sample preparation protocols. [38][39][40][41][42] Thus, the B-DLS process is not discussed any further in the following. Fig.…”