Bulk Nanobubbles or Not Nanobubbles: That is the Question.

Ananda J Jadhav,Mostafa Barigou

doi:10.1021/acs.langmuir.9b03532

Ananda J Jadhav, Mostafa Barigou

Open Access

https://doi.org/10.1021/acs.langmuir.9b03532

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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.

Highlights

Bulk nanobubbles are a novel nanoscale bubble system with unusual properties which challenge our understanding of bubble behavior
We used three different techniques to generate bulk nanobubble suspensions, namely, a continuous high-shear rotor-stator (HSRS), acoustic cavitation, and water-solvent mixing. These techniques between them cover the typical sources of possible contamination including nanoparticles, oil, or solvent nanodroplets and supramolecular structures, as discussed above, that have been associated in the literature with the observation of nano-entities which are at the center of the bulk nanobubble debate
The characteristics, in terms of bubble size distribution, bubble number density, mean bubble diameter and zeta potential, of the bulk nanobubble suspensions generated by a continuous HSRS, acoustic cavitation, and water−ethanol mixing at atmospheric pressure are presented in Figure 1 together with results obtained under a partial vacuum

Summary

Introduction

Bulk nanobubbles are a novel nanoscale bubble system with unusual properties which challenge our understanding of bubble behavior. Their existence has been reported in many recent experimental studies, and they have been the subject of a number of computational modeling studies. The most interesting theory perhaps is the ion-stabilized model proposed by Bunkin et al.[21] It conjectures that the presence of negative electrostatic pressure because of adsorption of OH− ions in the form of an electric double layer at the nanobubble interface, akin to that observed around solid nanoparticles, balances the internal Laplace pressure and, no net diffusion of gas occurs.[3]

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