Response to “Comment on Bulk Nanobubbles or Not Nanobubbles: That is the Question”

Abstract

Advanced techniques that combine high spatial resolution with chemical sensitivity to directly probe the observed nanoentities and provide direct evidence that they are truly gas-filled nanobubbles do not exist. Therefore, in our paper, we focused on providing, for the first time, multiple types of indirect evidence using a variety of physical and chemical techniques that the nanoentities are not due to contamination and, hence, they must be bulk nanobubbles (BNBs). It should be noted that such techniques require good experimental skills, sound protocols, good scientific expertise, and reliable equipment. While no single piece of indirect evidence on its own can be considered as conclusive proof, we estimate that our results combined provide strong evidence that bulk nanobubbles do exist and they are stable. The work presented in our paper is the culmination of a series of studies, and many authors have either directly or indirectly confirmed our findings. Nonetheless, in their Comment, Rak & Sedlak reject all of the work we reported. We here address their comments point by point and show that their criticisms are unwarranted and unfounded, as follows.

Influence of Bulk Nanobubbles Generated by Acoustic Cavitation on Powder Microstructure and Rehydration Characteristics of Spray-Dried Milk Protein Concentrate Powders

“…Over the last decade, researchers debated whether the nano entities created were BNBs or other nanoparticles [ 33 ]. Notably, BNB-incorporated DI water samples had the presence of titanium in only a trace amount (0.00017 μg/mL), implying the presence of titanium contamination could not be blamed for the detected nano-entities using NTA, which agrees with the results reported by another research group [ 9 , 34 ]. This observation further confirmed the presence of BNBs generated via acoustic cavitation.…”

Generation of Bulk Nanobubbles by Self-Developed Venturi-Type Circulation Hydrodynamic Cavitation Device

“…Freeze–thaw experiments are a valid method to qualitatively distinguish BNBs from solid nanoparticles and droplets by the gas-sensitive property since solid nanoparticles and nanodroplets do not vanish, while nanobubbles will elapse in the process. , As such, freeze–thaw experiments have been conducted to demonstrate that the colloidal particles, as observed by the Tyndall effect, in suspension are nanobubbles. The results of the freeze–thaw experiments are shown in Figure , where DLS detected no nanoentities after the first freeze- thaw, with the avg.…”

Influence of bulk nanobubble concentration on the intensity of sonoluminescence

“…Note that since the component of larger size did not necessarily increase with dilution, Ostwald ripening for growth mechanism of nanoparticles [25] meaning that smaller particles dissolve into surrounding liquid leading to the disappearance to cause larger ones to grow would not work. According to the literatures [5] , [26] , [27] , bulk nanobubbles probably disappeared by interaction with the free surface of the liquid or the wall of the container. This has been corroborated recently by Kanematsu et al [10] .…”

“…Long-lived floating bulk nanobubbles (fine bubbles smaller than 1 μm) in liquid have recently been studied [1] , [2] , and the existence of gas-filled bubbles is still controversial [3] , [4] , [5] . However, nanobubbles have the potential to serve as cavitation nuclei in sonochemistry [6] , [7] , [8] , [9] .…”