(Non)Existence of Bulk Nanobubbles: The Role of Ultrasonic Cavitation and Organic Solutes in Water.

A drawback of studies on bulk nanobubbles is the absence of direct proof that the nano-objects reported are really nanobubbles. The aim of our work was to provide such a proof or disproof. We focused on two effects (processes) commonly considered in research on nanobubbles: ultrasonic cavitation and addition of organic compounds to water, which could create in principle a barrier at the gas/water interface contributing to the stability of nanobubbles. We found that both of these processes lead to the generation of nano-objects, which are, however, not bulk nanobubbles. Ultrasonication leads to the formation of fine metal nanoparticles originating from the disintegration of the surface of the metal ultrasonic probe. Addition of organic solutes to water leads to the formation of a population of nanoparticles/nanodroplets originating from the so-called mesoscale solubilization of hydrophobic compounds present in the added solute as molecularly dissolved impurities. Subsequent ultrasonication of such mixtures adds metal nanoparticles and only slightly modifies the size distribution of mesoscale particles. While our results do not dismiss existence of nanobubbles in general, described effects must be seriously taken into account, especially in the case of biomedical applications where they can result in serious side effects.