We follow the history of nanobubbles from the earliest experiments pointing to their existence to recent years. We cover the effect of Laplace pressure on the thermodynamic stability of nanobubbles and why this implies that nanobubbles are thermodynamically never stable. Therefore, understanding bubble stability becomes a consideration of the rate of bubble dissolution, so the dominant approach to understanding this is discussed. Bulk nanobubbles (or fine bubbles) are treated separately from surface nanobubbles as this reflects their separate histories. For each class of nanobubbles, we look at the early evidence for their existence, methods for the production and characterization of nanobubbles, evidence that they are indeed gaseous, or otherwise, and theories for their stability. We also look at applications of both surface and bulk nanobubbles.

The electrolysis of aqueous solutions produces solutions that are supersaturated in oxygen and hydrogen gas. This results in the formation of gas bubbles, including nanobubbles ∼100 nm in size that are stable for ∼24 h. These aqueous solutions containing bubbles have been evaluated for cleaning efficacy in the removal of model contaminants bovine serum albumin and lysozyme from surfaces and in the prevention of the fouling of surfaces by these same proteins. Hydrophilic and hydrophobic surfaces were investigated. It is shown that nanobubbles can prevent the fouling of surfaces and that they can also clean already fouled surfaces. It is also argued that in practical applications where cleaning is carried out rapidly using a high degree of mechanical agitation the role of cleaning agents is not primarily in assisting the removal of soil but in suspending the soil that is removed by mechanical action and preventing it from redepositing onto surfaces. This may also be the primary mode of action of nanobubbles during cleaning.

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There are a growing number of reports in the literature of techniques to produce swarms of long-lived nanosized bubbles. These are of interest both because their stability is unexpected and because of the reported wide and growing range of applications for nanobubbles. These reports demonstrate the presence of nanoparticles but generally lack direct evidence that the particles are indeed nanobubbles. Here, we report two methods that are able to distinguish long-lived nanobubbles from other nanoparticles. First, the mean density of nanoparticles in dispersion is determined. Second, the influence of external pressure on the size of nanoparticles is measured. As the density and compressibility of a gas are very different from that of liquids and solids, these methods can differentiate between nanobubbles and other nanoparticles. The resonant mass measurement was adapted to measure the mean density of nanoparticles and showed that candidate nanoparticles were buoyant but with a density of 0.95 g/cm3. Light scattering was used to examine the influence of an applied external pressure of 10 atm on the diameter of candidate nanoparticles. An insignificant change was observed. These results demonstrate that the candidate nanoparticles investigated here are not nanobubbles and cast doubt on many reports of long-lived nanobubbles in bulk. These methods can be applied widely to distinguish nanobubbles from other nanoparticles.