Nanobubbles in bulk

Abstract

Currently, the existence of long-lived sub-micron bubbles in solution is not widely accepted as they should dissolve on a timescale of 1-100 microseconds, calculated through the use of a widely accepted theory of bubble dissolution. Despite this, bulk nanobubbles are reported to have applications in different fields, such as water treatment and remediation, seed germination, surface cleaning, froth flotation, and ultrasound imaging. It is therefore important to develop methods to test if nanoparticle dispersions contain nanobubbles.

Here, two methods are developed that are able to distinguish long-lived nanobubbles from nanoparticles. Firstly, the mean particle density of nanoparticles in a dispersion is determined. Secondly, the influence of external pressure on the size of nanoparticle dispersions is measured. As the density and compressibility of a gas are very different to the density and compressibility of liquids and solids, these methods can differentiate between nanobubbles and other nanoparticles.

The first part of my thesis focuses on nanobubbles that are armoured with a coating of insoluble surfactants. A novel technique for particle characterization that has the ability to distinguish positively buoyant particles (less dense than the solvent) from negatively buoyant particles (more dense than the solvent) was adapted to assess the density of nanoparticles. It revealed a significant population of lipid-coated gas nanobubbles in a commercial ultrasound contrast agent. These nanobubbles are proven to be gas entities by their response to application of pressure. These armoured nanobubbles have a complex response to the application of pressure due to the robust shell formed by the insoluble surfactants. The temperature at which the gas filled nanobubbles condenses to liquid filled nanodroplets is shifted to lower temperature, corresponding to a condensation at higher pressure due to the mechanical resistance of the lipid shell, which shields the bubble contents from some of the external pressure up to ~ 0.8 atm. The presence of lipids of low solubility at the nanobubble-solution interface effectively results in a negative Laplace pressure, which stabilizes these nanobubbles against dissolution.

Having developed protocols that can be used to demonstrate the existence of bulk nanobubbles, these methods were then applied to different systems reported to contain nanobubbles. These include nanoparticles produced by mechanical means, the mixing of ethanol and water and nitrogen supersaturation by chemical reaction. It was confirmed that nanoparticles were produced in these systems. However, the measured density of these nanoparticles was inconsistent with the nanoparticles being gas filled. Furthermore, the external pressure had only a minimal effect on the size of these nanoparticles. These experiments reveal that processes that lead to bubble formation can produce nanoparticles that result from the accumulation of material at the interface of the dissolving bubbles.

The results of this study demonstrate that the candidate nanoparticles investigated here are not nanobubbles unless they are coated with insoluble materials and casts doubt on many reports of long-lived nanobubbles in bulk. Many researchers have reported the production of stable long-lived nanobubbles in bulk without providing direct evidence that the nanoparticles being measured are indeed nanobubbles. It is recommended that the methods developed here be used as tests to determine if candidate nanoparticles are nanobubbles.