The existence of bulk nanobubbles is controversial as they should dissolve on a timescale of 1- 100 μs, calculated through the use of a widely accepted theory of bubble dissolution. Currently the existence of long-lived sub-micron bubbles in the bulk is not widely accepted. Despite this, nanobubbles reportedly have applications in water treatment, froth flotation, and ultrasound imaging. It is therefore important to develop methods to test if nanoparticle dispersions contain nanobubbles.
The aim of this work is to investigate systems that are reported to contain bulk nanobubbles in order to determine if the nanoparticles in these systems are indeed nanobubbles.

The first part of my talk will describe two novel approaches to investigate the nature of the nanoparticles. Firstly we develop a method to determine the density of nanoparticles using the resonant mass method. Secondly we develop the equipment to measure the influence of external pressure on the size of nanoparticle dispersions. As the density and compressibility of a gas is very different to the density and compressibility of liquids and solids these methods are able to determine whether nanoparticles are gaseous or otherwise.

The second part of my talk will investigate the presence of bulk nanobubbles in different systems, such as ultrasound contrast agents, mixtures of ethanol and water and nanoparticles produced by gas injection and shear, and address the following questions: Are we able to produce long-lived nanoparticles through supersaturation of gas? Are these particles gas-filled bubbles? If bubbles are present but armoured with a shell of insoluble material, how effective will the external pressure be when applied to these systems? We have performed a series of experiments aiming to answer these questions. The results and their implications will be presented in details.