UNDERSTANDING THE STABILITY AND PROPERTIES OF BULK NANOBUBBLES

Bulk nanobubbles are a novel type of nanoscale bubble system. They are spherical with a typical diameter of 100-200 nanometres and they exist in bulk liquid. The most peculiar characteristic of these bulk nanobubbles is their extraordinary longevity. Whilst the lifetime of macrobubbles (> 1 mm) is on the order of seconds and that of microbubbles (1-1000 microns) is on the order of minutes, nanobubbles do last for weeks and months. Existing theories, however, predict a huge inner gas pressure (typically around 30 atm) and, consequently, molecular diffusion theory would predict that they would dissolve extremely quickly - on a timescale of about 1 microsecond.

The existence of bulk nanobubbles has been reported by a number of academic researchers but due to their unusual behaviour there is still some controversy around the subject. In a preliminary study in collaboration with the IDEC Corporation in Osaka (Japan), we have managed to generate nanobubbles via two different techniques and, using advanced instrumentation, we have been able to visualise them and measure their size distribution.

Because of their unusual longevity bulk nanobubbles are already attracting a lot of industrial attention and many potential applications have been identified or tested, especially in Japan and USA. Thus, there is immense scope for nanobubbles to impact and even revolutionise many current industrial processes such as water treatment, industrial cleaning and the production of chemicals, biofuels, food as well as other important high value added applications including healthcare technologies.

There is, however, little academic or industrial activity taking place within Europe and the UK. As such, there is an urgent need for research on this subject so as to enable the UK to keep up with this emerging scientific field and so that UK industry can benefit from the vast potential of this novel technology.

From a scientific point of view, the mystery behind the longevity of bulk nanobubbles has led to many different speculations as to the reasons for this phenomenon. However, reports are sparse, and in the main conflicting and have not been independently validated. An aspect to be considered is that nanobubbles are not macroscopic systems and so everyday thermodynamics is not reliable. Furthermore, atomistic simulations on this scale are only now becoming feasible. To fully exploit the potential benefits of bulk nanobubbles, our understanding of the fundamental rules governing their existence and behaviour needs to be substantially improved.

Our hypothesis is that bulk nanobubbles do exist, they are filled with gas and they persist for a timescale at least ten orders of magnitude longer than expected. The aim of this proposal is to explore and study the underlying mechanisms by which they come to exist and persist, and to help explain some of the reported unusual properties of bulk nanobubble suspensions using a combination of experimental, theoretical and computational tools.

The work will address questions concerning the formation of nanobubbles, their coalescence, dynamic behaviour and stability, including their apparent immunity to the destabilising process of coarsening or disproportionation, also known as Ostwald ripening. The effects of liquid properties, gas properties, shear and temperature will be studied experimentally, modelled theoretically and simulated computationally by molecular dynamics. The practical aim of the present project is to develop robust predictive tools based on the knowledge gained from the experimental and modelling work, as an aid to industrial practitioners. These tools will provide a description of the structural and dynamical properties of bulk nanobubbles in terms of the liquid and gas intrinsic properties as well as external parameters like pressure and temperature. We will also work with our industrial partners to help them explore and develop novel applications.

Bulk nanobubbles challenge our understanding of bubble physics and behaviour. Their extraordinary persistence and their generally puzzling behaviour require us to revisit many aspects of existing bubble science. The issues at stake engage both academic researchers looking to probe, understand and model these entities, and industrialists seeking to exploit their remarkable properties to develop new applications or to enhance existing ones based on understanding rather than via trial and error.

Previous gas-liquid research at Birmingham has made substantial academic impact and has led to many national and international collaborations. The Atomistic Simulation Centre at QUB has generated a number of significant advances in the understanding of materials at the atomic scale by means of theory and computer simulation, whilst STFC Daresbury Laboratory has been a key international player in the development of advanced software for materials simulation. We expect that the combination of our strengths will result in new interdisciplinary views and tools for the study of gas-liquid systems and multiphase systems in general, delivering high impact fundamental research across disciplinary boundaries between several EPSRC areas, e.g. Fluid Dynamics, Process Engineering, Computational and Theoretical Physical Sciences, Particle Technology, Complex Fluids and Rheology, and Innovative Production Processes. This work also spans multiple EPSRC themes, e.g. Engineering, Manufacturing the Future, Physical Sciences, and Healthcare Technologies.

The computational methods used and developed in this project are generic and are thus applicable to other types of two-phase nanoscale systems, e.g. surface nanobubbles, nanoparticles and nanodroplets. In addition, we will further develop a range of new enhanced sampling methodologies introduced by one of us (Tribello), which are applicable to a very wide range of phenomena in biochemistry, chemistry, and materials science. These promising methods have not yet been adopted widely, mostly because they have only been tested on model systems. Here, we have an opportunity to use them to study a real complex problem of industrial relevance and to ensure that they work for system sizes that challenge both the software and the infrastructure of the UK's largest HPC resource.

There is immense scope for bulk nanobubbles to impact and revolutionise many current industrial processes, as reported by the 'Fine Bubbles Industries Association' in Japan (FBIA-Japan), including water treatment, industrial cleaning and the production of chemicals, biofuels, food and pharmaceuticals as well as other important high value added applications such as in the biotechnology and medical fields. This research has the potential to impact all these industries by improving our understanding of these nanobubble systems and by giving us the ability to predict their behaviour and, hence, to provide guidance to industrial practitioners involved in developing innovative applications.

Two of us (Barigou and Pacek) have been involved through FBIA-Japan and the BSI in the creation of the new ISO Technical Committee on Fine Bubble Technologies (ISO/TC281), and are currently involved in the creation of FBIA-Europe, to promote fine bubble technologies in Europe.

More specifically, this research is supported by Unilever (food/personal care), PepsiCo (food/drinks), Mondelez (confectionery/coffee), who are keen to explore and develop novel applications; Particle Technology, an SME who are keen to incorporate nanobubbles in their cleaning techniques for the aerospace, medical, pharmaceutical and telecommunications industries; and Malvern Instruments who are keen to enhance/develop instrumentation for nanoparticle characterisation. During the project, we will share results and ideas with all of them and we will also engage with them to help them evaluate and develop industrial applications specific to their own businesses.