UNDERSTANDING THE STABILITY AND PROPERTIES OF BULK NANOBUBBLES

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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.