Fine and ultra-fine bubbles - technical information

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Fine and ultra-fine bubbles - technical information

Fine and ultra-fine bubble technology is a rapidly developing area of science attracting a large amount of attention across the world. Applications currently in use and being explored cover many areas of science, industrial technologies and medicine.

Fine and ultra-fine bubbles

Modern bubble nomenclature can be confusing with the terms 'microbubble' and 'nanobubble' being used interchangeably, even in print.

In order to contribute to the recognition of a common standard, Aeration&Mixing© have adopted the terms used by Japan's Fine Bubble Industries Association (FBIA) who have greatly advanced international understanding in many areas of bubble research; see this video link... 

At Aeration&Mixing and in accordance with FBIA, we class fine bubbles as those with diameters from 1µm to 50µm, and ultra-fine bubbles as those with diameters of less than 1µm.

Nikuni KTM pumps are precision made devices which can create consistent fine bubble discharges in a carrier fluid at an average fine bubble diameter of 25µm.  Smaller fine bubbles can be produced by these pumps with additional equipment provided by A&M©.

Fine bubbles at a diameter of only 25µm have many uses particularly because, they not only provide an excellent bubble-to-particle ratio for DAF and IAF/IGF applications, but they are small enough to enable good mass-transfer of gasses to liquids.

Fine bubbles are best applied in tanks and vessels rather than in open lagoons or lakes, as their action is essentially gentle rather than the creating strong mixing needed for the treatment of active sludges.

A fine bubble in tap water at circa 15°C has a life of approximately 2 minutes, after which it disappears.  It should be noted that the rate of bubble collapse will vary according to the content and composition of the water or other fluid, the temperature, viscosity and level of surfactants present.  

When a fine bubble cloud in a carrier fluid disappears, two changes of state will have occurred;

a)  a proportion of the fine bubbles in the cloud will have collapsed and the gas content dissolved into the carrier liquid or collapsed at the surface; and

b) the remainder will have fractured into a very large number of ultra-fine bubbles, sometimes called nano-bubbles.

The ratio of bubbles that collapse to those that fracture is dependent on the charge present on the surface of the bubble whilst in the carrier fluid.

A typical analysis of ultra-fine bubbles in tap water will show circa 200 million bubbles per ml or cm3, with diameters ranging from 50 to 250nm.

Ultra-fine bubbles, which can only be measured with special instrumentation, have been measured with diameters as small as 50nm.  These resilient tiny spheres are completely invisible to the human eye and only visible at high levels of magnification in clusters under the backscattering of green laser light.  

One of the challenges in measuring ultra-fine bubbles is differentiating between a bubble and other particles present in a sample fluid.  One method is to note the characteristics evident under green laser light, as particles present in a fluid will reflect light whereas fine and ultra-fine bubbles will refract light.  

It should be noted that whereas fine bubbles have a life rarely extending beyond two minutes, ultra-fine bubbles evidence Brownian Motion (neutral buoyancy) and can exist for many months and, in some special cases, can remain intact for years.  This means that care should be taken in applications to ensure that where an ATEX rated gas is being sheared into fine bubbles and allowed to collapse, provision is made for the safe venting of any latent gas over an extended period of time. 

Research in Japan has demonstrated that ultra-fine bubbles at diameters of 200nm can elongate and pass through a membrane orifice only 100nm in diameter without collapsing.  This demonstrates the need for a viable commercial scale means of controlling the collapse of ultra-fine bubbles in industrial processes.  Current international research is seeking ways of collapsing ultra-fine bubbles at a time of a user's choosing, an area of research in which A&M is active.

The principal current uses of fine bubbles include:

Injecting gasses into liquids
Changing the state of fluids
Separating particles of waste by flotation e.g. DAF/IGF etc.
Cleaning and sterilization
Medical treatments and procedures

Future uses of ultra-fine bubbles include:

Advanced drug delivery
Further new medical treatments
Significant new cleaning techniques
Major advances in sterilization techniques and CIP technologies
Food and drink sector product developments
Algae farming

Fine and ultra-fine bubble technologies are a fascinating area of science constantly evolving yielding new discoveries. 

One of the most useful articles on fine and ultra-fine bubbles has been written by Professor Martin Chaplin and can be found via the following link... 

For those interested in a deeper understanding of recent research into bubble technology, one useful textbook is Micro- and Nanobubbles Fundamentals and Applications edited by Hideki Tsuge, ISBN978-981-4463-10-2.  This text book condenses 10 years of fine bubble research undertaken by leading researchers in Japan now published in English for the first time.