Having spent more than twenty years in the instrumentation arena and worked with a variety of different characterization techniques in lots of different application spaces, I have sometimes struggled to explain to my family what it is that I do. Particularly to my two young children who have never quite got the intricacies of nanomaterial characterization despite my attempts to stress the importance by referring to everyday products like computers and iPods that they take for granted.

So it was within this context that I found myself generating a presentation on the important and evolving sector of ultra fine bubbles or “nanobubbles”, when I was asked by my youngest what I was doing. When I said that I was putting something together for work on nanobubbles, I could see a slight recognition that had been absent in all my previous talks about my work. And so it has become that when my children are asked what their Dad does, they with confidence reply that “he measures nanobubbles.”

The arena itself is one of many that Malvern operates within and I got the opportunity to present a webinar on the nanobubbles on the 30th October, which aimed to separate the fact from the fiction on this contentious topic.  First off, we’re pretty confident that very small bubbles do exist but possibly not at sizes less than 100nm. Hence they are typically now referred to as ultra fine bubbles (<1um) rather than nanobubbles. An important driver to the activity in this area is the FBIA (Fine Bubble Industries Association) based out of Japan which is working with ISO (Technical Committee 281) to establish standards in the field focused on three areas:

1. Bubble generation techniques, and to define what are fine and ultra fine bubbles

2. Characterization techniques

3. Applications that bubbles can be used in and benefits expected.

As indicated in the webinar, the major difference between larger bubbles of the micro and millimeter scale is that you typically cannot see ultra fine bubbles with the naked eye and hence the importance of getting standards in place to ensure a consistency in the quality of the samples and what they are delivering in applications. I also indicated in the webinar that the exact mechanism that keeps these ultra fine bubbles stable for prolonged periods of time is not completely understood and requires further research.

Critical to the successful generation of standards in the arena will be characterization techniques and in the webinar we discussed three which are typically used in the space, which were DLS (Dynamic Light Scattering), NTA (Nanoparticle Tracking Analysis) and RMM (Resonant Mass Measurement). They all offer certain benefits in the measurement of ultra fine bubbles however RMM, the technology at the heart of the Archimedes instrument, is somewhat unique. As it is a mass based system it is the only technique that can differentiate between what might be bubbles in the sample and what be contamination from the generation process. This is an interesting application space which will see growth in research and applications over the next few years, and I am glad that Malvern is at the forefront of meeting some of the characterisation challenges in the arena…… if nothing else, it allows my kids to explain what their Dad does!

You can playback the webinar here: Nanobubbles – separating the fact from fiction

We were asked some questions that we didn’t get round to answering during the webinar, which I have answered below:

Q: ­How do you know if the Zetasizer is measuring nano-sized bubbles or nano-sized solid particles (impurities)?­  You don’t really as Zetasizer uses a light scattering technique and will scatter light from both nano bubbles and contaminants giving the size of each, however it cannot differentiate between the two. The only technique that can do this is Resonant Mass Measurement, which is used by our Archimedes instrument to differentiate the mass of the samples going through it.

Q: ­Could you change the gas to alter the zeta potential of the nanobubble?  One would expect the properties of CO2 or Cl2 to be different from N2 or Ar, for example…­  The exact mechanism for Zeta Potential (ZP) generation on the bubbles is not known so it is difficult to confirm or not if a change in the gas inside the bubbles would have an effect on the ZP or not. One would suspect that it would and it is worth further research as an ability to control and/or increase the absolute ZP could lead to improvements in the stability of these bubbles.

Q: ­For the Zeta Potential testing you had listed that you had noticed an increase in ZP as the concentration was decreased.  Was Conductivity and pH considered during this analysis?  I noticed that the conductivity was dropping with dilution of the samples.  ­The Zeta Potential actually appeared to increase with increasing concentration and was about -30mV at 10^8 particles/ml and dropped to about -23mV at 2^8 particles / ml. Yes, with all samples for ZP measurement the pH is important to monitor, however these were native samples and not just one sample diluted.

Q: ­Since Zeta Potential is directly affected by conductivity and pH shouldn’t this have been kept constant?­ As indicated the samples were as supplied by the manufacturer at different concentrations so they were real measurements on their particular manufacturing process. Of course for classic ZP comparison the sample conditions are critical to the results obtained.

Q: ­On slide 8, you say that ultrafine bubbles are ‘stagnant’, or neutrally buoyant, so why should the Archimedes system detect positive buoyancy? If these bubbles were positively buoyant wouldn’t they have risen out of the medium? ­ As indicated in the slides the ultra fine bubbles appear to stay stable in solution – at the larger scale bubbles rise to the surface – however the exact mechanism for their stability is not completely understood. I would refer to some of the research that has been completed in the area for more insight into the current understanding of stability.

Q: ­Since bubbles show negative zeta potential, when you measure the zeta potential of a mineral suspension that is well mixed by shaking, is your measurement affected by the gas bubbles? Is your zeta potential of mineral a mixture of the solids and bubbles? ­Many normal bubbles that actually get in by shaking have zero zeta potential, and you will see a peak at 0 if you have bubbles which does occasionally happen, and yes it will change average zeta’s etc., so best avoided if you can.

Q: ­How were the samples transported from Japan?­ By air

Q: ­Intense shaking of water solutions in Polypropylene (PP) tubes could cause the origin of particles from tube walls. PP has a density lower then water as well as nanobubble. Could you distinguish between PP particles and nanobubbles? ­Potentially yes as they will have differing densities, it doesn’t matter if they are higher or lower, just different. They would have differing charges potentially as well.

Q: ­Why do you have a minimum concentration of particles (10^4) for the Archimedes?­ This is really to do with a sensible time for experiment. The lower the concentration then the longer you have to wait to see a suitable number of particles / bubbles to give you a robust result and hence the decision to have a cut off on the concentration.

Q: ­How do you ensure that nanobubbles (NBs) do not shrink over a period of time or preparing a solution of NBs from their frozen state­? As indicated in the webinar the exact mechanism for stability is not understood, however it does appear that these ultra fine bubbles are stable over a period of time. Current research appears to indicate that a counter ion layer on the bubble acts as barrier to the gas inside leaking and hence maintaining their stability.

Q: ­Is Resonant Mass Measurement (RMM) able to measure the actual molecular weight (Mw) of particles? If so, what is a lower detection limit? I am interested in measuring Mw of large proteins like 100 kDa to 10 MDa.­ RMM measures the mass of particles, however if you are interested in the molecular weight of a protein then you should be looking at SEC (Size Exclusion Chromatography), linked to one of our light scattering detectors. We have two kinds of light scattering detectors that we can link with SEC and these are RALS/LALS (Right Angle and Low Angle Light Scattering) and MALS (Multi Angle Light Scattering). You can find brochures and videos on our website about these techniques – here’s a link to contact our sales team to discuss this in more detail.

Q: ­With regards to the contaminants you measured in your samples, where do you think that contamination came from and what time period is involved with the transport? ­We believe these come from the generation process and possibly the generators themselves.  The process entails passing water through membranes and there may be some slight shedding.

Q: ­What type of water was used to make the ‘nanobubbles’, e.g. was it degassed, filtered, deionised etc.? What was the dissolved gas content of this medium?­ Degassed DI, however we are not entirely sure on the dissolved gas content in the medium.

Q: ­Regarding the origin of contaminants: Do you know whether the different concentrations of the samples are made by diluting or by concentrating fresh batches?­ They were from different manufacturing batches and there was not just one sample diluted.

Q: ­What precautions we should take not to break bubbles during our experiments­? We’d advise you to avoid ultrasound, avoid high shear and store at a constant temperature and pressure.

Further Reading

Characterization of nanobubbles and other ultrafine bubbles by Nanoparticle Tracking Analysis (NTA)

Life in a (micro) bubble

A new use for empty space – Microbubbles

Introducing Archimedes, Particle Metrology system

Nanoscale Material Characterization: a Review of the use of Nanoparticle Tracking Analysis (NTA)