An Introduction to Nanobubbles

Hello, everyone, and welcome to the podcast series, Nanobubbles 101. And this is our first episode, which serves as an introduction to the topic of Nanobubbles that we'll be discussing in more detail in the course of this podcast series. So my name is Niall English, I'm the co-founder of AquaB, which was set up and has been going strong since April 2020.

These days, I'm the CTO of the company, but I served as CEO for many years in the earlier days of AquaB. I am also a professor in chemical engineering at University College Dublin, and I have been an academic there since 2007, climbing the ranks on the academic career ladder, as it were. So my research interests at UCD and indeed in AquaB are in how electric fields can be used to modulate the behavior of gas and liquids.

And I have been researching how electric fields affect gas and liquids since the year 2000, during my PhD research and since I first really became aware of and interested in Nanobubbles by a rather staggering discovery and lead invention I made in the lab back in July 2017. I had asked a postdoctoral fellow to carry out some research on trying to form gas hydrates, which are types of ice really, but it's not particularly ice. It's actually hollow ice, and these contain or entrap or imprison gas molecules.

So a gas hydrate is something that you can put atop a Bunsen burner and actually start burning what appears to be ice theatrically in a wonderful parlor trick on top of a Bunsen burner because of the entrap gas inside the ice burning. So the experiment was carried out by my colleague, Dr. Mahamud Ratzeghani, and there was a problem in setting the thermostat. So the temperature wasn't at the low temperature it should have been, it was effectively at room temperature.

And I had asked that this experiment be done using an applied electric field, which I had hypothesized based on my PhD on electric field effects on hydrates might enhance the formation of hydrates. Now, what I found instead was that the pressure dropped very, very substantially at the gas. So effectively the liquid water instead of the ice or hydrate was swallowing up the gas.

And in this particular case, the gas was methane. So this was a puzzle, and we were stumped. But I quickly, within seconds of learning this, had a sudden vision.

And the vision I had was almost a cartoon-like vision of tiny, tiny bubbles that weren't even visible. And for them not to be visible, they would have to be smaller than the wavelength of light. In other words, on the nanoscale.

And I hypothesized that the application of the electric field generated very, very vast quantities of nanobubbles. But this, if you like, was hypothesis, speculation. So therefore, I was naturally quite excited.

And this was the most exhilarating moment of my professional career, that sudden cartoon-like vision I just saw in pictures and saw pictures of tiny bubbles shorter than the wavelength of light, which couldn't be seen by the naked eye, and that the liquid must have been full of these nanobubbles, the bulk liquid. So this was an exciting novel hypothesis based upon this potential discovery, potential invention. So I demanded and suggested that we get about doing laser light scattering as soon as we possibly could.

While I was already getting excited thinking about potential implications for water treatment, aeration of water, also other gases, carbon dioxide and water, oxygen, nitrogen, for example, fish farms, irrigation, oil and gas recovery, potentially even in fuels. So I was very excited by the boundless possibilities that the application of electric fields could bring to bring about massive quantities of Nanobubbles. If this hypothesis of mine, my hypothesis was true, and if it was, then this would have been a very important discovery.

So months passed, and we did manage to get laser light confirmation of my hypothesis. So then I set about busily embellishing this invention and having important work and reduction to practice work done to set about applying for a patent and several patents. And that took quite a bit of work, and others, especially Mehmed Ratzeghani, did a good job in following all of these directions and in setting about this important R&D work to formulate a patent for this important lead invention of mine.

So that really was how I got interested in Nanobubbles based upon my own invention, based upon my own hunch, my own hypothesis. Was I right? Was I wrong?

It turns out I was right, and that has led to effectively an obsession of mine for the past seven years really into carrying out more fundamental R&D and research into Nanobubbles, as well as, of course, exploring the myriad of commercial and environmental applications. So here we are talking about Nanobubbles. Well, it will be useful if I defined what they are really in a more relatable way for us all.

So that would help. So let me think a little bit about that. So picture a glass of carbonated water.

The bubbles are quite large, of the order of one or two millimetres, and they rise within seconds and tens of seconds, and that rising upwards of bubbles is known as Stokes Law, the tendency for bubbles to rise, also known as buoyancy. Now, if you go to, let's say, a glass of beer or champagne and look at the tiniest bubbles that form at the very bottom of the glass, those are called Maser scale bubbles, perhaps one-tenth, one-twentieth of a millimetre, and they're just about visible to the naked eye. Now, we all know from everyday experience that those Maser scale bubbles rise more slowly.

So, there we have, in a nutshell, what Stokes Law is. The general gist of it is, the smaller the bubble, the more slowly it rises. Now, if we go to smaller bubbles still, microbubbles, you can no longer distinguish an individual bubble in and of itself, rather you perceive them as a swarm of bubbles, a general sort of haziness or cloudiness in the water itself.

And that would be what would define a population or a swarm of microbubbles, a general cloudiness or haziness. Now, those bubbles rise more slowly still, it might take of the order of half an hour to one hour for those microbubbles to rise out of the water by Stokes law. Now, imagine if I were to tell you that if we could generate nanobubbles, which are actually by definition smaller than the wavelength of sunlight or artificial light, therefore we can't actually see them with the naked eye, because they're smaller than the wavelength of visible light, those bubbles are so terribly small that they effectively wouldn't rise.

They're not particularly subject to buoyancy. So that means that in principle, they can stay in the water or the mother liquid almost indefinitely. And they're really limited by their own structural stability or indeed structural instability.

And that would be very important if we could realize such large populations of nanobubbles, because then we could sidestep not only Stokes law, but also Henry's law. Now, what is Henry's law? Well, Henry's law gives us the solubility of a dissolved gas in water.

Let's take dissolved oxygen as an obvious starting point. Now, if we think about fish kills during the summertime, once the temperature goes up to, let's say, 30, 35, 40 degrees, it's getting very warm in the summer, we can often see fish kills happening, for example, in pools and reservoirs. And the reason why that happens is as the temperature goes up, the solubility level of oxygen goes down.

In other words, the amount of oxygen that can be accommodated by Henry's law decreases as temperature goes up. So what we can effectively do is if these tiny nanobubbles don't escape by buoyancy, in other words, they evade Stokes law, they can help us to also circumvent Henry's law, and we can have more gas accommodated in the liquid, whether the liquid is water or petroleum or etc., than would otherwise be the case under thermodynamic dissolution and under Henry's law.

So we can have often several times the Henry's law level, although it does depend on the liquid and depends on the gas as well. How much in excess of Henry's law the total level of gas storage is in the regularly dissolved state and as well as the nano dissolved state. So by nano dissolved state, I'm talking about extra gas that's accommodated in the liquid in the form of nanobubbles.

And effectively, this extra gas in the form of nanobubbles are storage or reservoirs of gas or batteries, if you will, that can be called upon whenever the regular level of dissolved gas, such as oxygen or carbon dioxide, etc. goes down below a certain prescribed level that's needed to satisfy some biological or chemical process. So effectively, these reservoirs of extra dissolved gas and nanobubbles help to replenish the stock of dissolved gas to keep processes that are chemical or biological going, for example, water treatment or irrigation or algae cultivation, etc.

So this makes nanobubbles very interesting, the possibility that they can stay and reside enough in the mother liquid to evade Stokes law and then to effectively sidestep Henry's law to give extra levels of gas dissolution. And naturally, this effectively is a platform technology in that having such gas-laden liquid can really help with many, many processes from oil and gas recovery, irrigation, aquaculture, water treatment, of course, fermentation. And boosting calorific efficiency of engines, et cetera.

The list is almost limitless and goes on and goes on. So in terms of Nanobubbles environmental impact, this is also very important. One thing that I will say is a huge problem with the environment is the increasing level of eutrophication in water.

So eutrophication arises when we have under aerated waterways, for example, a reservoir or a lake or canals, rivers, estuaries, et cetera. And this is particularly problematic during summer when we see algae, blights, cyanobacteria, et cetera. And that stench of methane that we get whenever we see water blighted by terrible cyanobacteria or blue-green algae arises from eutrophication.

In other words, lack of oxygen deep in the so-called benthic zone where it was radiated oxygen four, five, six, seven meters down under the water. And with such anoxic conditions and eutrophication, lots of methane is released up into the atmosphere with that terrible stench that we can perceive, piercing our nostrils as we walk by. So fully one third, perhaps 40% of methane emissions to the atmosphere arise from undererated waterways.

Now, of course, a large part of the other ones is a bovine agriculture, cattle and so forth. But that's for a different days discussion, a different days podcast. But it's quite staggering, really.

I mean, one third to 40% of world methane emissions, with methane being a worse global warming, a fender gas, ton for ton or kilo for kilo, and CO2 arise from undererated waterways. So what can Nanobubbles do to help with that? Well, if we can aerate such waterways with Nanobubbles, for example, using low energy submersible generators powered by solar energy and solar power, which we can do with certain types of Nanobubble generators in an energy-friendly way off grid, then we can really try to stamp out the problems of undererated waterways and algae blights that that would bring and really try to suppress eutrophication.

And of course, this could be done using carbon and carbon subsidy economics and indeed carbon capture economics can of course be used to track all of this as well if that's desired. In terms of environmental impact, that's something very important to bear in mind in terms of Nanobubbles. Also, we, and by we, I mean mankind, we're using about, I would say, of the order of several percent of global electricity supply for aeration of water.

And we're creating these massive large bubbles that rise up in activated sludge tanks at water treatment plants, and they rise up within within fractions of a minute, within 10s of seconds, the water may have left the aeration tank. So if we could use nanobubbles instead, particularly in a low energy way with some breakthrough nanobubble generator designs, we can really reduce the energy demand that we need in water treatment and water aeration. And the same goes for fish farms, irrigation, oil and gas, and many other areas really to reduce the amount that we're putting on aeration or perhaps carbonation of water.

To do this in a vastly more energy efficient way, that can really boost the efficiency and reduce the carbon and electricity and energy footprint of industry. So this can really improve many industries, water treatment, for example, being a huge one, but also really help the environment in terms of suppressing harmful eutrophication. In terms of the history of Nanobubbles, we can do a little whistle stop tour, I suppose.

Well, I'm sorry to disappoint you folks, but there ain't a lot of history to discuss because it's really only been in the past, I would say, 15 to 20 years and really only since around 2010 that there has been a lot of laboratory and scientific emphasis on studying Nanobubbles and their fundamental properties. And quite a lot of this goes back to the Epstein-Plessé theory of thermodynamics, which states that Nanobubbles cannot exist thermodynamically. And of course, that's perfectly true.

We all know that they're not thermodynamically stable, but the Epstein-Plessé theory of bubbles in general hypothesizes and posits that Nanobubbles can't really exist for more than micro to milliseconds. However, that has not really been what has been observed experimentally, but it's only really been in the past 10 to 15 years that the advent and sophistication of laser light scattering, for example, dynamic light scattering, has allowed us, and by us I mean the scientific community, to really establish the definite presence of Nanobubbles kinetically beyond mere seconds, and sometimes even into days, weeks, and months with a sort of negatively decaying population. So really, there hasn't been a great deal of historical work done on high quality, unambiguous research into Nanobubbles.

The property that's been studied most, I would say, has been the Zeta potential and the electrokinetic properties of Nanobubbles, which is certainly a very logical place to start, given the analytic chemistry's familiarity with Zeta potentials and so forth. But we've made a lot of progress in the past, I would say, 10, 15 years in laser light scattering analysis of Nanobubble populations and typical sizes. And there's yet more progress that can be made in many areas of Nanobubble science and analysis.

But that's something that we'll discuss in future episodes. I suppose the future of Nanobubbles, where we're going. And also, I suppose, in our next episode, which is going to focus more on the basic scientific fundamentals and principles of Nanobubbles.

Briefly, in terms of how Nanobubbles are made. Well, we're opening a Pandora's box there. There's so many different methods of doing it.

I mean, essentially, up until the invention of the electric field approach, my lead invention about seven years ago, there was just really mechanical ways of generating Nanobubbles, where we would apply a lot of turbulence to the liquid, for example, by a hammer mill rotation or ball mill rotation, or else by putting at high pressure gas, for example, essentially pure oxygen from a pressure swing adsorption system or even a gas cylinder through microporous and indeed nanoporous membranes. Now, the problem about membranes is they get blocked and biofouled, and there's nothing much that we can do about that. I mean, there's things we can do with the side, et cetera, et cetera, but it's a fundamental problem of biofouling and blocking of membranes.

Also, of course, there's a problem of high energy costs, massive pressure drop, pushing gas through a granular solid by the Karman-Kazanian equation, and of course the maintenance problems of moving parts and pumps that break down. So the essential advantage, I suppose, of the electric field approach is that there's no moving parts, and we can apply an electric field, which effectively, the electrostriction there effectively gobbles up extra gas molecules from an upstream population of mother macro bubbles or meso bubbles. And effectively, we cannibalize the outer periphery of the large bubbles to create a subpopulation of baby daughter nanobubbles.

So that's an exciting innovation that's very low in energy, so low, in fact, it can be powered by solar methods. And that's something that can be discussed later in our podcast series. So in any event, I suppose I want to thank you a lot for listening to this introductory episode.

I've outlined a story on how I first became interested in nanobubbles, essentially stumbling upon the area after working in electric fields and gas and water for whatever 20 years. I've explained a bit what nanobubbles are and how they're different from macro bubbles and mesobubbles, evading Stokes law, supplanting, subversing Henry's law, if you like. I've spoken a bit about what makes nanobubbles quite interesting for the environment, for industry, about energy savings.

I've spoken a bit about the history of nanobubbles. Not that there was too much to say there. We've explored briefly how nanobubbles are generated.

In the next episode, we're going to be exploring more deeply the science of nanobubbles, including various scientific properties that give them their rather unusual and standout properties. I'm excited to be looking at that. That's going to be very interesting.

Anyway, thanks very much for your attention today. Enjoy your next episode.