Generating Nanobubbles

Welcome back, everyone. Here we are in our fourth episode in our Nanobubbles 101 series. And today we're going to discuss Nanobubble generation and all of the challenges and opportunities that that brings.

Once again, I'm Niall English, your host, and I am the co-founder of iQubi and CTO, and I'm also a professor in chemical engineering at UCD. So without further ado, let's explore some of the challenges of Nanobubble generation. And where better than to start off with the status quo, or at least the status quo ante, before my lead invention of electric field enhancements of Nanobubble generation.

So that very much is or was mechanical generation of Nanobubbles. Quite often, this would be generating Nanobubbles in a Venturi section, or generating Nanobubbles from a mechanical screw section, or from rapidly rotating blades. In other words, mechanical methods of severe high turbulence agitation of liquid with some level of existing gas there in the form of larger macroscopic bubbles.

And the essential idea is to break down those bubbles to make smaller Nanobubbles by energy-intensive turbulence. Another approach is to simply push high-pressure gas through membranes or, they might be called, microporous solid beds as well, or adsorbent beds, it might be couched in such language. But fundamentally, it's a micro or nanoporous solid, where we have liquid flow going through it governed by the Karman-Kazanian equation of granular bed fluid flow.

And in this case, as I alluded to in the first introductory episode in our podcast series, the high-pressure gas that's been pumped in, for example, from pressure swing adsorption or from a gas cylinder, has to penetrate through the porous network alongside the liquid. And this can lead to very large pressure drops that are given by, let's say, the Karman-Kazanian equation of granular bed fluid flow. Quite often, because of fouling or biofouling issues, these membranes tend to get blocked up in practice, which is a severe operational problem.

But all of these methods, as I've discussed briefly in the past, I suppose, two or three minutes, they all hinge on mechanical generation approaches, whereby we've got the breakage in a top-down sense of a larger microscopic bubble into a smaller nanoscale bubble. You might also call this bubble fragmentation. And this is taking in high amounts of energies and is achieved by mechanical methods in all of these methods I've just described.

And quite often, these can lead to the generation of a large distribution of bubble sizes in the nanomicro, meso, and even macroscale regime. And it may be that the actual tail of the distribution in the nanoscale region may often be quite small. It may typically be less than 1% and usually less than one-tenth of a percent of the total bubble population is actually in the nanoscale region, especially by way of mass, of the overall bubble.

So quite often, what other people are talking about when they talk about nanobubbles and what mechanical generation approaches do is they generate large numbers of bubbles, not quite indiscriminately, but of a wide range of sizes. A telltale sign is that if you see a great deal of cloudiness or milkiness in the water and you're told that this is a nanobubble generator, the generator may well be generating some tail of a distribution of bubbles in the nanoscale region, but in many cases, the very fact that the bubbles are visible shows that many of these bubbles, there is a very large portion of the bubbles which are not in the nanoscale region because nanobubbles by definition are not visible to the naked eye. You typically need laser light methods to probe those.

So if you're seeing these cloudiness and fuzziness and turbidity of the water, that's the typical hallmark or calling card of mass production of micro bubbles, which are much shorter lived and certainly do not evade or escape Stokes' Law. But they may be around for long enough, perhaps for the order of an hour or so, until they disappear into the atmosphere, sometimes much shorter. So you always have to be very careful about the actual production, I suppose, by mass of nanobubbles compared to other bubbles and what the ensuing lifetime of that minority of the bubble population would be.

So with any nanobubble generation method, you really have to look at the nanobubble population in terms of bubbles per buck. So what's the population of bubbles, but also what's the actual mass in nanobubble form as opposed to micro or mesobubble form? Where do you draw the line between a nanobubble and a small micro bubble?

Is it at 200 nanometers, as I advocate? Might it be at 400 or 500 nanometers? These are all slightly subjective questions.

There's no perhaps right or wrong answer. But really, in terms of trying to diagnose the efficiency of any nanobubble generation approach, I would be interested in looking at the mass of nanobubbles produced. Density, of course, and population are not the same thing.

We have to be careful about that. So the mass of nanobubbles being produced, and what is that mass of nanobubbles? A, as a proportion of the total mass of bubbles being produced, and B, in terms of the operational energy cost of achieving that.

And then, of course, how sustainable is this approach before it might break down and need maintenance, intervention, et cetera, et cetera, in terms of operational feasibility, which of course are very important patterns. If you're the operational manager of a manufacturing or a production plant or a water treatment plant or whatever the type of plant is. So I so far spoken in rather general terms about how I would think about the efficiency and effectiveness of nanobubble generation, as well as looking at the overall mass of nanobubbles being produced.

The next logical thing would be in terms of population and quantity and mass would be, well, what's the lifetime of those bubbles? Do those bubbles collapse in and of themselves within minutes? Could they be stable for hours?

How are they stable in clean water? How are they then stable in water? Where does the high chemical or biological gas demand like COD, BOD in the case of oxygen?

These are all questions that one would need to use to rank which nanobubble production method would be best for given applications or even in more general terms. But anyway, I will indulge myself a little bit and talk a bit about my own lead invention. I did mention this briefly in terms of the first introductory episode.

So in terms of the electric field approach, where we have an externally applied electric field to generate nanobubbles, what generally happens is that we have an upstream population of macro or mesoscale bubbles that have been generated. For example, we might be using a venturi section or a compressor section or some type of very coarse bubble generator. And then that population of macroscopic and mesoscopic bubbles comes into contact with an electric field.

And the electric field may be in the form of, say, some type of wire meshes or some type of electrode of some particular geometry. It depends on if we're doing a submersible generator that's simply submerged in the body of water, or if we have a flow-based generator where we're actively flowing through a channel and through a residence contacting area where the electric field and the mesobubble population can come into contact. Anyway, what we effectively do, then, is we effectively...

The electric field fragments and breaks up from the outside inwards, from the outer periphery inwards, and effectively cleaves off a subpopulation of baby, daughter, and nanobubbles from these mother macrobubbles. Now, you might ask, well, how does this happen? Well, the answer is electrostriction.

So let me define what that is. And I certainly didn't invent electrostriction. Electrostriction is a more obscure topic in condensed matter physics, but it essentially goes as follows.

If you would apply an electric field to, for example, water, petrol, or a liquid, it can make the liquid itself more dense because the molecules effectively pack together more efficiently. Now, when we have a larger bubble, say a macromesous bubble, passing through the electric field, passing by electrodes, by the way, electrolysis does not occur here because of sheathing of the electrodes. It's deliberate and willful.

We're not trying to make hydrogen or do electrolysis. That's what we're trying to avoid because it's a high energy process. The electric field makes the water immediately surrounding the larger bubble get more dense.

So therefore, the bubble water interface retreats. It goes back a little bit into the mother liquid itself. Then you're left with this temporary vacuum on the outer ridge of the gas mother bubble.

Now, mother nature abhors a vacuum, and therefore, to restore mechanical equilibrium, in other words, the pressure in the large gas bubble with the surrounding liquid water or liquid solvent around the bubble, we effectively suck in gas from the outer periphery, and these are sucked into, I suppose, an emergency basis to reestablish mechanical pressure. These are sucked into the surrounding liquid water in the form of Nanobubbles, or sort of baby daughter bubbles from the mother macro bubbles. So we effectively have a subpopulation of daughter Nanobubbles that have formed.

And in this case, the mechanical work is being done by the turning force or torque on the solvent or water molecules themselves, as electrostriction makes them pack in a more oriented way. But this is a very low energy process, and the actual nucleation energy of the bubble in terms of the mechanical work done, the level of efficiency can typically be of the order of 30 to 40% energy efficient, which is a lot better than mechanical generation approaches, where the Nanobubble population implies that the level of nucleation efficiency is substantially less than 1% efficient in terms of the nucleation energy efficiency if we look at bubble nucleation theory. So this means that typically we can get perhaps mass conversion, so the mass of daughtered Nanobubbles from upstream mother bubbles can often be of the order of perhaps a quarter to a third, so 25 to perhaps 35% efficiency, sometimes I've seen higher, and this efficiency of mass conversion is staggeringly high compared to typically less than one-tenth of a percent that one would see with mechanical generation approaches.

So the level of energetic viability is quite striking, and of course operationally we don't need a membrane to achieve this. In fact, a membrane would slow us down and wouldn't particularly help us. We just mentioned the energy cost.

So in terms of my own particular breakthrough, of course I did have help with others in my group. In terms of reduction to practice efforts under my instruction, we were able to develop some strong patents, and we are dealing with the prosecution of those now at present. So that's interesting and exciting to get essentially a good level of global cover.

On a personal note, aside from the EIC Accelerator grant to Aqua B, I have also been awarded an EORC Advanced grant on Nanobubbles, starting soon in 2024. And that will be focused upon lifetime engineering and population engineering, with applications in carbon capture, for example, water treatment, aquaculture and agriculture. And the essential idea is, can we dial up and control and manipulate the lifetime, be it shorter or longer, that we wish to have of Nanobubbles, and as well as the overall population?

Because being able to control that and have that longer or shorter, larger or smaller, will allow us to manipulate and get the best performance efficiency for those various different unit operations, as we might call them, water treatment, carbon capture, etc. So that's interesting, but also very challenging in view of the energy challenges and other methods that I've discussed with more classical Nanobubble generation methods. And in many cases, the electric field approach does tend to overcome, at least in large part.

So those are certainly things to bear in mind, these important principles of lifetime engineering and population engineering in the wider arena of Nanobubble engineering, we might call it. So thanks very much for listening to this particular episode, the fourth in our series on Nanobubble generation in particular. I hope that you found it as interesting and enjoyable as I have.

To give a quick recap on what we discussed just now, we've discussed some of the problems and challenges that we have with classical bubble generation, essentially based on mechanical methods, for example, membranes blocking up, the high energy costs, perhaps the limited lifetime and bubble instability. I think I've tried to give you an insight into why this can hamper real world applications. And then I've introduced some of the electric field approaches, which I feel help to overcome many of these limitations.

Now, in the next episode, the fifth in our series, we're going to be considering more deeply one of the most important applications, and that is water treatment. So Nanobubbles in water treatment and water aeration, water carbonation, and we'll certainly have plenty to say there, plenty to discuss. And I do hope you'll join me, and I'm looking forward to that.

And thank you very much again for your attention today.