The Science of Nanobubbles

Hello again, and welcome back to Nanobubbles 101. And this is the second episode where we're going to discuss some of the more fundamental scientific properties and some of the underlying physicochemical mechanisms of Nanobubbles. Once again, I am Niall English.

I am the co-founder of AquaB, which has been going since year 2020. I served for several years as CEO, and now these days I'm CTO. And I'm also a professor in chemical engineering at UCD, as well as the holder of an ERC advanced grant on Nanobubbles.

Now, as I alluded to in the earlier introductory episode on Nanobubbles, they have many intriguing, unique properties, not least of which is being able to, I suppose, try to evade, to some extent, buoyancy or Stokes law, and then, in a sense, sidestepping, at least for a limited time, Henry's law. In other words, the total level of thermodynamic solubility. And those are two very important properties.

So we're going to delve into that in more detail in this current episode on more fundamental scientific properties of Nanobubbles. And where better to start than Stokes law? So in the last episode, I spoke about a glass of carbonated water with bubbles rising quickly, then taking a glass of champagne or beer and looking at the very bottom to see the small bubbles being generated there.

So the small bubbles that we see at the very bottom of a glass of beer or wine, perhaps one tenth, one twentieth of a millimeter, just about visible to the naked eye are mesobubbles in the mesoscale region. And typically mesoscale bubbles are less than about 100 microns or one tenth of a millimeter in size. Then once we get into the microbubble regime, which is smaller still, typically lower than about 10 microns in bubble diameter, depending on how good your eyesight is, we're not going to be able to see smaller mesobubbles, smaller than typically about 40, 50 microns.

And once we get really into the microbubble regime, we don't perceive those individual microbubbles, but we see them rather as a general sort of cloudiness or turbidity, or perhaps milkiness quality to the water. So if you ever see water that's very cloudy, and folks describing this as large amounts of nanobubbles, I think you should be a little bit wary, because as I said before in the last episode, nanobubbles are not visible to the naked eye, because by definition, they're smaller in diameter than the wavelength of light. So, yeah, so that's something to be aware of.

Now, once you go smaller still into the nanobubble regime, anything that's defined as being less than typically 1,000 nanometers or one micron is classified by many people as a nanobubble. I personally prefer to classify bubbles that are smaller than about 200 nanometers or 250 nanometers into nanobubbles, because they tend to be more stable or more metastable nanobubbles with a lifetime that's more meaningful. Typically, something in the sort of no-man's land between several hundred nanometers and a micron, I would just personally describe as a small micro-bubble, even though it's comparable to the wavelength of light, perhaps a bit smaller, perhaps a bit bigger.

Typically, light is a wavelength of the order of 500 odd nanometers, visible light, that is, from the sun or from an artificial light in a room, indoor lighting. So that's something to be aware of in the bubble kingdom, as it were, or distribution of bubble sizes. Now, typically, once you get down into the small micro-bubble regime of several hundred nanometers or more than, or of the order of one micron, the rising times by Stokes law might be of the order potentially of a couple of hours in terms of a lifetime in the mother liquid, or typically water, or it could be petrol, or and so forth.

But really, the nanobubble then, if the nanobubble is typically less than one or 200 nanometers, then the scant buoyancy and Stokes law will be, to all intents and purposes, evaded, because the rising time will be slow, terribly slow, that's more comparable with just general convection currents in the liquid. So then really, the lifespan of the nanobubble and the stability or meta-stability or instability, however you call it, however long or short that nanobubble lives, is effectively divorced and disentangled from its Stokes law rising time. And whether that nanobubble then lasts for a minute to a month, then depends on how stable or unstable the nanobubble is.

So we really have to start thinking about the electrostatic and density properties of the nanobubble itself to start making commentary on to how stable or unstable we may think its meta-stability is. And once again, I remind folk that nanobubbles are not thermodynamically stable. All they are is meta-stable.

So any additional presence of gas beyond thermodynamic solubility, as defined by, say, what percentage we are of Henry's law at a given temperature, what percentage the conventional solubility is, anything that I say is in the nanobubble phase or nanodissolved phase, as I explained in the introductory episode, well, that level of solubility is meta-stable. The question is, and the key question is, is it meta-stable to the extent it may only be stable for minutes or fractions of a minute, or could it be stable to potentially weeks or months if there wasn't some biological or chemical demand for the gas, for example, oxygen or CO2 in that solvent, for example, water or petroleum or whatever the solvent is? And that really is the key question.

So let me remind ourselves once more of this key question. What is the typical kinetics of demand of gas in the liquid, of the chemical or biological process? For example, are we burning fuel in an engine?

Are we having Nanobubbles in water in an activated sludge water treatment plant or dissolved air flotation, where the demand time, instead of demanding fuel to be burned within seconds, could be demand for it in minutes to hours? Or are we maybe in a slower moving, long irrigation system where we might require Nanobubbles to be present, even with some turbulent pumping, potentially for many, many hours or one or two days? Or are we in a reservoir situation where we would hope that Nanobubbles generated could be stable, potentially for many days into weeks?

So really, the kinetics of demand and how quickly the demand for gas is really starts to dictate for how long we need the Nanobubbles to be metastable. Is it minutes? Is it months?

Is it somewhere in between? And then we're really getting into the whole art of population and lifetime engineering. Can we try to manipulate the lifetime?

Have it deliberately short, deliberately long? Do we want to have a larger population or a smaller population? How does that feed into, as it were, our Nanobubble generation strategy?

So these are real questions that we have in terms of Nanobubble engineering. What sort of physicochemical properties do we want to dial up, and what indeed can we dial up, bearing in mind what generation approaches we have at our disposal? So really, something to bear in mind is the density of the bubbles and the demand kinetics we have for them.

Now, mechanically generated Nanobubbles, by definition, have typically a shorter lifetime. That's because typically the density of them tends to be a bit lower, as given by the Laplace pressure from the Epstein-Plessé bubble theory, thermodynamic bubble theory. So the Laplace pressure posits that the pressure of the gas inside the bubble might tend to be slightly higher than that of the surrounding liquid, due to thermodynamic curvature effects of the bubbles.

And the scientific community is a little bit divided about that and implications for mechanical equilibrium that that might have for having slightly distributions of pressure throughout the body of a liquid. We have tend to found, in the case of electric field generated nanobubbles, by we, I mean my research group and I, that there tends to be sometimes, well, often, a much greater mass than expected by the Laplace pressure in the form of nanobubbles. And we've established this in various ways, for example, titration, gas chromatography, and various other methods compared to the light scattering population that we have.

So really, the density and electrostatic personality of the nanobubbles can really dictate how stable or unstable they are. And as I mentioned in the previous episode, the Zeta potential is one area where we can try to establish how stable nanobubbles are. Quite often, folk in the chemistry and analytical chemistry community are trained to think of Zeta potential as definitely meaning a surface charge.

As in, this is a reflection of electrophoresis. And that's very important, and in many cases, that's true, that there may well be apparently a slight negative surface charge on the nanobubbles in terms of their electrostatic potential. However, there is also the concept of dielectrophoresis, that dipoles and dipolar phenomena of the surrounding water and dipolar accumulation, if you like, in the hydration layers around nanobubbles means that nanobubbles can also move in apparent electrophoresis, but it's dipole-driven, so dielectrophoresis.

So the presence of us measuring a zeta potential, for example, with laser light scattering methods, does not necessarily always mean, in my view, that this originates from an explicit surface charge on nanobubbles. In many cases, that is an important effect, but it can also be a background dipole orientation effect of say dipolar solvent molecules like water in the hydration or solvation layers immediately around the nanobubble. And this really defines the electrostatic potential and electrostatic personality, if you will, of the nanobubbles.

So typically, electric field generated nanobubbles as compared to nanobubbles generated by other methods, for example, mechanical methods, typically have a larger magnitude of a zeta potential. So the zeta potential could be perhaps sometimes, in some cases, two or three times larger. Other cases, perhaps one and a half times larger than the nanobubbles generated by classical mechanical generation methods.

So this gives us an early clue that nanobubbles can be longer lived when generated with electric field methods and also higher in terms of population per unit volume per milliliter. So for example, in my laboratory, we've done quite a lot of work at gauging and measuring nanobubble populations by laser light scattering over time and how those populations shift over days, weeks, and indeed into months. And we've done some work in terms of trying to characterize the negative exponential decay of the population of nanobubbles over time and of how nanobubbles might grow over so slightly and coalesce.

Typically, we wouldn't really expect nanobubble populations to get above, much above 10 to the 9 nanobubbles per milliliter, because if they do, then you start to see crowding effects. Nanobubbles at levels of populations of order 10 to the 9 per milliliter, even high 10 to the 8, they tend to see each other or see each other's reflection. And the nanobubble population begins to get so crowded that the electric field being emitted, if you like, from one nanobubble is, I suppose, beginning to enter into the surrounding electric field of another nanobubble, and they're beginning to interfere a bit with each other.

So there is typically a natural upper limit on the nanobubble population we found to be of the order of high 10 to the 8, lowish 10 to the 9 nanobubbles per milliliter. And really being able to gauge nanobubble population levels in industrial generators where you may be producing hundreds or even of the order of one or two thousand liters per minute, that's very important, being able to gauge and compare and contrast the population and how the population may decline slightly over time to really get a good idea on the population dynamics of industrial scale nanobubble generators. And that's something that AquaB is working on.

Not that I particularly want to speak at great length in the current podcast about AquaB's activities, but I'm more interested in general terms. So one other scientific aspect I think that is important about nanobubbles is their temperature and pressure sensitivity, as well as their lifetime kinetics and survival kinetics. So being able to measure nanobubble populations at elevated temperatures is important to characterize their time decay.

Quite often, if a nanobubble population is sort of structurally unstable and would collapse upon itself or merge with each other within maybe minutes or so, and certainly within less than an hour or half an hour, that can be a key indicator. If it's doing that at an ambient temperature, then it would clearly be even more unstable at higher temperatures like 60, 70, 80 degrees. And also it's also important to establish for many industrial applications, which we're going to discuss more in the next episode, the third episode, the pressure dependence as well.

Because in some cases, it may be desired for Nanobubbles to be temporarily stable or metastable at high pressures. So really, quite often, the method that we will use to generate Nanobubbles, or that should be used, needs to be gauged carefully as to whatever the situation is, the temperature, the pressure, the pH of the solution that's desired, the level of salt, and so forth. So I think that we've given a fairly broad overview of some of the key scientific concepts, the physicochemical stability or instability, depending on how you think of it.

So I want to thank you again for listening to this episode. As a quick recap, we've gone through some of the main physicochemical properties. We've discussed Stokes' law, Henry's law.

In particular, we've discussed bubble size distributions, the whole concept of a zeta potential, residence times, the whole idea of population and lifetime engineering, why they're important for different particular applications, different contexts. And I think we can look forward to the next episode, the third episode, where we're going to be examining how and why some of these properties that we've discussed here are of interest for a rather a wide range of environmental and industrial applications. So thanks once again, I'm Niall English, and I do hope you'll join me in the next episode and indeed further episodes beyond.

Thank you very much.