The secret life of shampoo: Bubbles and droplets through the eyes of engineers
VOICEOVER
Welcome to Up Close, the research, opinion and analysis podcast from the University of Melbourne, Australia.
SHANE HUNTINGTON
I’m Shane Huntington. Thanks for joining us. Bubbles and droplets are found not only in nature but in many of the materials used in industry and in our homes. Despite their prevalence, our understanding of how they behave has been somewhat limited; until recently that is. We’re now beginning to understand that even subtle chemical changes often measurable only on the nano scale can have substantial impact on how bubbles and droplets behave. For gases such as carbon dioxide, the behaviour impacts the capacity of our oceans to absorb the greenhouse gas, a major concern for climate change. To tell us more about our growing knowledge of bubbles and droplets and their role in our world, we’re joined by Associate Professor Ray Dagastine and Dr Rico Tabor from the Department of Chemical and Biomolecular Engineering at the University of Melbourne, Australia. Welcome to Up Close Ray and Rico.
RICO TABOR
Hello.
RAY DAGASTINE
Well thank you for having us.
SHANE HUNTINGTON
Guys, when chemical engineers like yourselves talk about bubbles and droplets, are you talking about the same sorts of things that the rest of us understand as bubbles and droplets?
RAY DAGASTINE
Well yes and no. Every time someone says bubble, they immediately think of blowing bubbles. While it’s not that that isn’t relevant, that’s not actually most of the time what we mean about bubbles. What we’re talking about are when you have a bubble under water; so it’s just a single bubble. When we blow bubbles with a soap film, that’s actually you could call that a double bubble because it’s actually got two sets of interfaces so it’s actually more relevant to a foam which is one use of bubbles.
But when we’re talking about bubbles, we normally mean if you blow bubbles underwater say the way you might see someone scuba diving and you see bubbles coming up. We’re concerned about bubbles in that context.
SHANE HUNTINGTON
When we talk about bubbles versus droplets, what is the difference there in what we’re describing?
RAY DAGASTINE
Well so a bubble is really just a gas pocket in a liquid. It’s kind of fleeting. They can dissolve. They can disappear. They never hang around for a lot and because it’s full of gas, if you put it underwater it wants to go to the surface pretty quickly. But what it is, is that water and gas aren’t really mixable. You get a gas pocket and the water hangs out somewhere else. The same is true for oil droplets. Oil droplets are compartments of oil within a liquid. Their uses differ a lot because when we have two phases we can dissolve things in two phases, we can get chemical reactions to occur in either the oil or the water, as opposed to bubbles, not quite as much.
SHANE HUNTINGTON
Rico, let me ask you about the interfaces that we see in some of these bubbles and droplets. In the sort of work you’ve been doing, what is that interface because presumably rather than having an internal gas, a bubble or an interface in an external gas, you’re just talking about a single gas bubble inside a fluid or something of that type. What is the interface? What prevents it from collapsing?
RICO TABOR
That’s a great question. So when you’ve got a soap bubble as you pointed out, you’ve got a film of molecules and chemicals at the interface which basically do the work of stabilising the bubble. When you’ve got a pure bubble, you might have some ions absorbed at the interface but most of the time you haven’t really got very much there at all. You’ve got a definition between the gas and the liquid and sometimes that can be quite poor and diffuse. The reason the bubble is stable for as long as it is, is the surface tension. So this is the property of liquids that causes, for example, on your Teflon saucepan causes water to form beads. It’s a force that pulls in the interface and that’s what causes bubbles to be stable. It’s got this force which is really just interactions between water molecules at the interface summed over all of the water molecules that are there.
SHANE HUNTINGTON
This force must be, although it’s on a small scale, incredibly strong because we see pictures of divers at great depths under the ocean where the pressure is incredible but the bubbles still form and maintain their size as they rise to the surface?
RICO TABOR
Well the pressure within a bubble is actually dependent on its size. So small bubbles, very, very tiny bubbles have absolutely astronomical pressures within them. In fact it’s been suggested that you can cause controlled nuclear fusion in bubbles because the pressure is so great they just have a massive amount of energy. Larger bubbles have a much lower pressure.
SHANE HUNGTINTON
What ultimately causes a bubble to collapse or for a droplet to disperse?
RICO TABOR
It can be caused by a number of factors. You can either coalesce with another bubble; gas bubbles can just spontaneously dissolve. Gas does have a solubility in water and in fact different gases have different solubilities which always complicates things. So gas bubbles can just disappear.
SHANE HUNTINGTON
Let me ask when we look at droplets and bubbles in sort of everyday life where we would perhaps see them, can you give us some examples of where they are critical to the function of some of the things that we’re using in our everyday life?
RICO TABOR
Absolutely so they occur in almost every liquid system so they’re particularly relevant to things like carbonated drinks; Coke, other caffeinated beverages are available, champagne for example. The way that the flavour and the scent is delivered to you is completely dependent on the population of bubbles that rise through the liquid and burst.
They’re also very relevant in lots of food so ice cream is full of bubbles. That’s what gives it the interesting texture. Also things like bread. When bread dough rise the loose texture is caused by CO2 bubbles in the mixture. Droplets are in all emulsions so things like mayonnaise and salad cream are typical emulsions that you’d have in your own house and a lot of consumer care products so things like cosmetics are filled with emulsions as well.
SHANE HUNTINGTON
What’s the actual function that’s being performed by them in these various materials? I can imagine in bread you know the idea of bread rising of these bubbles and so forth is something that we can get our head around but in the emulsions and some of the other examples you gave, what are the bubbles actually doing?
RICO TABOR
So bubbles and drops, they give you the opportunity to have two different phases present; so you can have two different materials with different characteristics in very close proximity. So it’s like having a mixture but that can be much more stable and can deliver things in a completely different way. So it can change the thickness or viscosity as well as the properties and what you can dissolve in them, so emulsion is very important because you can have two different things dissolved in the water in the oil phase.
RAY DAGASTINE
Going on from what Rico was talking about in terms of where you would droplets or emulsions or bubbles and foams. While we’ve got great examples of bread and ice cream; who doesn’t love ice cream especially in the summertime. Other examples that are very important to our lives that we don’t see are in things like minerals processing which is a very important industry to Australia but worldwide as well. A lot of the steps to actually purify minerals - when we dig a rock out of the ground, we grind it up and then we need to get the mineral; it could be copper, gold, silver, nickel, magnesium, whatever you’d like actually out of the rocks and grinding only gets us so far.
A lot of the processes that then we use to separate the mineral from the rock involve processes that use either foams so bubbles or emulsions with droplets. One of them is called froth flotation. They use bubbles to separate out one mineral from a rock you don’t want. They’re just ground up into fine powders. Then you also use solvent extraction which is basically dissolving rocks in acid and then mixing them with an oil. So when you do that, some of the dissolved rock stays in the acid and perhaps the metal atom or molecule you want actually goes into the oil. So it’s a way to separate out on the molecular scale things you don’t want from the metal you are trying to recover.
It is very important in processes that we don’t see and the reason I’m bringing up minerals processing particularly is about 70 per cent of the world’s ores are refined using these processes. So it’s a pretty significant step to our everyday lives; it’s kind of a hidden use of emulsions and foams.
SHANE HUNTINGTON
Ray, although this use is obviously widespread in industry and in many other areas of our lives, is the area of research around bubbles and droplets relatively new? Many of our listeners I suspect have not heard much out in the research sector from this area.
RAY DAGASTINE
What amazes me about drops and bubbles is we’ve been working on them and researching them since the time of Leonardo da Vinci. He’s credited with some of the first bubble research actually. But because of their nature that inherently they’re a little fleeting, they’re actually quite difficult to study. At any given time when you’re trying to study in a controlled lab condition, the time scales that a bubble lasts can be quite small. They’re hard to manipulate. They’re difficult to study their surface chemistry or the nature of their surfaces. Not until recently have we really probably gotten good at taking hold of bubbles and being able to learn a bit more about them. Doesn’t me that bubbles aren’t studied everyday by a whole range of people but really I think possibly focussing on the nano scale nature of bubbles is something that’s much more recent.
SHANE HUNTINGTON
When we pull all this understanding together it gives us a different ability to design emulsions and bubbles. What kind of parameters do you control when you’re trying to design a good emulsion?
RAY DAGASTINE
So for instance bubbles as I said they’re pretty time dependent in their existence and in any process or product what you really care about controlling in the first instance is how many bubbles or drops and their size. So you get a distribution. Are they small? Are they large? There’s a lot of interesting things that go on just because of their size. For example, if your bubbles are too big in your ice cream it tastes terrible. If your bubbles are just the right size in your soft drink you get great mouth feel; the texture you actually feel in your mouth. In food science they actually call that mouth feel.
So getting a bubble or drop at the right size and the right time so when you design an emulsion or foam you care about how long do they need to be at this size, are they going to be stable, are they going to hang around. If I’m going to be mixing it or putting it in some type of chemical processes, is the process going to change their size?
So a lot of what you care about designing bubbles or drops in an emulsion or a foam is, are they going to be stable? If they bounce into each other, are they going to stick together and make a bigger bubble or a bigger drop or are they going to bounce off each other and they’re going to stay stable in the size range that say I want them to be in?
So the stability of bubbles and drops is very sensitive to how they interact on the nano scale because when they bump into each other, it’s that last 100 nano metres between them or maybe micron to 100 nano metres that really matters. Now when I say a micron, your hair is maybe about 50 microns wide. So when bubbles and drops maybe even a millimetre in size of smaller get close enough that they’re a fiftieth of the thickness of your hair apart or smaller than that, that’s when it really starts to matter whether or not bubbles and drops are going to stick together or bounce off each other. That’s really the key to stability. Studying that is much more recent that the time of Leonardo da Vinci.
SHANE HUNTINGTON
Rico can you tell us, what sort of parameters affect the way these bubbles and droplets interact when we’re talking about the actual interface between I guess the inside and the outside of the bubble? Can we control these parameters?
RICO TABOR
Probably the most important thing is whether you’ve got a stabiliser present. So in most applications where we want bubbles to hang around. So in foams, we normally put a stabiliser on the bubble so some sort of surface active chemical which sticks to the surface and stabilises it so it gives some kind of steric barrier.
RAY DAGASTINE
Like a soap.
RICO TABOR
Like a soap film. So soap molecules absorb onto the interface; they stick to the interface like glue and they give a steric barrier that stops bubbles coalescing when they bounce into each other. In other applications, you don’t have that. So for example bubbles in the ocean or bubbles in soft drinks they will stick together based on other parameters within the system. We found really sort by accident that pH is one of the most important things. pH is controlled obviously by whether you’ve got acid or base in your solution but also if you’ve got carbon dioxide present in your gas.
SHANE HUNTINGTON
I’m Shane Huntington and my guests today on Up Close are Professor Ray Dagastine and Dr Rico Tabor. We’re talking about bubbles and droplets here on Up Close coming to you from the University of Melbourne, Australia.
Gentlemen, in order to interrogate the parameters that affect the way bubbles form, the way they work and how long they’ll last and the like, what scales of measurement are we talking about? On what sort of size range do we have to be able to measure things and what sort of forces are there? Ray?
RAY DAGASTINE
Well the bubbles and drops themselves that we tend to work with are probably around the 30 to 50 micron in size so diameter so about the thickness of your hair. But the scales of the things that you’re actually looking at, you’re driving these things down to 10 nano metres apart so 10,000 is the thickness of your hair.
In terms of forces, we’re really looking at the forces that are about the weight of a bubble or a drop on that scale so really tiny. I think if you scale it up by something like 30 million times it’s like the weight of a sheet of paper, so they’re quite small in that sense. But that’s for an individual bubble or drop. If you look at a lot of bubbles, things always scale up.
One of the interesting things about being able to look at these very tiny forces over these very tiny distances is we’re able to do that because of the nano technology revolution. I have a little chuckle in my voice because just calling anything a revolution out of the political context is always a little odd to me. But it is perceived that the advancements around nano technology and this can be everything from semi conductors in computers to hard drives to actually just how engineered our laundry soaps are. Everything is engineered on the nano scale. So that has driven a huge amount of technological innovations and that’s actually really developed a set of tools that we’ve been able to use and extend and harness in the realm of studying drops and bubbles.
SHANE HUNTINGTON
Before we get to those particular tools, is it possible to do these studies en masse in sort of ensembles where you have large numbers of bubbles and droplets or do you need to look at them individually to get the sort of understanding we’re talking about?
RAY DAGASTINE
This is really reliant on being able to look at individual drops and bubbles. It’s very difficult to get a group of drops or bubbles all to be exactly the same size and then come up with what you would call an indirect method to be able to look at these things. So what sets our research apart from pretty much anything that’s been done before is our ability to directly measure the forces between two individual bubbles. If we collide two bubbles together or two drops and it’s about being able to directly measure and quantify those forces on this nanometre scale.
Most bubble and drop research has previously been focussed on large swarms or much larger scale ensemble averages and while that tells you some information, it doesn’t really get down to these key fundamental questions. So it’s the single bubble or two bubbles together is really where the breakthrough lies.
SHANE HUNTINGTON
When you’re in your lab and you’ve got these two bubbles in a certain apparatus, how do you go about physically measuring these very small ports? What’s happening? What sort of equipment and measurements are you making?
RAY DAGASTINE
The tool we use primarily is something called the atomic force microscope. To give you some context, it’s an extension of the scanning tunneling microscope which isn’t any more informative but I can tell you someone got the Noble Prize for inventing the scanning tunneling microscope.
The atomic force microscope was designed to actually take pictures of surfaces on the nano scale. That’s actually its primary application. But what we use it for and the mode we use AFM, it’s a very fancy positioning device with a very fine spring or force sensor on the end of it. So if you want to measure the forces between two drops or bubbles, as you bring them together what you really need is a very accurate, highly sensitive distance positioning device and a very sensitive spring or force sensor on the end of it. That’s really the aspects of AFM that we use.
So while AFM can do an awful lot of things in taking pictures, what we’re trying to do is get a look at not just bringing two drops or bubbles together is we do simulated collisions. One of the first steps to this is, how do you make two drops or bubbles collide against each other? Normally people think wait, if I’ve got a bubble in water, it’s floating around, it’s going every which way. We have to take a drop or bubble and we get it to attach to a surface on the back of it and then we push it against another bubble on the front. Being able to hold onto drops or bubbles is quite a tricky thing and we’ve developed most of the methods to do that.
Sometimes we use specially modified surfaces. Effectively we use a surface that to bubbles it looks like we’ve got Velcro on it. One of the other ways we can generate bubbles is we actually use sound; we use ultrasound to make bubbles on surfaces which is kind of neat that you can use sound to make a bubble. By having these two bubbles or drops mobilised on surfaces we’re able to then simulate collisions in labs at different speeds. The speeds are set by how they might move around in nature under Brownian motion all the way up to speeds that they might collide if they were being processed in solvent extraction or something like that.
SHANE HUNTINGTON
Rico, how do you determine all the other parameters around the bubbles that you’ve created that you would need to sort of match the experiments up to theory? So for example you’ve got these two bubbles in the device and you’re - as Ray pointed out, you’re driving them together and you’re measuring the forces between them. But presumably you need to know their chemical composition precisely; you need to know their size precisely in order to use that information. How do you go about determining that?
RICO TABOR
That’s a very good question. A lot of the times we can infer the experiment of parameters from our data. So we’ve got our force data. So if you imagine our AFM is like if you take a ruler and balance it on the end of a table and imagine putting an orange on the end of a ruler. Any force we put on that orange if we push it up or down are going to deflect the ruler. It’s going to bend it. What we measure is that bending and so from that data, we can actually get a lot of information.
Other parameters we need to know are pH and how much gas is in there because that’s going to determine what happens to the surface of the bubbles and what our bubbles are made from to begin with.
SHANE HUNTINGTON
This is Up Close coming to you from the University of Melbourne, Australia. I’m Shane Huntington and our guests today are Ray Dagastine and Rico Tabor and we’re talking about the latest research into bubbles and droplets.
Rico following on from these measurements, I assume at some stage this data has to be valuable to industry or valuable to someone who’s using bubbles in some sort of specific application? How scalable is the information that you’re determining about bubbles to those sorts of processes?
RICO TABOR
What we do is very fundamental but it does have a great deal of relevance. In industrial processes such as soft drinks, the bubbles we look at are of comparable scale and we look at them as a function of pH which is very relevant for soft drinks. Most drinks are quite acidic which is what the tangy flavour comes from in things like Coke and champagne.
Also, we think our bubbles might be of interest to environmental scientists as well because we think they might have an important role to play in the ocean. Again, these are exactly the scale of bubbles that we measure directly.
SHANE HUNTINGTON
Ray can you tell us a bit about how our physical understanding of bubbles and droplets has changed recently in particular with the work you’ve been doing and how that might be applied further out in the sort of general population?
RAY DAGASTINE
So two of the big outcomes from our recent work is our understanding of how bubbles are stable in two different scenarios. One of the scenarios is when bubbles are acted on by outside forces. Just from water flowing around bubbles and dragging it along, bubbles can bounce into each other and whether or not they’re stable is really dependent on how quickly they bounce into each other and how quickly they’re pulled apart. One of the key things we’ve seen in our measurements, which is scalable to some other emerging technologies such as microfluidics, is examples where bubbles and drops can actually coalesce or stick together when you actually try and pull them apart. That when you drive them together as they’re colliding they’re fine but when they go to pull apart from each other is actually when they start to stick. That’s controlled by fluid flow around the bubble. But that’s scalable through processes like microfluidics.
I think one of the more fundamental things that we’ve looked at recently is to try and understand the nature of the surface of a bubble. When I say that I’m talking about whether or not a bubble is charged. All bubbles, for reasons that even we’re still trying to understand or there’s a little bit of a debate over, are charged which means a bubble isn’t a neutral surface. It actually has ions on it and this is rather important for how bubbles interact with other bubbles and in most processes.
There’s a raging debate in the literature in chemistry as to whether or not a bubble is positive or negative and you’d think well it wouldn’t be that hard to measure. Interestingly, there are people that have measured bubbles being positive and there are people that have measured bubbles being negative. What’s even more confusing is then we get a bunch of people who do molecular and modelling and quantum mechanics and they said, we’re going to solve this problem. We’re going to model a bubble and we’ve got modellers saying bubbles are positive and we’ve modellers saying bubbles are negative.
So we’re in a bit of a conundrum there. A lot of the measurements would be called indirect. Nobody’s really been able to grab two bubbles and drive them together and what Rico had said earlier is that one of the key things we found is that bubbles are very sensitive to pH. What we’ve really found is that the surface of the bubble, whether or not it’s positive or negative, is very sensitive to pH. What’s even more interesting is it’s not just pH is important, the type of gas is very crucial to that.
So we’ve actually figured out where bubbles are positive and where bubbles are negative and we think we’ve found the key variables in terms of pH and the type of gas and the amounts of different types of gases that really explain some of the disparity in the observations that others have had. It’s also a guide to suggest to modellers if you’re going to model a bubble, these parameters are important and if you don’t take them into account, you’re probably not going to reproduce what experimentalists say.
SHANE HUNTINGTON
So would it be fair to say then Ray that given this sort of lack of clarity around exactly how these bubbles are working that things like our determinations of the ocean’s ability to absorb CO2 and so forth could be way off?
RAY DAGASTINE
So when you talk about the ocean’s ability to absorb CO2, that’s really about how much gas can dissolve into the ocean. One of the mechanisms for gases to be dissolved is actually through bubbles that get trapped in waves and not big waves just the little everyday waves that happen across the ocean.
But the mechanism for what we’ve learned about bubbles might affect things might actually be more relevant to how aerosols are generated. Most of the marine aerosols so we’re talking about little particles floating in the air that go up in the atmosphere and interact with - help form clouds and bounce of light. Most of the marine aerosols off the ocean are from little bubbles getting trapped in waves and then the bubble rising back up to the surface and popping. When that happens, you get a little spray of water droplets. As the water dries out, you’re left with little salt particles that go up and become the aerosol.
Now that whole process of bubbles getting trapped and then bursting is really dependent on the surface charge of the bubble and the amount of CO2 that’s there. What we’ve found is trace amounts of CO2 really affect the nature and surface of a bubble. So as we start to change the amount of CO2 that we have in bubbles, we may actually change that whole process. Now I don’t know which way it’s going to change but I think it’s something worth looking at for atmospheric chemists say maybe we need to look at how aerosols are generated as a function of CO2. Because if we start changing that in our atmosphere, that might actually change how aerosols are changed.
Now for those of you that don’t know, aerosols are a rather fascinating aspect of climate change because they tend to bounce light away from the earth. We get a certain amount of reflection of sunlight just from aerosols and it’s called the indirect effect on global warming; that aerosols - well they’re great for causing chemistry in the atmosphere and you get a lot of chemical reactions, but they also affect the amount of like that makes it to the earth. They’re very complicated to incorporate into global change models. But we know when we start to change the character of aerosols in our climate, it really does start to affect the whole global climate change effect.
So bubbles really, in our case, will probably affect more aerosol generation than actually what’s sequestered in the ocean.
SHANE HUNTINGTON
Gentlemen, this seems like a fairly complex area that brings in a degree of mathematics, modelling, chemistry, chemical engineering and the whole lot. Is it a large collaborative team that you’re working with on these projects?
RAY DAGASTINE
Actually, this is only possible because we’re actually a large group of inter-disciplinary researchers. Our research team has researchers from Department of Mathematics and Statistics, the School of Chemistry, other chemical engineers and it’s really reflective of a larger inter-disciplinary team within this University called the Particulate Fluids Processing Centre which is a special research centre of the Australian Research Council. So it’s through this bringing together of engineers and chemists and mathematicians that work in a very integrated team fashion, we’re able to come up with these very strong fundamental outcomes.
SHANE HUNTINGTON
Associate Professor Ray Dagastine and Dr Rico Tabor from the Department of Chemical and Biomolecular Engineering at the University of Melbourne. Thank you very much for being our guests today on Up Close.
RAY DAGASTINE
Thank you.
RICO TABOR
Thank you.
SHANE HUNTINGTON
Relevant links, a full transcript and more info on this episode can be found at our website at upclose.unimelb.equ.au. Up Close is brought to you by Marketing and Communications of the University of Melbourne, Australia. This episode was recorded on October 8, 2010. Our producers for this episode were Kelvin Param and Eric van Bemmel. Audio engineering by Gavin Nebauer. Background research was conducted by Christine Bailey. Up Close is created by Eric van Bemmel and Kelvin Param. I’m Shane Huntington. Until next time, good-bye.
VOICEOVER
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