Christine Henry and Vince Craig with the apparatus
for measuring bubble coalescence. “With bubble
coalescence we see really rich complex behaviour
from a very simple system,” says Dr Craig.

Researchers at the Australian National University (ANU) are measuring how dissolved salts are affecting bubble coalescence (how bubbles merge). Now, while that might sound like an arcane and esoteric quest, the scientists believe the data they are collecting will be critical for testing theories on how air/solvent interfaces operate. This in turn may transform many industrial processes connected with soft matter systems (for example, emulsions, gels and colloids) and revolutionise biology.

“Anyone who has taken a dip in the Australian surf will have observed the white foam produced by a breaking wave,” says Dr Vince Craig. “It’s a bright effervescent mix that persists for some five to 10 seconds and then it’s gone. What most people don’t realise is that the behaviour of this foam, the size of the bubbles and their persistence, is largely governed by the salt in the water. If there was no salt in those waves, the foam would look completely different!”

Dr Craig is a colloid scientist in the Department of Applied Maths at ANU. He’s convinced that what we can learn from this behaviour is basic to fundamental processes that drive all biology as well as having valuable applications to many industries.

“If you have water with low salt, that is water with a low concentration of salt, all the interactions between the particles are very simple,” says Dr Craig. “The dissolved salt exists as ions that can be described as just a simple charge. The forces at work are long range and repulsive, and nothing much complicated can happen.

“However, if you increase the salt concentrations the electrostatics are screened, the forces become much shorter ranged and they become much more complicated because different salts do different things. So, you suddenly have these specific ion effects and this opens up an enormously rich complexity of what can happen but there’s no real theoretical framework to help us understand this.

“It’s a really difficult challenge. And one of the limitations faced by theoreticians is that they don’t have a lot of experimental data to test their theories with.

Which is where measuring the effect of different salts on bubble coalescence comes in. It’s a study that several colloid scientists at Applied Maths have been grappling with over many years. At the moment, Christine Henry is the researcher at the lab bench making the measurements.

“It’s been known for a long time that dissolved salts will affect how bubbles coalesce in water,” says Ms Henry. “The salt in the water is in some way stopping the bubbles from melding or coalescing together as they do in pure water.

“The presence of the salt ions in the water is keeping the bubbles separate, making them stable for a longer time. But it’s not just any salt that causes this effect, because some salts have no effect and nobody understands why that is and why some behave differently to others.”

To measure the effect of different salt solutions (and the same salt solutions at different concentrations), Ms Henry creates bubbles by feeding nitrogen through a glass tube filled with the salt solution.

“We shine a laser beam through the bubbles as they move up the column and measure the turbidity of the solution. If you have a lot of small bubbles the laser light is scattered more and the signal coming through to the photodiode detector is lower.

“The traditional view of colloid science and surface chemistry has been that you just look at ions as point charges and it doesn’t really matter what types of ions they are. However, our research is demonstrating that it does matter what ions are present. By looking at the ions as real bodies, with a size and a polarisation and so on, we can get much more accurate information about what’s happening at the interface of a liquid and gas or a liquid and a solid. Indeed, this seems to control a lot of the behaviour down at that very fine scale.”

The applications of this work are still many years off and it’s difficult to know exactly what they’ll be though Dr Craig points out there are many industrial processes based on bubbles and high salt solutions where this understanding could be critical.

And just to underline how critical salt is to the processes of life on Earth, Dr Craig is happy to throw in a few observations on what this means for the history of life.

“For all the complex stuff that happens in our cells you need to understand and be able to describe what the ions do at interfaces,” explains Dr Craig. “Unfortunately, at the moment it’s too complicated to describe this in the complex environment inside a cell but we have this lovely simple model system represented by the bubble interface where we do have some level of understanding.

“There’s an evolutionary aspect to this as well. We’re saying that complex interactions can’t take place unless you’ve got a lot of salt. If we accept that life probably started in the ocean then it gives you a time limit as to when it was possible for life to take off – the oceans first needed to build up their salt levels to some critical level.

“And flowing on from this, I’m currently looking for someone to help me search the literature to survey what salt levels are found in all living creatures to find out what’s the lowest salt level any creature can live with. I’m even prepared to speculate on what that level might be, around 0.1 molar salt concentration. That’s where the threshold lies for the inhibition of bubble coalescence. We’re not sure why it applies to bubbles but I have a feeling that this is also the minimum level needed to sustain complexity.

“Life needs salty water, and a lot of salt. And that’s why all living creatures carry around in their cells quite a lot of salt. If they didn’t need it, it wouldn’t be there.”

So, next time you’re down on the coast watching the surf bubble and froth, take a second to consider that you’re observing a process that depends on high levels of dissolved salt and that your very existence might also depend on those same processes.

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