ScienceWise - Jan/Feb 2008

Bubbles and Salt

Article Illustration
Christine Henry and Dr Vince Craig in the “bubble lab”
Article Illustration
Sea foam. The presence of salt makes enormous differences to bubble properties.

The huge difference ions make to bubble coalescence

Researchers are measuring how dissolved salts are affecting bubble coalescence. 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, RSPSE, and 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. There’s a lot of data in biology, for example, that reveals that by adding a different salt or a different concentration of the same salt you can produce a very different outcome but there’s no theory to explain why. “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 frit placed at the bottom of a glass tube filled with the salt solution. “We shine a laser beam through the bubbles as they move up the column. The beam is collected by a photodiode detector on the other side of the column and the light signal is converted into a voltage. In effect, we’re measuring 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.

“It’s a simple, reproducible method that works well, and it’s been used by previous investigators, including Vince Craig, Barry Ninham and Ric Pashley, to measure the effect of single salts in solution on bubble coalescence. I’m extending these studies by studying mixtures of salts and the effect of salts in non-aqueous systems. “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. We’re seeing that the size and the symmetry of the ion makes a difference on bubble coalescence, and this understanding has wide repercussions."

“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.”

“With bubble coalescence, we see really rich complex behaviour where different salts do very different things,” says Dr Craig. “So, here’s an opportunity where you can provide some really high quality data with which the theoreticians can use to test their theories in a simple system; a system that is more accessible than say a really complex protein system in a cell.” “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. 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.” 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.

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