This image shows a stabilizer forming a net-like structure on a tiny air bubble and thus supporting it. Image: Group Jan Vermant, ETH Zurich.
This image shows a stabilizer forming a net-like structure on a tiny air bubble and thus supporting it. Image: Group Jan Vermant, ETH Zurich.

Oktoberfest in Munich, Germany, is not only an exciting cultural event, but also a source of inspiration for materials scientists and engineers. Not the beer itself, but the beer foam.

A good head of foam – generally measuring about 1.5cm and containing an impressive 1,500,000 bubbles – is supposed to be a sign of quality and freshness. Ideally, this foamy head remains stable, but several processes can act to destabilize the bubbles: liquid drainage of the foam, merging bubbles or popping can all cause rapid destabilization. These are generic problems common to all types of foam, whether in food and drink or technologically advanced materials.

One destabilization process, in which large bubbles become larger and smaller ones shrink and eventually disappear, is particularly difficult to stop. Experts call this process ‘Ostwald ripening’, after a German chemist and 1909 Nobel Prize winner Wilhelm Ostwald, who first described the phenomenon over 100 years ago.

Ostwald ripening causes an undesirable change in the texture of beer foam and foamed food products, and weakens product performance in many other situations as well. Achieving foam and emulsion stability thus presents a challenge across a wide range of materials science applications, from personal care products to advanced functional materials.

"Foams – whether beer foam, ice cream or foam for insulation – tend to coarsen due to their bubbles merging or ripening," explains Jan Vermant, professor for soft materials at ETH Zurich in Switzerland.

Surface-active components, such as certain proteins in beer foams, can typically prevent or slow down ripening by lowering surface tension, at least in the short term. But these components cannot ensure the long-term stability of the foams: they can only slow down the ripening process, not stop it once it has begun.

Vermant and his group have now taken a new approach to the foam stability problem, which they report in a paper in the Proceedings of the National Academy of Sciences.

"For the first time, we have succeeded in quantitatively controlling the dissolution arrest of foam bubbles and formulating novel, yet universally valid strategies. These will help the food and materials industries to develop controlled and more effective stabilizers in order to prevent or stop Ostwald ripening," says Vermant.

In their study, the ETH materials researchers showed how particular particles can act as a stabilizer and protect small bubbles against shrinkage. For testing purposes, the scientists used micrometer-sized latex particles and particles shaped like rice grains. These particles were chosen because they form an irregular network structure at the bubble interface.

The researchers tested whether this network can support the bubbles in a special microfluidic arrangement. They coated individual bubbles with a controlled amount of the particle stabilizer and then gradually exposed them to changing pressure conditions in a mini pressure chamber, thus simulating Ostwald ripening.

"This allowed us to determine precisely the pressure at which the bubble begins to shrink and finally collapses," says Peter Beltramo, a postdoc in Vermant's group. This particular experimental design allowed them to vary the number and nature of particles coating the bubble, and then relate the number of particles to the surface rheological properties. They identified surface yield stress as the prime parameter that needs to be controlled.

The researchers found that even partially covered bubbles could be just as stable as bubbles that were completely covered with particles. As a result, the required quantity of stabilizer can be predicted accurately. "Our findings will save a lot of materials and thus reduce costs," emphasizes Beltramo. Furthermore, the researchers found that a coated bubble could withstand a much higher pressure than an uncoated one.

These findings about the role of interfacial mechanics are universally valid for all materials with large surfaces or for applications in which surfaces play an important role, says Vermant. For example, the ideas and measurement techniques they developed can be applied to other cases of thin film stability, such as the films lining the alveoli in the lungs or tear films on the eyes. "These films are very stable, with the stability being imparted by similar mechanisms – developed by nature," says Vermant.

Although the findings are general, they could be of particular benefit to the food industry: food scientists can now search for edible stabilizers to make foamy foods such as ice cream or even bread dough last longer. "We provide the food industry and other companies with development guidelines and quantification tools that they can use to develop new products," says Vermant. And indeed ice cream helped to initiate this research, which was co-financed by Nestlé. Yet, what is good for beer foam or ice cream may also be good for making concrete better: incorporating small, stable bubbles can make it lighter and better able to resist thaw-freeze cycles.

This story is adapted from material from ETH Zurich, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.