"Aerophilic" surface minimizes foam by absorbing bubbles
Although some people may like a good frothy head on their beer, the foaming of liquids is considered problematic in many fields. A new MIT-designed material could help, as it keeps such foam from forming.
In various industrial processes, unwanted foam can both clog up machinery and occupy space that would otherwise be put to better use. Additionally, when foam forms in bioreactors that are being used to grow cells, those cells may be damaged or killed when the foam bubbles burst.
Ordinarily, substances such as alcohols or glycols are added to liquids in order to minimize foam formation. Those chemicals subsequently have to be filtered back out, however, adding to the cost and complexity of the overall process. Additionally, they may alter the chemistry of the finished product – whatever it is.
With these limitations in mind, a team led by Prof. Kripa Varanasi started by watching slow-motion video of bubbles rising up through a liquid and accumulating on a submerged surface, forming a foam. Among other things, the scientists noticed that each bubble would initially bounce back down from the surface a few times, before eventually floating back up and settling in place against it.
This led the researchers to create what's known as an "aerophilic" surface. In the same way that hydrophilic surfaces attract water, aerophilic surfaces attract and then spread out gas bubbles.
The MIT surface is made up of three layers, each one with a more finely-detailed texture than the one before. This structure, called a plastron, keeps a thin layer of air trapped against the surface. As a result, when bubbles rise up and hit that surface, they soon spread across it and dissipate, instead of repeatedly bouncing off. This means that they can't accumulate on it, so very little foam is formed.
In lab tests, two sheets of material were placed in beakers full of a foaming liquid – one of those sheets had been coated with the aerophilic surface, and the other had not. While the "bounce time" of the bubbles on the untreated sheet was in the hundreds of milliseconds, that figure was reduced to just a few milliseconds on the treated sheet. This difference resulted in a full layer of foam forming on the untreated material, but almost no foam forming on its aerophilic counterpart.
The technology is reportedly ready for commercialization, with few if any further refinements needed. It's described in a paper that was recently published in the journal Advanced Materials Interfaces.